Including Citizens’ Perspective in Advancing Urban Green Infrastructure: A Design-Toolkit for Private Open Spaces

Abstract

In response to the growing pressure from urban expansion and the risks associated with climate change, adapting cities and reintroducing nature into the urban environment are becoming increasingly urgent. The implementation of Urban Green Infrastructure plays a key role in adaptation strategies, thanks to its ability to offer a wide range of ecosystem services. However, the adaptation process must necessarily be trans-scalar and include collective participation to ensure a real improvement in both physical and social urban resilience. For this reason, citizen participation in planning processes is essential, but even more important is the recognition of their active role and the scope for private citizens’ involvement. The transformation of private spaces according to UGI principles and with a view to the common interest, however, is still a theme that is scarcely included in urban resilience plans. This study presents the results of a survey conducted in the case study of Milan through questionnaires and focus groups to record preferences, concerns, and priorities in the sustainable transformation of private open spaces. The outcome of the study is the definition of a toolkit for the design of private open spaces, integrating Nature-Based Solutions and adhering to UGI principles.

1. Introduction

Urban Green Infrastructure (UGI) and Nature-Based Solutions (NbSs) have emerged in recent years as a key topic in the debate on urban adaptation to climate change [1,2,3]. The growing evidence of climate change [4], the intensification of extreme weather events, and the exacerbation of urban risks have made it increasingly urgent to make cities more resilient and better prepared [5]. The emphasis on these approaches has grown significantly alongside the evolution of research on urban resilience and adaptation [6,7]. In this regard, the strategic and designed reintroduction of natural components and processes into urbanized environments can offer numerous benefits [8]: from mitigating the urban heat island effect [9,10] to managing stormwater drainage [11], from reducing air pollution [12] to positively impacting the psychological and physical well-being of citizens [13]. Actions aimed at reintroducing nature into cities respond both to the perceived need to restore proximity to nature in fully anthropized environments [14], and to the practical need to mitigate the limitations of urban settings in addressing increasing risks [15,16].

Various interrelated concepts and notions populate the field of urban adaptation and support this process of reintroducing nature into cities. The European Commission defines Green Infrastructure (GI) as “a strategically planned network of natural and semi-natural areas with other environmental features designed and managed to deliver a wide range of ecosystem services”. This definition refers to another fundamental concept, namely that of Ecosystem Services (ESs), which are “the benefits societies obtain from ecosystems” [17], including direct resource consumption (provisioning ESs), experiential benefits (cultural ESs), and contributions to the overall regulation and maintenance of fundamental environmental conditions for human life (supporting and regulating ESs). Based on this understanding of the interactions between humankind and the environment, approaches such as ecosystem-based adaptation (or mitigation) have become increasingly widespread [18]. Indeed, the aim of NbSs is exactly to “bring more, and more diverse, nature and natural features and processes into cities, through locally adapted, resource-efficient and systemic interventions” [19]. Similarly, the UGI approach supports the systemic planning of urban greenery as part of the key infrastructural networks of a city, responding to the key principles of Connectivity, Multifunctionality, Integration of Green and Gray Components, and Multi-Scale Implementation [20,21].

The substantial efforts of scientific research have progressively informed the definition of local policies and strategies. Currently, many international institutions are actively engaged in promoting UGI and NbSs at various levels, and this approach has increasingly gained prominence in international agreements concerning environmental protection, climate change mitigation, biodiversity support, and risk management [22,23,24].

However, limitations and barriers to the effective and systematic integration of UGI and NbSs into urban planning practice persist. This study specifically focuses on the importance of enhancing citizen engagement—not only through consultative activities in participatory public programs, but also by recognizing citizens as key actors in promoting the expansion of UGI within urban areas. It also highlights the value of private open spaces as critical areas for the diffuse, small-scale deployment of UGI across different spatial levels. The paper is organized as follows. In the next section, the theoretical background is presented, discussing the main barriers to the advancement of UGI and the role of citizens in this process. Section 2 also introduces the case study of Milan and the research framework provided by the YADES project. In Section 3, the mixed-method approach—combining focus groups and questionnaires—and the resulting design toolkit are described. The results of the study are presented and discussed in Section 4 and Section 5, respectively, followed by the Conclusions in Section 6, which reframe the research contribution within a broader context.

2. Theoretical Background and Research Framework

2.1. Barriers to the Advancement of Urban Green Infrastructure

Despite the intense research effort of the past decades, the effective integration of UGI and NbSs into urban transformation programs remains highly limited and fragmented [25], and a systematic adoption of this approach in urban contexts still faces several obstacles. Referring to Nelson et al. [26], five main challenges can be identified: (i) participation and equity, which emphasize the need to consider the dynamic and often uneven perceptions of risk and benefit across different social groups; (ii) governance, as the inherently complex and systemic nature of NbSs demands coordination across institutional levels and sectors, posing significant challenges to existing governance frameworks; (iii) valuation, highlighting the need for consistent methodologies to assess and communicate the multifaceted benefits of NbSs in economic, environmental, and social terms; (iv) infrastructure integration, which calls for moving beyond the traditional separation between green and gray infrastructure and toward a more holistic and multifunctional understanding of urban systems; (v) scale and feedback, which stress the importance of trans-scalar planning approaches and the involvement of a broad range of actors to ensure responsiveness to local conditions while aligning with broader sustainability goals. Similarly, Kabisch et al. identify the persistence of institutional inertia, sectoral silos, limited policy coordination, and a lack of public awareness as central barriers. Moreover, socio-spatial justice is a key aspect to evaluate how UGI is planned, distributed, and experienced within the urban fabric, in order to prevent the risk that green interventions may inadvertently reinforce existing inequalities or trigger processes of green gentrification [27]. Additional barriers can also be identified, such as financial constraints [28,29].

2.2. Managing the UGI as an Urban Common

Understanding UGI implementation as a complex socio-political process, rather than solely a technical challenge, calls for updated institutional models and inclusive governance frameworks [30]. In recent years, research has increasingly focused on how to combine traditional top-down decision-making models with greater responsiveness to bottom-up initiatives and needs. In this respect, collaborative governance models offer a useful perspective on how to structure co-decision and co-management arrangements among public authorities, private actors, and civil society [31]. Collaborative governance can be defined as the functions and structures of policy decision-making and administration that involve the public beneficially across the limitations of public institutions [32]. By adopting different kinds of horizontal decision-making structures, it aims at achieving consensus-based decision-making. The presence of a multi-stakeholder governance model, in which the community emerges as an active partner alongside other urban actors, is also identified by Foster and Iaione as a key principle in their work on translating Ostrom’s design principles to the context of urban commons [33]. Urban commons do not strictly correspond to public spaces or to a specific land use, but they are “rather something often formed politically, and certainly morally and ethically, in response to outside pressures that often seek to enclose it” [34]. Indeed, the recognition of shared urban resources as commons draws attention to the governance models of such resources and to the question of who benefits from urban transformation processes [35]. In their extensive review of collaborative governance across a range of policy sectors, Ansell and Gash identified some critical factors to ensure the successful collaboration, including face-to-face dialogue, trust building, and the development of commitment and shared understanding [36].

This perspective can also be applied to UGI, which serves as a key element in adapting cities to climate change and improving urban livability. Unlike other urban infrastructure, the implementation of green infrastructure cannot be entrusted to a single urban actor—typically the public administration—but rather requires multi-actor approaches and trans-scalar, trans-disciplinary collaboration. Several researchers have highlighted how conventional, large-scale, and centralized intervention are insufficient to address the challenges posed by urban growth and climate change [30,37,38]. In contrast, they argue that localized NbSs offer a more sustainable and adaptive alternative [39], scaling interventions to the level of a neighborhood street or even to private spaces, as in the case of this study [40,41]. This shift in perspective supports the reconfiguration of citizens from consumers to stewards of ecosystem services within the urban environment [42]. In the context of private open spaces, stewardship dilemmas also arise, including residents’ long-term commitment in managing private greenery and the potential conflict between individual preferences and community benefits. Additionally, the logic of exclusion versus inclusion becomes particularly relevant: while private spaces are, by nature, exclusive, their integration within the urban green system responds to a collective need for adaptation. Thus, the engagement of residents in the sustainable transformation and management of private land can be properly framed within a shared-governance framework. This approach aligns with the evolving understanding of urban commons as not only physically shared resources but also as socially constructed spaces, shaped by norms, agreements, and governance arrangements that facilitate civic stewardship and public–private collaboration.

This process is still highly experimental and cities can be seen as “laboratories where management strategies and governance structures for ecosystem stewardship are tested and evaluated” [42]. This study focuses specifically on the barriers and opportunities related to residents’ engagement in private space greening, seeking to bridge the differing perspectives of public and private actors and to lay the foundation for improved dialogue.

2.3. Advancing UGI Through Citizen Engagement

In light of the challenges highlighted in the literature, increased citizen involvement—going beyond traditional forms of participation—can represent a strategic approach to support the advancement of UGI [43]. Beyond fostering widespread awareness of the benefits of nature in urban environments, citizen support and their perceptions of risks and benefits are crucial in underpinning innovative policies and forward-looking planning decisions.

Since the 1970s, a gradual cultural transformation has led an increasing number of people—even beyond experts and scholars—to develop a deeper awareness of the value of natural ecosystems and the importance of reintegrating and protecting nature in urban environments. More recently, the COVID-19 pandemic acted as a disruptive event, deeply impacting habits and perceptions [44,45,46]. During lockdown periods, the ability to physically or visually access urban green spaces was rediscovered as a valuable asset and a source of well-being [47,48].

On the other hand, urban planning approaches have, for years, increasingly sought to include citizens through participatory processes and public consultations [49]. Today, new scenarios for citizen participation and active inclusion can be explored, drawing on both the progress of research and a collective mindset that is increasingly open to and engaged with issues related to urban green spaces.

At the European level, there are still relatively few cities actively experimenting with effective and systematic methods for involving both public and private actors in the multi-scalar planning processes of UGI. The review conducted by Mahmoud et al. highlighted that public participation and collaborative governance are rarely considered an integral part from the beginning of UGI plans despite EU guidelines advocating for the adoption of co-creation approaches [50]. Among the most noteworthy examples are Barcelona, which, in response to the consequences of intense urbanization and climate change, has developed a detailed strategy to reclaim every vacant urban space as public green space, including the mapping of courtyard interiors and private areas [51,52,53,54]; Hamburg, whose Open Space Requirement Analysis (2012) classified all building typologies in the city and identified the potential for integrating new urban greenery in proximity to residential functions [55,56,57]; and Edinburgh, which since 2009 has developed an Open Space Strategy aimed at improving the quality and accessibility of existing green spaces, minimizing their loss to urban development, and ensuring adequate open space provision in new developments [58]. Above all, the example of Rotterdam proved to be of particular interest for this study.

The city of Rotterdam is experimenting a strategy combining “densification”—to achieve a mix of uses and functions—and “greenification”—to compensate for previously unmet or future demands of urban green provision [59,60]. Since the 1970s and more intensively in the last 20 years, the redevelopment of the central area has focused on reintroducing the residential function, promoting mixed-used and vertical growth [61]. This process went hand in hand with constant attention to the public space (the so-called “groundscraper” strategy). Additionally, Rotterdam has been facing sea-level rise, higher rainfall, increasing heat waves, and flooding, which pushed the city to adopt pioneer initiatives for urban adaptation, including GI as a key part of the urban program [60]. Citizen engagement and the inclusion of private actors was highly enhanced. Indeed, the scarcity of public transformable space—only 40% of Rotterdam’s area is public land—and the objective of balancing densification and accessible green spaces made it necessary to include private open spaces in each spatial planning action [61]. Many of the foreseen greening interventions apply to private areas and properties, such as green roofs and walls. Additionally, private initiatives are encouraged and supported both directly, through public financing of private actions (e.g., the Climate Adaptation Grant adopted in 2012), and indirectly, by deregulating certain initiatives and reducing bureaucratic restrictions.

2.4. Case Study and Research Framework

This study focused on the case of the city of Milan. This city combines the challenges of a large urban area with high population density, the risks associated with a territory increasingly affected by environmental issues and CC (impacting on air quality, water management issues, and heat peaks), and the potential of a dynamic metropolis long engaged in ecological change and the modernization of its governance structures [62,63,64,65,66]. Since 2019, the city has updated its internal organization by establishing the Directorate for Environmental Transition. Additionally, the Territorial Governance Plan Milano 2030 (PGT Milano 2030), adopted in 2020 and currently under review, outlined a roadmap to strengthen the municipal ecological network through the urban green and blue infrastructure approach and emphasized participatory processes and the inclusion of citizens in strategic decision-making [67]. Due to these characteristics, the city offers a particularly relevant context for investigating citizens’ perceptions and exploring opportunities to engage them more actively in the implementation of UGI—moving beyond purely consultative roles toward genuine citizen empowerment.

This study was conducted within the framework of the EU-funded YADES project, which focused on the adaptation of historic urban areas to climate change and concluded in March 2025. As part of the project, the author carried out research on the integration of UGI in historic and dense urban contexts, which also included the social investigation presented in this article. The project provided a relevant network for engaging experts in the discussion and collecting their feedback. This study is primarily focused on the social dimension, active participation, and the potential role of private actors in the implementation of UGI in the case of Milan. It was preceded by previous explorations that had already highlighted the spatial and strategic relevance of private open spaces in Milan, both for preserving existing green areas and for planning new ones. Previously published research on the spatial character of Milan’s green system [68] highlighted the presence of a relevant amount of privately-owned small-sized green spaces. In the central district of the city, private open spaces mainly consist of courtyards inside the urban blocks and cover an area of 1.6 km², which corresponds to 19% of the entire area of the historic center [69]. The study area specifically focuses on Municipality 1, the historic center of the city, which is characterized by high density and a historically layered urban fabric where the complementarity between public and private open spaces is particularly evident (Figure 1).

3. Materials and Methods

3.1. Scope of the Research

Researchers and practitioners are increasingly aware of the pivotal role that citizen engagement plays in the processes of renaturing cities. Citizens are now widely recognized as key stakeholders, with valuable local knowledge and a sense of attachment to their neighborhoods [70]. This study aims to explore new approaches to citizen engagement and empowerment in the development of UGI. In particular, it investigates the potential role of residents in transforming their own private open spaces, in line with the principles of UGI and NbS. When proposing such transformations, it is crucial to understand current citizen attitudes, perceived priorities, and needs and to raise awareness of the importance of private spaces in both preserving urban green heritage and contributing to future UGI networks [71,72,73].

This research seeks to address the following questions, investigated in the case study of Milan city center (Municipality 1):

• What are the perceived priorities regarding urban adaptation to climate change and the role of private spaces?
• What are the perceived risks associated with the implementation of NbSs in private open spaces?
• To what extent are citizens interested in playing an active role?

These research questions were investigated through a social inquiry targeting both a group of experts and a broader general audience. The findings informed the development of a design toolkit for NbS implementation in private open spaces within the Milan case study. This toolkit aims to align local citizens’ priorities and preferences with climate adaptation needs and Sustainable Development Goals (SDGs).

3.2. Methodology

The study was performed with two different approaches, each one responding to a specific goal:

• Focus groups were conducted within a selected community in early stages of the research to frame a preliminary investigation of the research topic with a target audience of experts or interested individuals.
• Questionnaires were distributed to a wide audience in the study area of Municipality 1 to record preferences and concerns about alternative solutions.

Last, the results emerging from the questionnaires were integrated into a design toolkit for including NbSs in private open spaces. The various steps of the study are outlined in Figure 2.

3.2.1. Focus Groups

The focus groups were conducted as part of the annual summer schools of the YADES project, in 2021 and 2022. The focus groups were interviewer-administrated, meaning that the collection of answers and feedbacks happened in a face-to-face situation where it was possible to make additional questions and have a discussion about the raised topics.

The first summer school, due to pandemic-related reasons, was an online event limited to project participants. The audience involved in the focus group consisted of individuals specialized in urban risk management, with experience in urban resilience and adaptation strategies, although not necessarily experts in green solutions. Additionally, most participants were not specifically familiar with the Milan case study. Therefore, the survey referred to historic centers and dense urban areas in a broader way.

Differently, the second summer school took place in person in Milan and was an open event. The audience that participated consisted of interested individuals with a scientific background and a moderately high sensitivity to environmental issues. In this case, the investigation focused more on the daily experience of greenery in the case of Milan and included a specific focus on private open spaces in this city. The detailed structure of the focus groups is reported in Appendix A.

3.2.2. Questionnaires

While the focus groups were targeted at an expert or interested audience, the questionnaires were aimed at investigating the general perception of the population of Milan, particularly in Municipality 1. The questionnaire was self-administrated, avoiding respondents being subject to the interviewer effect, and cross-sectional, aiming to explore the population’s attitude towards the current state of UGI in Milan’s historic center and possible future scenarios [74].

The questionnaire has been structured into three sections: (1) courtyards of Milan; (2) transformation scenarios; (3) general information. The detailed structure is reported in Appendix B. The first part aims to gather objective and subjective information about the current situation of private open spaces. Objective information includes courtyard accessibility, current functions, and the presence and integration of vegetation. Subjective information, on the other hand, refers to respondents’ evaluations of the adequacy of the current situation, along with some insights into the modes and frequency of private outdoor space utilization. The second section introduces the reference to future transformation scenarios for which respondents are asked to provide feedback regarding possible interventions to be implemented, priorities to be followed, and potential issues. It also investigates respondents’ inclination towards their direct engagement and the opening up of private spaces. Last, the third section collects some basic information about the respondents while ensuring anonymity and privacy, in order to verify the composition of the sample

3.2.3. Design Toolkit

The final step of the study involves the development of a design toolkit to integrate NbSs into private open spaces. While inspired by existing NbS catalogs—such as the classifications provided by the World Bank [75] and LabSimUrb at Politecnico di Milano [76]—this toolkit differs significantly in both scope and purpose. These previous catalogs offer broad taxonomies of NbS interventions across scales, ranging from territorial to building-level strategies. In contrast, this toolkit intentionally narrows its focus to small- and medium-scale interventions suitable for residential private open spaces.

Beyond its practical orientation, the toolkit is grounded conceptually in co-design methodologies. Referring to previous participatory experiences of NbS co-creation [77] and considering the guidelines drawn from EU-funded projects [78], the toolkit is not simply a technical compilation, but an instrument intended to empower residents as active agents in shaping their environments. Drawing from real-world examples—including Rotterdam’s adaptive design practices [79] and decentralized stormwater initiatives in several U.S. cities [80,81,82]—the toolkit is aimed to promote distributed agency in the development of UGI, emphasizing the role of private citizens as stewards and co-producers of ecological value. Without claiming to be a universal or comprehensive tool, the toolkit aims to connect the technical perspective of NbSs with the social dimension of the case study area. It embraces a bottom-up logic, incorporating residents’ perceptions and priorities—collected through the questionnaires—as key inputs in the structuring of design options. In this way, it serves not only as a practical guide but also as a mediating device between expert-driven urban policy and the lived experiences of citizens.

4. Results

4.1. Analytic Results of Social Investigation

4.1.1. Focus Groups

The focus groups were aimed to set the groundwork for an exploratory analysis of the topic in collaboration with knowledgeable or interested individuals. As illustrated in Figure 2, the two focus groups pursued distinct objectives, reflecting the different profiles of the participants involved. In brief, the first focus group confirmed that the limited pervasiveness and distribution of green spaces in historic and high-density urban contexts is a widespread issue, thereby highlighting the broader relevance of this study. In contrast, the second focus group contributed to a deeper understanding of the Milan case study and subsequently informed the development of the questionnaire. Participation data are reported in

Table 1. Focus groups’ information summary.

Focus Group	Mode	N. Participants	Participants Type
1st YADES Summer School	Online	38	Specialized/international
2nd YADES Summer School	Hybrid in presence in Milan and online	62	Specialized or interested in the topic/mostly based in Milan

. The analytical data from the responses are provided in the Supplementary Materials.

The first focus group included 38 international participants with proven expertise in urban resilience, mainly partners of the YADES project or representatives of the scientific community. This session served as a preliminary exploration of the value of integrating green features in historic and high-density urban contexts as a strategy for climate adaptation. In particular, the discussion focused on identifying the most effective approaches to promote this integration, as perceived by the participants, as well as the main barriers that hinder its implementation. From the starting general questions about the relationship between the historic/dense urban areas and the availability of urban green spaces, it emerged that, even when considering cities other than Milan, the lack of greenery is a widespread issue, with 31% of respondents reporting a severe lack and 63% a low presence. However, when asked about the distribution of green spaces between public and private areas, the responses contradicted the data from the Milan case. Indeed, 88% of the respondents believe that the existing greenery is predominantly public. It can be hypothesized that this perception is influenced by the accessibility of green spaces. Still, this highlights a lack of awareness of the role of private green spaces, which, while not directly accessible to everyone, provide a collective service in terms of ecosystem benefits. Regarding the perceived barriers to more effective integration of green infrastructure in historic contexts, respondents assigned significant importance to the scarcity of transformable public space (28%) and the inadequacy of existing planning tools (31%). Finally, when asked about the relevance of certain proposed actions to support the integration of green infrastructure in historic areas, respondents attributed slightly greater importance to coordinating private initiatives with public ones and differentiating green solutions.

The second focus group collected feedback from 62 participants, mostly based in Milan or at least familiar with the case study. The starting questions revolved around the perception of nature’s role in the daily experiences of the urban space. In total, 82% of the respondents claimed to have daily contact with nature, mainly in the form of visual or passing contact (e.g., passing through a park or walking along a tree-lined avenue during their daily commute). Activities involving stationary interaction with nature (such as spending time in the park to relax, exercise, or socialize) are less common. Respondents unanimously emphasized the fundamental importance of daily contact with nature, particularly highlighting its impact on mood improvement and stress reduction. Moving on to specific questions about private courtyard spaces, over half of the respondents reported not having any vegetation (26%) or having only potted plants (29%) in their courtyards, while 45% had ground-integrated vegetation. In the context of future transformation of these spaces, respondents attached greater importance to the “structural” incorporation of vegetation through tree planting and interventions to restore soil permeability. This was followed in perceived importance by the inclusion of furnishings for space utilization, while the inclusion of playground equipment for children was not strongly supported. Lastly, regarding barriers to implementing courtyard renaturation projects, respondents expressed more concern about increased maintenance costs, followed by the inconveniences caused by transformation work. Concerns about the loss of parking spaces were relatively mild, and the issue of insects due to increased vegetation ranked much lower.

Further insights were obtained by applying an inductive coding approach on the open-ended questions of both groups. Using ATLAS.ti software (version 25), recurring themes and patterns have been identified based on the context and meaning drawn from participants’ responses. Given the exploratory nature of the focus groups and limited size of the qualitative dataset, a single-coder strategy was adopted. A cross-reading of the open-ended responses from the two focus groups reveals a meaningful correspondence between the strategic directions identified for integrating UGI in dense urban areas (first focus group) and the types of interventions prioritized by participants in the context of private courtyards in Milan (second focus group). In particular, the first group emphasized the importance of coordinating private initiatives with strategic planning objectives and adapting green solutions to the historical and spatial context. These strategies align with the preferences expressed in the second focus group for structural and context-sensitive interventions, rather than generic or purely ornamental additions. While the subjectivity inherent in the coding process undoubtedly represents a limitation in the interpretation of these data, the thematic coding and resulting mapping supported a preliminary understanding that proved useful for informing the subsequent phase of the study. Figure 3 presents the coding of themes emerging from the open responses and the mapping of relationships between strategies and intervention types.

4.1.2. Questionnaires

Taking advantage of the relevant insights provided by the focus groups, a broader social investigation was carried out to investigate the general perception and attitude of Milan’s citizens towards including private open spaces in the municipal UGI strategy. The target of the questionnaire was specifically the population of Municipality 1, consisting of 111.560 people [83]. Setting a confidence level of 95% and a margin of error of 5%, the sample size was set at 383 respondents. The questionnaire distribution took place between June and September 2023. After closing the questionnaire distribution, a total number of 394 answers was recorded. Out of the total, 39 respondents declared not to have a courtyard or a garden in their dwellings and consequently did not fill out Section 2. Thus, the number of useful answers was limited to the 355 of those who declared to have a courtyard and entirely filled out the questionnaire. This still guaranteed a margin of error limited to 5.19% (see

Table 2. Sample size and margin of error of the questionnaire in Municipality 1.

Population Size: 111,560	Target	Total Answers	Complete Answers
Sample size	383	394	355
Confidence level	95%	95%	95%
Margin of error	5%	4.93%	5.19%

). The sample has a balanced gender distribution, with 51% of female respondents and 47% male. Regarding the age distribution, there is an imbalance towards the younger age groups, especially the 26–35 age range, and inadequate representation of the over-65 age group. This is certainly attributable to the online administration mode of the questionnaire.

As shown in Figure 4a, 90% of participants (355 answers) claim to have a courtyard/garden/space related to their residence. The vast majority of them live in apartments within residential buildings (92%), while a smaller number have different housing solutions. Regarding the private open space location, internal courtyards (answers “c” or “d” to Q3) were the most common type (84%), raising potential challenges in evaluating the visual and physical accessibility of private greenery [84]. Regarding the presence of vegetation of any kind (Figure 4b), 85% of the responses are positive. Out of the 303 positive answers, 26% reported to have only potted plants, and a significant 60% at least one tree. Natural surfaces, such as flowerbeds or grassy areas, are less common.

Regardless of whether there is vegetation at present, the significance of greenery in private open spaces is unanimously acknowledged, with an average score of 4.7 out of 5. Finally, respondents were asked to evaluate the greenery currently present in their courtyards with respect to quantity, quality, impact, and accessibility. Question Q10 records peaks of dissatisfaction, especially regarding quantity and quality, thus opening the possibility of taking greater and better actions for the integration of greenery in private spaces (Figure 5).

The assessment of the current state is strongly influenced by the frequency of use of private open spaces. A cross-analysis of responses to Q6 (How often do you use the courtyard space?) and Q10 (a—Satisfaction about the quantity of existing greenery; b—Satisfaction about the quality of existing greenery) reveals that respondents who use the courtyard space daily or several times a week tend to evaluate both the quantity (

Table 3. Cross-analysis of Q6 and Q10a.

	Is the Quantity of Existing Greenery Satisfactory? (Q10a)
	No n. Answers (Row%)	Yes n. Answers (Row%)	TOT
Frequency of use (Q6)
Daily—Several times a week	181 (77%)	55 (23%)	236 (100%)
Sporadically—Rarely or never	67 (56%)	52 (56%)	119 (100%)
TOT	248 (70%)	107 (30%)	355

) and quality (

Table 4. Cross-analysis of Q6 and Q10b.

	Is the Quality of Existing Greenery Satisfactory? (Q10b)
	No n. Answers (Row%)	Yes n. Answers (Row%)	TOT
Frequency of use (Q6)
Daily—Several times a week	155 (77%)	81 (23%)	236 (100%)
Sporadically—Rarely or never	61 (51%)	58 (49%)	119 (100%)
TOT	216 (61%)	139 (39%)	355

) of existing greenery more negatively compared to those who use the space more sporadically.

At the same time, the questionnaire responses also highlight how the presence of greenery encourages the use of the courtyard. Indeed, among respondents who reported the presence of greenery in their courtyard, 69% use the space on a daily or weekly basis, whereas in courtyards without any form of greenery, the proportion of frequent users drops to 52% (

Table 5. Cross analysis of Q6 and Q8.

	Is There any Greenery in Your Courtyard? (Q8)
	No n. Answers (Col.%)	Yes n. Answers (Col.%)	TOT
Frequency of use (Q6)
Daily—Several times a week	27 (52%)	209 (69%)	236 (66%)
Sporadically—Rarely or never	25 (48%)	94 (31%)	119 (34%)
TOT	52 (100%)	303 (100%)	355

).

The most relevant results obtained from the questionnaire are those concerning future transformation scenarios, included in Section 2. In particular, questions Q11, Q12, and Q13 investigated the preferences, priorities, and concerns about greening private courtyards. The questionnaire results highlight a strong preference for “structural” greening interventions, such as green walls and surfaces which are able to deeply change the perception and functionality of the space (Figure 6). To that point, respondents’ preference is aligned with the opinion expressed within the focus group and definitely with the direction of transformation highlighted by the analysis conducted so far. The preferences expressed are also consistent with the assignment of priority to air quality improvement and microclimate mitigation, which are actually severe problems in Milan directly affecting people (Figure 7).

Among the concerns, the increase in the cost of maintenance ranks first, as in the focus groups, stressing again the need to continue working on low-maintenance solutions and circular processes on the research front and to strengthen awareness-raising activities about the long-term sustainability of green interventions on the educational front (Figure 8).

About people’s attitude on their direct engagement, positive feedback was registered on participating in greening activities, even though there was a high rate of undecided respondents (Figure 9a), while there is a much more negative attitude towards the regulated opening of private courtyards to non-resident people (Figure 9b).

Last, a logistic regression analysis was conducted to investigate the effect of age on responses to three different questions concerning satisfaction with the current amount of greenery (Q10a), concern about increased maintenance costs (Q13e), and willingness to participate in greening activities (Q14). The response options for each question were dichotomized by aggregating positive answers (for Q10a and Q13e: “agree” and “strongly agree”; for Q14: “yes”) into YES = 1, and neutral or negative answers (for Q10a and Q13e: “neutral”, “disagree”, “strongly disagree”; for Q14: “I don’t know” and “no”) into NO = 0. As shown in

Table 6. Statistical significance of age as an explanatory variable for satisfaction with the quantity of existing greenery (Q10a), concern about green space maintenance costs (Q13e), and willingness to participate in greening activities (Q14). Significance codes: * 0.01 < p ≤ 0.05 (Statistically significant); ** 0.001 < p ≤ 0.01 (Very statistically significant); *** p ≤ 0.001 (Highly statistically significant).

		Estimate	S.E.	Odds Ratio	p-Value
Q10a	18–25	−0.4336	0.2170		0.04568 *
	26–35	−0.4827	0.2897	0.6171149	0.09572
	36–50	−1.2246	0.3825	0.2938752	0.00137 **
	51–65	0.1574	0.3564	1.170464	0.65879
	>65	−15.3770	621.6501	2.098233 × 10−7	0.98027
Q13e	18–25	−0.4336	0.2170		0.045684 *
	26–35	0.9405	0.2813	2.561262	0.000829 ***
	36–50	2.8760	0.4778	17.74316	1.75 × 10−9 ***
	51–65	1.6123	0.3950	5.014331	4.48 × 10−5 ***
	>65	16.2443	621.6501	11,345,140	0.979153
Q14	18–25	0.24846	0.21364		0.245
	26–35	0.29054	0.27920	1.337149	0.298
	36–50	1.62334	0.40128	5.069996	5.22 × 10−5 ***
	51–65	−0.20924	0.35228	0.8112005	0.553
	>65	0.03922	0.79308	1.039999	0.961

, taking the 18–25 age group as the reference category, age proves to be statistically significant in all cases, with some noteworthy differences. In the case of satisfaction with the amount of existing greenery (Q10a), respondents aged 36–50 exhibited the highest level of dissatisfaction, with an odds ratio of 0.294—meaning that the odds of being satisfied versus dissatisfied were approximately 3.4-times lower in this group compared to the reference category. The same age group also stood out in terms of interest in active involvement (Q14), with an estimated coefficient of 1.62334, significant at the 0.1% level, corresponding to an odds ratio of 5.069996—i.e., the odds ratio of being willing to participate instead of not being interested in direct engagement is about five-times higher than in the reference group. Lastly, with regard to concern over increasing maintenance costs (Q13e), all age groups showed statistically significant differences compared to the reference category, probably because the youngest group (18–25) is the least likely to be directly responsible for such expenses.

4.2. Integrating Citizens’ Perspectives into a Design Toolkit

The results obtained from the questionnaire suggest the need to base a viable strategy for the inclusion of private space in the UGI program on participatory processes and public consultations, in order to involve citizens in the definition of effective ways and tools for the sustainable management of private open space. The final step of this study reinterpreted the input provided by citizens and proposed a toolkit to integrate NbSs into private open spaces. This tool supports citizen empowerment by making them aware of the practical actions they can implement within the spaces under their control. The overarching logic remains that of UGI, as each individual action not only brings local benefits but also contributes to the broader ecological network at the municipal level.

The proposed toolkit is a catalog of possible actions selected for their compatibility with the focus on residential private spaces. The chosen solutions can be implemented by individual citizens owning an undeveloped area, by a group of citizens (such as residents in the same building deciding to act on the common courtyard), or even by municipal decision-makers to develop coordinated programs for intervention in private spaces.

The proposed classification introduces various parameters to support informed choices, including the following:

• The scale of application, classified into three classes defined on the quantiles of the size distribution of private open spaces within the study area: S < 125 m²; M 125–225 m²; L > 225 m².
• The impact on a scale from 1 to 3: (1) solutions requiring integrated application with others for full functionality; (2) solutions applicable individually; (3) solutions individually applicable with high impact.
• The mode of application with respect to the courtyard space, i.e., whether it involves interventions on the ground, on the surrounding facades, on the building, or the incorporation of a furniture/technological element.
• The perceived priority to which the solution responds, referring to the five options recorded through the questionnaires (see Figure 6): (1) air quality improvement; (2) aesthetic improvement; (3) microclimate regulation; (4) biodiversity improvement; (5) fostering social cohesion; (6) support for food self-production.
• The contribution in terms of adaptation to CC: (1) contribution to ground water infiltration; (2) contribution to rainwater management; (3) enhancement of biodiversity; (4) mitigation of micro-climate; (5) contribution to circularity; (6) contrast to drought.
• The type of solution distinguished between green, gray, or hybrid.
• The contribution to the Sustainable Development Goals (SDGs).

The toolkit aims to facilitate the identification of context-appropriate solutions. For instance, in cases where private open spaces are affected by flooding, priority can be given to actions that contribute to water management, such as vegetated surfaces, rain gardens, and permeable pavements. In contrast, in situations where the limited size of the space constrains certain types of interventions, the toolkit allows for the selection of actions compatible with small-scale settings, such as climbing plants or vertical greening solutions. Figure 10 shows the resulting toolkit, with 18 NbSs applicable in private open spaces and the corresponding data.

5. Discussion

The investigation conducted in the Milan case study enabled the exploration of both the potential and limitations in proposing new forms of active citizen involvement in the development of UGI. The results that emerged from the focus groups—particularly the second one—highlighted the importance of gaining a deeper understanding of local risk perceptions and community preferences in order to guide locally appropriate strategies and engagement models. Additionally, the questionnaire findings offered an insightful snapshot of Milanese residents’ attitudes toward integrating NbSs into private open spaces, as well as their preferences regarding forms of active participation. Milanese citizens have long been accustomed to being involved in urban decision-making, at least in consultive ways, and are generally receptive to more proactive forms of civic engagement. However, what was more unexpected was the general reluctance toward regulated forms of opening up private open spaces for broader community use. Similar initiatives—such as opening schoolyards outside school hours for collective activities—have already been piloted in Milan [85], as well as in other cities [86,87,88]. Nevertheless, extending this model to residential private open spaces has not, at present, met with widespread public approval, possibly due to concerns about noise and safety.

The active involvement of residents in incorporating the strategies and principles of NbSs into the spaces under their stewardship opens up promising avenues for the development of UGI. It allows for the envisioning of a complementary system of small green spaces that, collectively, can make a significant contribution to the urban ecosystem [89]. This strategic direction aligns with the core principles of UGI by reinforcing the connectivity of urban green spaces and multi-scalar implementation—particularly at the micro scale—enhancing social inclusion and the engagement of diverse stakeholders, and promoting the integration of green and gray components into cohesive and scalable solutions.

The opportunity to more systematically include private open spaces and to empower citizens represents a promising direction for a city like Milan, which is actively engaged in climate change adaptation. The city has already seized several collateral opportunities to strengthen its ecological network and promote the development of urban green areas —from the construction of new metro lines that led to the creation of linear parks on the surface, to the reuse of disused railway yards as new public parks [90,91]. However, in order to truly achieve the goals of green space accessibility and distribution, it is essential to experiment with innovative approaches. In this context, new models of public–private collaboration represent an opportunity that should not be overlooked [92,93].

As for the limitations of this study, it is important to highlight that the questionnaire targeted residents of Municipality 1. The reference sample is therefore representative of the population residing in a limited and distinctive portion of the city of Milan—namely, its historic center. Additionally, the online distribution method likely introduced a bias toward digitally connected respondents. Regarding the limited scope of the study area, it is worth noting that other studies have shown that citizens’ willingness to participate in the implementation of UGI tends to be higher in inner-city areas than in peripheral ones [94]. This may be due to the relative lack of green space in central neighborhoods, to demographic characteristics, or to other contextual factors. The study area of Milan’s Municipality 1 shares several characteristics with historic and densely built districts in other European cities—such as limited availability of public green space and a high proportion of impervious surfaces. In contrast, civic participation patterns are very site-specific and might vary even within the same city. However, previous participatory experiences, such as those of the Clever Cities Action Labs, which involved Milan, London, and Hamburg, provide a relevant example on how a flexible methodology can be applied and adapted to different local contexts [95]. Therefore, the results of this study are more likely to be applicable to other dense urban centers in European cities, rather than to suburban or peri-urban areas where the urban fabric, green space configuration, and social composition differ significantly.

Nevertheless, the study yields replicable and transferable results. In particular, the questionnaire can be easily reproduced in other case studies with only minimal adjustments. Furthermore, the statistical relevance of respondents’ age—especially the key role played by the 35–50 age group in shaping attitudes—offers a concrete and actionable insight, both for the municipality of Milan and for other urban contexts facing similar challenges. In this sense, the case study can serve as a useful reference for similar historical high-density urban areas seeking to engage residents in greening initiatives. While the findings of this study—including the specific NbS included in the toolkit—are context-specific, the methodological approach, grounding the operational tool into the social investigation, can be replicated in other urban areas. The external validity of the results is therefore conditional on contextual similarity, particularly in terms of socio-demographic composition, housing typologies, and local governance frameworks.

Finally, the developed toolkit comprises a critically selected set of NbSs applicable in private open spaces, aimed at enhancing citizens’ agency and positioning them as active contributors to the realization of UGI. Compared to most existing NbS catalogs, the proposed toolkit presents some innovative aspects. The first concerns the target users: Unlike traditional catalogs primarily intended for planners or public authorities, this toolkit is designed with residents as the primary implementers of the proposed solutions. While it may also serve as a useful reference for Municipality 1 and the city administration, its main audience consists of private actors and local residents’ communities. The second innovative aspect lies in the perspective adopted in its development: rather than offering an encyclopedic or knowledge-based overview, the toolkit adopts a practical and implementation-oriented approach, translating existing comprehensive NbS catalogs into an actionable tool tailored to the specific characteristics of the urban context and aligned with the priorities expressed by citizens. However, for this contribution to be fully integrated within a systemic UGI approach, a stronger public–private collaboration is needed, as in the successful case of Rotterdam [61,96,97,98]. This should include the following:

• Greater recognition by public authorities of the strategic role of private open spaces and of active citizenship;
• Administrative and financial tools to facilitate and support private initiatives;
• Technical assistance and monitoring tools to ensure that private interventions are carried out properly;
• Ultimately, a long-term strategy in which public and private actions are planned in a complementary manner to achieve clearly defined urban adaptation goals.

6. Conclusions

This study aims to contribute to the ongoing research effort focused on the systematic integration of NbSs into urban environments, with particular attention to the often-overlooked role of private open spaces and the potential for active citizen engagement in advancing UGI. The social survey conducted among residents of Milan’s Municipality 1 and the design toolkit for greening private open spaces represent a further step in the direction outlined by previous studies, which have emphasized the need for a multi-scalar integration of NbS. Wang et al. have already pointed out that, while most scholars have focused on the macro dimensions of large-scale, predominantly state-led or market-driven projects, in reality, a majority of urban regeneration efforts actually take place at the street level, involving small-scale interventions with limited funding and scope [41]. This perspective is reinforced by Pauleit’s identification of multi-scalar thinking as one of the key principles of UGI [21]. By focusing on residential private spaces, this study aims to fill a research gap regarding small-scale interventions, which are the ones that most directly touch upon residents’ everyday lives. For this reason, the investigation of citizens’ attitudes become fundamental to support multi-actor processes and inform the design of collaborative governance models.

In terms of governance, this study also suggests a potentially novel approach compared to the forms of participation typically explored in Milan and most other European cities. It proposes a shift whereby citizens move from being consulted or supporting actors in public-led initiatives, to becoming active agents capable of intervening directly in private spaces under their stewardship. While this approach can be included under the broader umbrella of public–private partnerships (PPPs) [93], it reorients the focus: Whereas traditional PPPs have primarily been used to attract private investment and delegate the management of public green spaces, the model proposed here centers on private space, with the public sector playing a supporting role by coordinating or aligning these actions with broader urban priorities. Indeed, the toolkit developed through this study is an operational instrument that can support resident-led greening initiatives. This means that it can be used by residents or local groups independently, making it actionable at the grassroots level. However, its transformative potential is significantly amplified when public institutions act as enablers—by embedding these citizen-led efforts within broader multi-scalar green infrastructure strategies, as in the case of Rotterdam [59,96]. Institutional actors can strengthen alignment by providing guidance—such as recommending plant species or setting minimum soil depaving targets—and by introducing incentive schemes or fiscal benefits. The role of public institutions is also to actively trigger a transformative process. While the study revealed a generally positive attitude among citizens towards participating in greening actions, it is equally clear that the transition from intention to real initiatives requires a launch strategy and sustained support from public authorities. In this way, the toolkit helps to bridge top-down policy goals with bottom-up action, fostering coherence in urban climate adaptation efforts.

Furthermore, the inclusion of private spaces within the UGI calls for a reflection on the nature and distribution of the benefits associated with this approach. The provision of ES is closely linked to the accessibility of green spaces. In this regard, the reluctance of residents to open private green spaces to non-residents tends to position such spaces not as urban commons, but rather as club goods, following the classification proposed by Casprini et al. [31]. However, it is important to note that ecosystem service provision operates across multiple levels: while the direct enjoyment of private green spaces may be limited to residents, indirect benefits—such as visual enjoyment—extend to the surrounding urban block, and broader environmental benefits—such as increased permeable surfaces, the creation of urban cool spots, and enhanced biodiversity—contribute to the well-being of the wider community [16]. This multilayered distribution of benefits reinforces the need to address socio-spatial justice in the planning and governance of NbSs, ensuring that both direct and indirect beneficiaries are considered. In the specific context of dense and historic urban areas, the systematic use of private spaces can also provide a valuable complement to public green space [68], particularly where available land is scarce. A recent study on the city of Naples highlighted “the need to update the transformation rules of areas of high historical-architectural value with the criteria that take into account the resilience essentials cities require” [99]. In this sense, integrating private green spaces into urban ecological strategies represents a viable pathway toward increasing urban resilience.

Finally, this study reaffirms that understanding citizens’ perspectives is essential both for advancing research and for designing effective collaborative governance frameworks. Even in widely cited cases such as Barcelona’s Superblock initiative—associated with a substantial increase in neighborhood green space—implementation has encountered significant opposition, including concerns about increased traffic on the remaining car-accessible streets and other unintended consequences [100,101]. Acknowledging practical concerns and systematically identifying residents’ preferences and objections represent the first crucial steps toward establishing a constructive, multi-actor dialogue. Such inclusive dialogue not only improves the legitimacy of planning processes but also contributes to the emergence of urban commons, where shared governance mechanisms can balance individual interests with collective benefits.

In light of the findings, several recommendations for future research and policy development can be outlined, including the following:

• Further exploration of collaborative governance frameworks and public–private cooperation models that can support long-term co-management of green interventions on private land;
• Development of tools for monitoring and evaluating the effectiveness of small-scale NbSs in terms of ecosystem services, social impact, and cost-efficiency;
• Comparative studies across different urban contexts to assess how socio-demographic, cultural, and spatial variables affect residents’ willingness to engage in NbS implementation;
• Investigation of the conditions under which private spaces may evolve into forms of urban commons, through shared management or recognition of collective benefits.

Ultimately, this study reinforces the idea that systemic change in urban ecological planning depends not only on visionary policy or technical innovation, but also on empowering local communities to take an active role in shaping greener, more resilient cities.