A Practical Framework for the Design of Low-Carbon and Circular Building Structures

Abstract

The construction sector is responsible for nearly 40% of annual global carbon emissions. This includes approximately 28% from operational carbon, 23% from transportation, and 11% from building and infrastructure materials. Following a review of the literature and a survey, a Low-Carbon Circular Design Framework was developed. The Framework was piloted to develop specific design guidance for low-rise steel, steel–concrete, and steel–timber hybrid structures. The specific guidance is targeted at industry experts, researchers, and building designers. Additionally, a Low-carbon Circular Design Hierarchy and associated flowchart was proposed. The effectiveness of the Framework, specific guidance, and proposed design flowchart was evaluated through a real-world case study involving a three-story commercial building typical of this typology in Aotearoa New Zealand. The results demonstrated that a 57% reduction in carbon using the proposed hierarchy is readily achievable within the Aotearoa New Zealand context.

1. Introduction

Addressing the climate change crisis requires a reduction in greenhouse gas emissions [1]. The construction sector has emerged as a significant contributor to greenhouse gas emissions [2]. Buildings alone are responsible for approximately 40% of global CO₂ emissions [3]. The construction sector in Aotearoa New Zealand accounts for 20% of the nation’s total greenhouse gas emissions [4]. Similarly to numerous other nations, the New Zealand Government has pledged to reduce carbon emissions by 50% by the year 2030 and to attain net-zero carbon emissions by 2050. Hence, there is a critical need to develop innovative tools, technologies, and frameworks to revolutionize the construction and building sector, enabling the attainment of carbon reduction targets within specified timelines [5]. To support these targets, a circular and low-carbon design guidance framework was developed by the authors to assist building designers to reduce carbon emissions [6]. Alongside this framework, specific guidance was created for designing low-carbon steel, steel–concrete, and steel–timber hybrid structures [7]. Low-carbon circular design aims to transition the sector from a linear model to a circular one, emphasizing reduced resource consumption, minimized waste, and lowered upfront and whole-life carbon emissions. Grounded in principles that seek to minimize waste and emissions while extending the lifecycle of products and materials, circular design employs strategies such as refurbishment, repurposing, and reuse of existing buildings, alongside designing new buildings for longevity, adaptability, and easy deconstruction. This approach advocates for minimal use of new resources by maximizing the utility of existing ones and regenerating products and materials at the end of their service life. By implementing circular economy principles, along with efficient material use and low-carbon alternatives, the built environment can strategically address global challenges of resource scarcity, climate emergency, and ecological deterioration [8,9]. Circularity and carbon reduction often complement each other, although circular design benefits extend beyond carbon reduction to include biodiversity, resource limitations and ecosystem integrity. However, efforts to minimize carbon emissions, upfront emissions can occasionally conflict with circular design goals. For instance, a highly circular building design may require more initial carbon investment to ensure flexibility, adaptability, and ease of disassembly, potentially resulting in higher upfront and lifecycle carbon assessments compared to alternatives with fewer circular features. This approach, emitting more carbon today in anticipation of future benefits, requires careful consideration of the likelihood that the building will be used differently in the future. This is especially critical given the urgent need to reduce global emissions to address the climate crisis. Thus, it is essential to transparently detail the reasoning behind design decisions, especially when they conflict, by outlining their impacts on circular design, whole-life carbon, and upfront carbon [10].

LETI (2022) [11] has developed a hierarchy of design strategies specifically tailored for existing buildings, building upon the principles of the EU waste hierarchy [12]. In contrast, Instructed [13], influenced by PAS 2080 [14], has introduced a distinct hierarchy that integrates a combination of approaches for the further use of existing buildings and the design of new low-carbon buildings [14]. This includes the addition of five key elements: “building cleverly”, “building efficiently”, “minimizing waste”, “specifying low-carbon” materials, and implementing “carbon offsetting”.

Figure 1 illustrates the elements and terminologies within each hierarchy, emphasizing the shared elements. Notably, both LETI and IStructE have excluded energy recovery and disposal from their frameworks, as these processes are more relevant to waste management than to Low-carbon circular building design strategies. Beyond the strategies depicted in Figure 1, further circular economy principles for new construction—whether expanding an existing building or creating a new one—have been advocated by various sources [15,16,17,18]. These principles, namely Design for Longevity, Adaptability, and Disassembly (Deconstruction), reinforce a circular economy in construction.

While the existing hierarchies have laid a strong foundation and significantly contribute to the state-of-the-art of circular and low-carbon design approaches, a more comprehensive hierarchy would provide a more advanced tool to assist building designers in proactively reducing carbon in buildings. This research represents a more comprehensive, yet simple, hierarchy by incorporating all impactful design strategies into a unified chart for new building designs and alterations/removal of existing buildings. The proposed hierarchy makes a connection between current circular building design strategies and their future implementations by incorporating Construction 4.0. This concept merges Industry 4.0 technologies to profoundly reshape the construction sector, thereby improving efficiency, sustainability, and safety in construction endeavors. The Construction 4.0 transformation is propelled by advancements in digital technologies, automation, and enhanced broadband connectivity.

Previous research has identified several critical challenges in the design of low-carbon buildings. One major challenge is the absence of a standardized global system for design perspectives, highlighting the need for design approaches that integrate multiple factors and adapt to varied characteristics [19,20]. Another significant challenge is the inadequacy of methods for calculating carbon emissions during the design phase [20,21]. Moreover, there is a crucial requirement to enhance the existing database of carbon emission factors to extend its applicability across a broader range of building types [22].

In addition to the challenges related to the low-carbon design, the impact of these solutions on the entire life cycle of buildings poses another significant barrier in practice. Hence, there is a need for a practical life cycle assessment (LCA) methodology so that building designers can evaluate the impact of various solutions on the whole life cycle carbon of a building.

A recent study in university of Waikato investigated the application of LCA practices in the Aotearoa New Zealand building and construction sector [23]. The study utilized a survey to identify barriers and potential solutions for wider LCA adoption. Results showed that the most critical barrier to LCA implementation is a lack of awareness and expertise in conducting LCA. To address these challenges, the study suggests implementing guidance, workshops, and training programs on LCA for building practitioners. These results underscore the importance of providing practical guidance and workshops on LCA, which is a key step in embracing circular and low-carbon design principles. Building designers require comprehensive sustainable strategies, relevant solutions, and practical methodologies to assess the impact of these solutions on the whole life cycle of buildings not only to reduce carbon in buildings but also to assess the environmental impact of their building design decisions. Additionally, recent assessments of ESG performance in construction companies show that organizational barriers, such as lack of strategy, awareness, or governance structures, continue to limit the practical adoption of sustainability practices in this sector [24].

To address these critical issues, this research project aimed to introduce a novel, material-agnostic framework for circular and low-carbon design, along with a pilot application of the framework informed by case studies. The goal is to provide building designers with essential resources to comprehensively address carbon reduction in building design.

2. Research Methodology Toward the Development of the Low-Carbon Circular Design Framework and Specific Guidance

Figure 2 illustrates the methodology employed in this research. A literature review of the current low-carbon strategies, solutions, and recent research and development was conducted to identify existing gaps. To address the gaps, a design guide framework was developed to serve as the basis for creating specific guidelines aimed at assisting building designers in reducing carbon emissions and construction waste. Both the framework and the specific guidance will undergo revisions based on feedback from users, including industry experts including building designers and researchers. Case studies were utilized to demonstrate the efficacy of the framework and specific guidelines in reducing carbon emissions in real-world buildings.

3. Industry Feedback

The study incorporated a survey of sectoral practitioners to identify the main barriers to low-carbon building design and assess the capability of building designers and construction industry experts to implement circular strategies and solutions in their projects. The survey, which garnered 75 responses, was analyzed using Microsoft Excel to perform descriptive statistical analysis and frequency evaluation of key response trends. The survey design included a mix of Likert-scale and multiple-choice questions, enabling structured and quantifiable feedback on the perceived importance and implementation challenges of circular strategies.

Results reveal that 76% of the participants consider the integration of sustainability in building design as important. The demographic composition of the respondents included 21% structural engineers, 43% architects or engineers in multidisciplinary consultancies, 19% main contractors, 5% public sector professionals, and 5% from academia. The survey results indicate that a majority of the participants are unable, either independently or under supervision, to implement strategies such as waste reduction, specification of low-carbon materials, circular economy practices, and measurement of embodied carbon in building projects. Figure 3 illustrates that the most favored solution among participants for overcoming these barriers is the provision of training and guidance on designing low-carbon buildings.

4. Framework

Informed by an extensive review of circular and low-carbon solutions and strategies in the literature spanning the past two decades, the Framework is structured to facilitate circular and low-carbon design by emphasizing areas with significant potential for carbon emissions reduction and promoting circularity within the construction sector [6]. It includes:

• A visual representation of generic strategies for creating Low-carbon circular designs, applicable to both greenfield and brownfield sites. This presentation follows a hierarchical structure familiar to most designers, inspired by aforementioned waste minimization and circular design hierarchies;
• An accessible overview of LCA and circularity, organized into Modules A–D, to introduce the Framework’s layout (to provide a simple guide to carbon calculation for the design engineers;
• A set of generic solutions aimed at achieving low-carbon circular design in alignment with the strategies outlined.

Key features of the Framework include its generic nature, making it applicable to any material, typology, or system, and its evidence-based approach, drawing on evidence from the literature. Furthermore, the Framework identifies the LCA modules affected by each solution, enhancing understanding of LCA. It adopts a whole-of-life perspective, considering Modules A–D of EN 15978:2011 [25], and supports cradle-to-cradle thinking, aiming to prepare the sector for the future adoption of advanced LCA methodologies and to promote LCA as a design-informing tool rather than merely an assessment tool. Thus, the Framework serves as a foundation for developing specific guidance for different materials, typologies, and structural systems.

Furthermore, it is important to note that the Framework is designed to be dynamic, allowing for updates to incorporate emerging methodologies, strategies, and solutions. This ensures its continued relevance and effectiveness in promoting sustainable building design. Future research should also examine the temporal aspect of life cycle assessments in the Framework.

4.1. Hierarchy of Design Strategies

Figure 4 illustrates the hierarchy of design strategies, presenting a comprehensive array of Low-carbon circular design strategies designed to:

• Simplify language and avoid ambiguity, ensuring self-explanatory clarity;
• Integrate impactful design strategies into a unified chart, distinguishing between circular and low-carbon strategies instead of merging them, to provide distinct and clearly defined guidance;
• Offer strategies for both new building designs and alterations/removal of existing buildings, tailored to greenfield (new building) and brownfield (existing building) sites, respectively, emphasizing the unique considerations of each;
• Mirror LETI’s (2022) hierarchy for new buildings, “Design for longevity” to achieve a lifespan exceeding 50 years with minimal initial carbon investment. This is accomplished through maintenance, refurbishment, and retention of buildings, aligning with IStructE’s principle of “Build nothing”. Next, “Design for adaptability” is emphasized for buildings intended to accommodate future changes, recognizing that this may require higher upfront carbon investment. When future adaptability is not viable, “design for disassembly and deconstruction” is recommended to facilitate the reuse of components or elements, consistent with LETI’s fourth hierarchy stage. If reuse is not possible, recycling and remanufacturing after demolition become the last resort. Nevertheless, selecting recyclable materials during the design phase ensures that, even without reuse, materials can still be processed sustainably at the end of their lifecycle;
• Highlight the connection between current circular building strategies and their future implementations, facilitated by incorporating Construction 4.0 elements into the chart, advocating for the integration of advanced circular and low-carbon solutions using Industry 4.0 technologies. This ensures comprehensive data management throughout the building lifecycle, optimizing efficiency, and sustainability.

Figure 4. Low-carbon circular strategies’ chart for designing new buildings and reusing existing buildings, developed by ShahMohammadi et al. (2025) [6].

Figure 4. Low-carbon circular strategies’ chart for designing new buildings and reusing existing buildings, developed by ShahMohammadi et al. (2025) [6].

As shown in Figure 4, The chart is structured into four main sections. The first section, represented by the left green leg, focuses on circular strategies for the “Design” of new buildings on a greenfield site, where no existing buildings are present. This section is organized in a synergistic hierarchy, referred to as the ‘4D’s. The second section, depicted as the right brown leg, addresses strategies for “Reusing/Recycling” existing buildings on a brownfield site, where buildings already exist. This section is termed the ‘4R’s. The third section, centrally positioned, is dedicated to strategies for “Lowering” the upfront and whole-life carbon intensity, known as the ‘2L’s. The final strategy involves implementing Construction 4.0, which links all low-carbon and circular strategies in both brownfield and greenfield contexts. Construction 4.0 serves not only as a low-carbon and circular strategy but also as a bridging element at the top center, highlighting the importance of systematically recording information from the design stage through to the end of the building’s life. Construction 4.0’s current capabilities facilitate optimized building design aligned with low-carbon principles and future building reuse in accordance with circular economy principles.

Construction 4.0 Strategy in Hierarchy of Design Strategies

Construction 4.0 integrates Industry 4.0 technologies to significantly transform the construction industry, enhancing efficiency, sustainability, and safety in construction projects [5]. This transformation is driven by digital technologies, automation, and broadband connectivity.

As an individual strategy, Construction 4.0 encompasses solutions that significantly contribute to both low-carbon and circular design solutions. For example, advanced structural health monitoring (Monitoring 4.0) revolutionizes the building and construction sector. Monitoring 4.0, with its advanced sensors, data analytics, data assimilation, stochastic system identification, and machine learning algorithms, stands at the forefront of this revolution, offering comprehensive insights into the health of structures from inception through to their operational lifespan. Information obtained from Monitoring 4.0 facilitates the early detection of potential issues long before they escalate into serious problems. This predictive maintenance capability enables more efficient resource allocation, extending the lifespan of structures and optimizing maintenance budgets. Monitoring 4.0 significantly contributes to minimizing environmental impact while maximizing the resource efficiency of buildings and infrastructure by ensuring that materials and resources are used optimally throughout the construction process and the lifecycle of the structure [26].

As shown in Figure 4, Construction 4.0 plays a hybrid role, as follows:

• Supports circular and low-carbon strategies: As an individual strategy, Construction 4.0 encompasses many novel solutions such as Monitoring 4.0, circular design, and advanced LCA significantly enhancing construction sustainability and critically contributing to circular, low-carbon, and sustainable building design. For example, Monitoring 4.0 can inform decisions about the type and amount of materials needed, reducing waste and promoting the use of sustainable alternatives. Moreover, by prolonging the lifespan of structures through effective maintenance, Monitoring 4.0 reduces the need for new constructions, thereby conserving resources and minimizing environmental impacts.
• Bridges greenfield and brownfield strategies: Construction 4.0 will facilitate the transformation of brownfield to greenfield buildings, linking all strategies together and allowing advanced optimization methodologies to achieve the best possible solution efficiently. As a result, the advantages of circular and low-carbon strategies in designing sustainable buildings will be greater than the sum of their parts. For example, when Monitoring 4.0 becomes more widely available, the safety factors used in the design codes can be safely lowered, as those safety factors are suggested using conservative assumptions and rare event scenarios that might not be relevant to the majority of structures. On the other hand, special-purpose future new structures with elevated demands will benefit from a deeper understanding of the response of existing buildings to loads. Another example is that Construction 4.0 offers advanced technologies (e.g., advanced BIM and digital twin) as a platform carrying digital material passports that facilitate the deconstruction of existing buildings and the reuse of elements from brownfield buildings in greenfield buildings. This goes beyond extending the lifespan of existing buildings or refurbishing or repurposing existing buildings. This means that elements of existing buildings will be disassembled in a sustainable way and used in new construction projects elsewhere. In this case, in an ideal situation where the deconstruction rate is not less than the new build rate, and existing elements remain undamaged during the deconstruction and reuse process, there is no need for virgin materials. Carbon emissions from existing buildings will be significantly reduced (or net zero achieved), demolition will be replaced by deconstruction, and construction and demolition waste will be significantly reduced.

4.2. Low-Carbon and Circular Design Solutions

To integrate these strategies early in the design process, each strategy is expanded into actionable solutions accompanied by implementation steps to guide designers in realizing and achieving low-carbon circular building designs. Furthermore, in the solutions section of the Framework, each solution is described along with its impact on the LCA modules, thereby enhancing the understanding of LCA and illustrating the contribution of the selected solutions to reducing the life cycle carbon footprint of buildings.

Table 1. Definitions of low-carbon circular strategies and associated solutions [6].

Field	Strategy	Strategy Definition	Solutions
Low- Carbon (New and Existing)	Less Material Usage	This strategy focuses on minimizing material usage to reduce upfront carbon emissions, without compromising the performance and integrity of the structure.	Optimize Concrete Strength Class
			Maximize Utilization and Minimize Over-specifying
			Minimize Irregularities and Transfers
			Efficient Structural Choices
			Utilize Higher-Strength Steel
	Lower Carbon Intensity	This strategy focuses on maximizing the utilization of materials with lower embodied carbon content.	Optimize Concrete Strength Class
			Maximize Utilization and Minimize Over-specifying
			Minimize Irregularities and Transfers
			Efficient Structural Choices
Circular (New buildings)	Design for Longevity	Design that maximizes building lifetimes through maintenance, repair, or refurbishment without compromising structural integrity by using durable materials and seismic/fire-resilient design principles.	Durability
			Seismic resilience
			Fire resilience
	Design for Adoptability	Design that ensures buildings can be easily repurposed to meet future needs by incorporating adaptability and anticipating changes, extending their useful life and reducing the need for new construction. It accommodates potential alterations within the building’s design life.	Spatial flexibility and increased loading allowance
			Adoptable to future vertical expansion
	Design for Disassembly	Design that enables non-destructive dismantling of buildings for component and material reclamation, prioritizing mechanical connections over permanent bonds and allowing easy replacement or upgrades, in synergy with Design for adaptability.	Reversible connections
			Design in layers
	Design with Circular Materials	Design focused on non-virgin sources such as reclaimed or highly recycled materials, prioritizing reusability and recyclability for virgin materials, and emphasizing the use of EPDs and material passports for lifecycle transparency and traceability.	Utilize reclaimed materials
	Construction 4.0	Construction 4.0 is an innovative approach that uses advanced technologies, digitalization, and sustainable practices to reduce carbon emissions and improve environmental performance in building projects.	Use material passports
			Utilize BIM
			Prepare for digital twin
			Structural Health Monitoring
Circular (Existing Buildings)	Reuse through Retention or Refurbishment	Building life extension through restoration, refinishing, and futureproofing, while preserving most of the building’s fabric and minimizing major replacements. This may include retrofitting to enhance carbon efficiency.	A wide variety of solutions are available, but to leverage their advantages, it is crucial to: identify building potential, engage early with clients, and conduct a general and loading assessment of the building.
	Adaptive Reuse through Repurpose	Building life extension through significant alterations or refurbishment, transforming spaces by updating components with shorter lifespans to meet diverse needs, such as converting industrial areas into residential spaces or vice versa.
	Reclaim	Deconstruction for reclamation according to a disassembly plan, cleaning and repairing components, and aiming to minimize reprocessing or remanufacturing. This approach facilitates material reclamation for future use, with a preference for on-site reuse before considering off-site options.	Selective and non-destructive dismantling is used to disassemble buildings according to a planned process, followed by cleaning and repairing components to minimize reprocessing or remanufacturing.
	Recycle and Rebuild	Demolition with recycling is essential when other options are infeasible, recovering recyclable and reusable materials through sorting to minimize landfill waste. Recycling reprocesses end-of-life materials into new products or substances for original or alternative uses.	Partial demolition, and utilize deconstruction methodologies

presents strategies, their definitions, and related solutions.

5. Specific Guidance

While the Framework provides pathways, strategies, and relevant low-carbon design solutions, the successful implementation of these solutions in buildings requires specific design guidance. Unlike the Framework, specific design guidance is tailored to particular materials and structural systems, offering not only crucial technical information and design steps but also providing sufficient information to estimate environmental impact and potential carbon reduction associated with the chosen solutions. For additional details, refer to the specific guidance [7]. It is imperative to ensure that specific design guides align with design standards and building codes. Therefore, while the Framework may have global applicability, it is essential to verify the compliance of existing specific design guides developed overseas with the design standards, building codes, and regulatory Framework of the end user’s country. The specific guidance serves as a pilot to the Framework and will be utilized by industry experts, including building designers, and researchers. Both the Framework and the specific guidance will undergo revisions based on feedback from users to address any identified gaps and enhance their efficiency for sustainable building design in the future.

6. Balancing Sustainable Building Design Strategies and Addressing Conflicts

It should be noted that implementing certain solutions may impact other solutions, end-of-life scenarios, carbon emission in other building stage, or conflict with other objectives of sustainable building design, such as cost-effectiveness. In such cases, designers require additional guidelines or tools to navigate these complex decision problems. Therefore, it is crucial to assess whether the selected strategies and associated solutions:

• Conflict with each other;
• Affect the end-of-life scenario of the building; or
• Impact building performance (e.g., structural integrity, fire resistance, seismic resilience, durability, acoustic performance, etc.).

6.1. Navigating Complex Decision Making in Low-Carbon Circular Building Design

When a decision focuses on a single goal, setting a decision rule is usually simple: decision-makers want to improve the relevant performance metric for that goal. Essentially, something is optimal for a specific measure if no other value can improve its performance [27]. However, challenges in sustainable building design often stem from significant uncertainties about future impacts, driven by the complex nature of the climate system and the methodologies used for sustainability assessments, such as LCA and environmental modeling. Moreover, low-carbon and circular building design frequently involves multiple, and sometimes conflicting, objectives—such as reducing upfront emissions versus enabling long-term adaptability or reuse. These objectives may hold different levels of importance for various stakeholders, further complicating the decision-making process. Multi-criteria decision-making (MCDM) tools [27,28] offer valuable support by helping designers and stakeholders assess trade-offs, prioritize goals, and explore scenarios under uncertainty.

Yet, the implementation of such tools is not only challenged by technical and methodological gaps, but also by broader economic and institutional dynamics. Recent research highlights that interconnected investment ecosystems, particularly the centrality of venture capital networks, can significantly influence the adoption and diffusion of green technologies within industries, including construction [29]. These financial forces act as both enablers and barriers, indirectly steering design decisions and technological uptake in practice. This includes the availability (or absence) of financial incentives, regulatory signals, and market demand, all of which significantly influence whether sustainable strategies are adopted or deprioritized in real-world projects. Therefore, future research should not only advance decision-making tools but also explore how financial and institutional contexts affect the practical adoption of low-carbon and circular strategies.

6.2. Low-Carbon Circular Building Design Flowchart

To successfully implement the proposed framework and fully utilize the potential of specific guidance for reducing carbon in buildings, a low-carbon circular design flowchart is proposed, as illustrated in Figure 5. Low-carbon circular design begins with the building design phase. As depicted in Figure 6 and emphasized by previous studies [30], the early design stage offers a high potential for carbon reduction at a low cost. However, as the building progresses through its lifecycle, the potential for carbon reduction decreases, accompanied by increased costs. Sometimes, carbon reduction goals may conflict with waste minimization objectives, underscoring the importance of clearly defining project goals. The second stage involves utilizing the Framework to identify strategies, associated solutions, and specific guidance to implement the chosen solutions. Life cycle assessments are conducted, and waste for each scenario is estimated. If the selected solutions do not require changes to building materials or structural systems, the structural and building attributes will remain the same as the original (reference) option. In this case, the decision-making process is typically less complex. If the selected solutions meet the carbon and waste reduction targets, the results can be accepted. Otherwise, the project should be revisited, and the Framework and specific guidance should be consulted to add necessary strategies and solutions or to revise existing ones. This iterative process continues until the results meet the project requirements. If any selected solution has the potential to affect other solutions or impact building or structural performance (e.g., durability, seismic performance, etc.), the optimal decision likely involves solving a single or multi-objective optimization problem or utilizing MCDM, as discussed in the previous section.

7. Case Studies

To showcase the practical implementation of the Framework and specific guidance in reducing carbon emissions within structural building design, a real-world case study was selected. As illustrated in Figure 7, this case study focuses on a three-story office building situated in Christchurch, Aotearoa New Zealand. The building’s lateral resistance systems incorporate reinforced concrete shear walls in one direction and moment-resisting steel frames (MRSF) in the perpendicular direction. Steel–concrete composite flooring systems are utilized throughout the structure. The substructure of the building is composed of a raft foundation. Constructed in 2014, the building’s design adhered to New Zealand Standards and reflected prevalent engineering practices at that time.

7.1. Carbon and Waste Reduction Goals

Two carbon reduction goals including 50% reduction and net zero were defined.

While there are several strategies and solutions that can be chosen to reduce carbon, some strategies or solutions may not be easy to implement, or they are expensive. Moreover, some solutions affect other solutions and finding optimum decisions require employment of optimization of multi-criteria decision making.

7.2. Low-Carbon Strategies and Solutions Selection Criteria

The Framework is used to identify strategies and associated solutions aimed at achieving carbon and waste reduction goals. The criteria for selecting circular and low-carbon strategies, along with related solutions, are as follows:

• Retain the material and structural systems for building superstructures to achieve a 50% reduction in carbon emissions;
• Minimize material and structural systems replacement for achieving net-zero carbon;
• Ensure that the alternative substructure is suitable for the building site conditions;
• Confirm that the selected solutions are available and implementable in Aotearoa New Zealand.

Table 2. The selected low-carbon and circular strategies and associated solutions.

Strategy	Solutions	Definition of the Solution
Design for Disassembly	Reversible connections	Replacing welded shear studs by high-strength bolt studs to enable steel beams can be disassembled and reused.
Design for Longevity	Seismic resilience using reuseable low-damage MRF	Moment Resisting Steel Frame with OSHJs to enable the structure to withstand and recover from seismic events while minimizing damage and the environmental impact associated with their construction, maintenance, and repair.
	Seismic resilience using reuseable low-damage concrete shear walls	Post-tensioned dissipative rocking concrete shear walls to enable the structure to withstand and recover from seismic events while minimizing damage and the environmental impact associated with their construction, maintenance, and repair.
Low Carbon Intensity	Specify Low-Carbon Steel	Utilizing low embodied carbon structural steel
	Specify Low-Carbon Concrete	Utilizing low embodied carbon concrete materials
	Specify Low-carbon Reinforcement	Utilizing low embodied carbon steel reinforcement
	Specify Certified Sustainable Timber	Replace steel–concrete composite with steel–timber hybrid flooring systems

presents the selected circular and low-carbon strategies, associated solutions, and a brief definition of each utilized solution.

8. Life Cycle Assessment

This research project adheres to the guidelines provided by ISO 14040:2006 [31] and EN 15978:2011 for conducting carbon LCAs. ISO 14040 (2006) delineates the four phases of an LCA study: goal and scope definition, inventory analysis, life cycle assessment, and interpretation. Meanwhile, EN 15978:2011 incorporates references to the EN 15804 [32] methodology for inventory analysis, system boundary definition, and identification of relevant modules. The system boundary in EN 15978:2011encompasses either a cradle-to-grave approach (Modules A to C) or a Cradle-to-Cradle approach (Modules A to D), with calculation rules specified within the standard. This research utilizes a cradle-to-cradle approach (Modules A to D).

The LCA was conducted using a comprehensive spreadsheet developed by the authors. The use of available commercial LCA software was found to be cumbersome for this study, primarily due to the unavailability of most of the selected Environmental Product Declarations (EPDs) within the software libraries. However, for the purpose of validating the results obtained from the spreadsheet, the authors utilized and LCAQuick [33] V3.6 for multiple examples. LCAQuick, developed by BRANZ (Auckland, New Zealand), is a free tool used to assess the carbon footprint and other environmental impacts of building designs.

A summary of key boundary features considered in this LCA is given below:

• A1–A3: Manufacture of materials.
• A4: Transport of materials to site.
• A5: Construction works, and waste generated onsite.
• C1: Deconstruction and demolition
• C2: Transport of materials to waste facility.
• C3–C4: Waste processing and disposal of materials.
• D: Emissions associated with recycling of materials at end of life.

In this study, the materials considered have a service life that exceeds the 50-year timeframe assumed in the LCA. Consequently, considerations for Module B2 (Maintenance) and B4 (Replacement) are not critical. These modules gain particular importance in the LCA when including services, façades, and fit-outs, which require more frequent maintenance and replacement than structural elements. Additionally, the study did not account for the “carbon uptake” effect in exposed concrete due to uncertainties.

8.1. End of Life Scenarios

Modules C3 and C4 and D values are based on the following end-of-life scenarios for steel, concrete, and Timber used in the current version of LCAQuick:

Steel: 85% of normal structural steel and 20% of normal reinforcing bar steel is assumed to be recycled as per what is assumed in the current version of LCAQuick. The former aligns with findings within HERA’s Recycling Report (prepared by thinkstep-anz) [34]. The basis of the latter value appears to be a general assumption, noting that the HERA report identified 85% of structural steel is recycled but was not able to determine the specific composition of that by product (as it relies upon scrap data). Notably, the latter rates are also lower than those assumed in steel EPDs. Consequently, this discrepancy is likely leading to an underestimation of the carbon reduction impacts in Module D for steel products, particularly for rebars in reinforced concrete components. Therefore, future versions of LCAQuick (or indeed any such tool) should default to values used in EPDs unless there is a more reliable data source.

Concrete: The end-of-life emissions for concrete are considered as zero in the current EPDs for concrete products in Aotearoa New Zealand, because these modules have not yet been developed. Developing Modules C and D for concrete would assess the potential carbon uptake of crushed concrete in landfill [35] and the benefits beyond the system boundary due to potential concrete recycling. This inclusion could alter the LCA results for concrete buildings.

Timber: For timber landfilling is assumed as to be the predominant end-of-life scenario for EPDs of timber products from Aotearoa New Zealand.

8.2. Functional Unit

The functional unit used to compare the proposed and reference buildings is one (1) square meter of Gross Floor Area (GFA) over a period of fifty (50) years, following a Cradle-to-Cradle approach. GFA (1755 m²) is determined based on the RICS (2015) [36] definition of gross internal area, which is the space within a building measured to the internal face of the perimeter walls at each floor level.

8.3. Inventory Analysis and Life Cycle Impact Assessment

The life cycle inventory (LCI) analysis phase involves compiling input/output data for the system under study. In this study, inventory data includes material compositions such as structural steel members, steel reinforcement, concrete, and timber—along with their respective volumes used in the building. These data are used to determine the embodied carbon (or carbon intensity) for each material.

For the assessment of the superstructure and substructure, only structural materials incorporated into the structural frame and foundations are considered. Future expansions of the study could include non-structural materials and components. Table 1 presents the references used to establish the carbon intensity of each construction material/product. These references are selected in accordance with guidance from EN 15978:2011 and drawn from the latest version of EN 15804-compliant LCAQuick tool, BRANZ’s databases, and valid Environmental Product Declarations (EPDs).

In this research, only the global warming potential (GWP) categories of environmental impacts are reported.

Table 3. EPDs for structural materials used for reference building case study (EPD 2024) [37].

Key Inputs Material Category	Carbon Intensity Data Source
Concrete	Allied Concrete Ltd.: 2019 (Dunedin, New Zealand); EPD—ready mixed concrete using Holcim supplied cement (EPD Registration No. S-P-00555), version 1.0, accessed from www.epd-australasia.com on 1 May 2025. The Allied Concrete EPD 2019 for 30MPa concrete has been used in LCA modeling as it is close to the baseline (for Auckland, corresponding to 344 kgCO2eq/m3). The baseline is provided by the Infrastructure Sustainability Council (ISC) in 2020 from the Materials Calculator NZ 2.0.
Low-carbon concrete (EC40)	Firth Industries Ltd. (2020); EPD—for ready-mixed concrete (EPD Registration No. S-P-02050), accessed from www.epd-australasia.com on 1 May 2025. Not provided in Firth EPD (ci2). Provided by Firth based on outputs from EC3 Embodied Carbon Concrete Calculator tool.
Reinforcing steel	Pacific Steel (NZ) Ltd. (2018), EPD—Seismic (EPD Registration No. S-P-01002), accessed from www.epd-australasia.com
Low-carbon reinforcing steel	NatSteel Holdings Pte Ltd. (2023), EPD—(Berg EN EPD No. 000379), accessed from www.greenbooklive.com on 1 May 2025
Structural steel	Data source: Liberty (OneSteel Manufacturing Pty Ltd.) (2020), EPD—hot rolled structural and rail (EPD Registration No. S-P-01547); version 1, accessed from www.epd-australasia.com on 1 May 2025.
Low-carbon structural steel	Hyundai (2019): EPD—Section Shape Steel from UL Environment (declaration no. 4789119110.101.1), accessed from https://spot.ul.com/ on 1 May 2025.
CLT	Data source: Red Stag (2022); EPD—Cross-laminated timber (CLT) (EPD Registration No.: S-P-03711), accessed from www.epd-australasia.com on 1 May 2025

lists Environmental Product Declarations (EPDs) for concrete, reinforcing bars, structural steel, and cross laminated timber (CLT) used in the LCA of the reference building case study.

9. Results and Discussions

9.1. Circular and Low-Carbon Strategies and Solutions for Achieving a 50% Reduction in Whole-of-Life Carbon Emissions

9.1.1. Superstructure

As shown in

Table 4. LCA results for superstructure of reference building and circular and low-carbon solutions for +50% carbon reduction.

No.	Strategy	Solution	Superstructure Carbon Emission (kgCO2eq/m2)
			Life Cycle Modulus			Total (Non-Biogenic)	Biogenic	Carbon Reduction %	Cumulative Carbon Reduction %
			A	C	D
Reference Building			377	14	−143	248	0	Not Applicable
1	Design for disassembly	Reversible connection in flooring systems	377	13	−153	237	0	5	5
2	Design for longevity	Seismic resilience (steel frame design)	377	13	−169	221	0	6	11
3	Low carbon intensity	Specify Low-carbon concrete	356	14	−169	201	0	8	19
		Specify Low-carbon structural steel	166	17	−33	150	0	21	40
		Specify Low-carbon reinforcing rebs	80	17	9	106	0	17	57

and Figure 8, a combination of circular (D1: Design for longevity and D3: Design for disassembly) and low-carbon (L1: Lower carbon intensity) strategies have been chosen to achieve a 57% reduction in the whole-of-life carbon emissions of the building’s superstructure. Table 4 presents the results of the LCA for the reference building’s superstructure, along with the application of the selected circular and low-carbon solutions. The carbon reduction percentage and accumulated carbon reduction are provided in Table 4. The total non-biogenic carbon emissions of the reference building superstructure are 248 kgCO₂eq/m². Employing reversible connections in flooring systems, which allow steel beams supporting concrete slabs to be disassembled and reused, can achieve a 4% reduction. Furthermore, utilizing low damage optimized sliding hinge joints (OSHJ) in MRF lateral resisting systems can reduce the whole life cycle carbon of the building’s superstructure by 7%. Replacing the concrete, reinforcing bars, and structural steel with low-carbon products will result in reductions of 8%, 16%, and 17% in the whole life cycle carbon of the building’s superstructure, respectively. The results demonstrate that the selected strategies and associated solutions can reduce the whole life cycle carbon of the building’s superstructure by 57%. Importantly, these selected strategies and solutions will not impact on structural performance, operational carbon, fire resistant or durability, of the building. Furthermore, they will not affect any other solutions or life cycle modules of the low-carbon building compared to the reference building.

9.1.2. Substructure

As shown in

Table 5. LCA results for substructure of reference building and circular and low-carbon solutions for 50% carbon reduction.

No.	Strategy (Associated Solutions)	Type of Foundation [Site Specific Characteristics]	Substructure Carbon Emission (kgCO2eq/m2)
			Life Cycle Modulus			Total (Non-Biogenic)	Biogenic	Carbon Reduction %
			A	C	D
	Reference building	Raft Foundation [Shallow foundations are possible, Assumed gravels near surface, ultimate bearing capacity of 600 kPa, Minimal geotechnical risk]	236	0	0	236	0	Not Applicable
	L2: Less material usage (L2.4 Efficient structural choices)	32 $\times$ 9.3 m Screw Piles, ground beams and slab [Bearing capacity is less than option 1, but founding on shallow gravels is possible]	119	0	0	119	0	50

, the selected strategies, and the associated solutions to reduce 50% carbon emission of the building substructure are: “Efficient Structural Choice”, a solution under the “Less material usage”, which is a low-carbon strategy (L1), and the other one is “Low-carbon concrete and steel” solutions under “Lower carbon intensity” strategy (L2). Table 5 compares the whole life carbon emissions of the original reference building raft foundation substructure with an alternative option of screw piles and ground beams and slab. The results show the whole life cycle carbon of the alternative substructure is 50% of the raft foundation. The alternative option meets building design requirements and will not affect structural performance, superstructure or operational carbon of the building.

Table 6. Carbon reduction in foundation through the use of low-carbon concrete and reinforcement.

	A1–A3 (kg CO2 e)	A–D (kg CO2 e)
Normal Steel (Pacific Steel)	2.39 × 105	1.61 × 105
Normal Concrete (Baseline 2020)	1.43 × 105	1.43 × 105
Sum (normal)	3.82 × 105	3.04 × 105
Low Carbon Rebars (Natsteel)	3.30 × 104	3.16 × 104
Low Carbon Concrete (EC40%)	8.55 × 104	8.55 × 104
Sum (low-carbon)	1.19 × 105	1.17 × 105
Reduction by using low-carbon steel and concrete simultaneously	69%	62%

Note: The analysis has been conducted for a raft foundation consisting of 411 m³ Concrete and 64.5 ton steel reinforcement.

illustrates the impact of using EC40 low-carbon concrete (with a 40% reduction in carbon) and Natsteel reinforcement on the whole-life carbon emissions of the raft foundation of the reference building. As shown, the use of low-carbon alternatives has resulted in a 69% reduction in A–C and a 62% reduction in A–D. The influence of low-carbon concrete has been more significant than that of reinforcement; specifically, in A–C, 22% of the emissions are attributed to concrete and 78% to steel, while in A–D, 31% are attributed to concrete and 69% to steel.

The results presented in Table 4 and Table 5 demonstrate that achieving a 50% reduction in the whole-of-life carbon emissions of building structures using conventional structural materials (e.g., steel and concrete) is easily achievable without compromising structural or building performance or affecting operational or other modules of the building life cycle. Therefore, these results can be directly utilized, and optimization or MCDM is not required for this case study to determine the optimum decision.

9.2. Low-Carbon and Circular Strategies and Solutions for Achieving a Net Zero Whole-of-Life Carbon Emissions

Table 7. LCA results for superstructure of reference building and circular and low-carbon solutions for net zero emissions.

No.	Strategy	Solution	Superstructure Carbon Emission (kgCO2eq/m2)
			Life Cycle Modulus			Total (Non-Biogenic)	Biogenic	Carbon Reduction %	Cumulative Carbon Reduction %
			A	C	D
Reference Building			377	14	−143	248	0	Not Applicable
1	Design for disassembly	Reversible connection in flooring systems	377	13	−153	237	0	5	5
2	Design for longevity	Seismic resilience (steel frame design)	377	13	−169	221	0	6	11
		Seismic resilience (concrete wall design)	377	13	−186	204	0	9	20
3	Low carbon intensity	ow-carbon concrete in superstructure	356	14	−186	184	0	8	28
		Specify low-carbon structural steel	166	17	−50	133	0	21	49
		Specify low-carbon reinforcing rebs	80	17	−5	92	0	17	66
		Steel–timber hybrid floor	77	13	−5	85	−93	44	103

presents all strategies and solutions, including those necessary for achieving a 50% reduction, as well as additional strategies and solutions required to achieve net-zero whole-of-life carbon emissions. It also displays the results of the LCA for the reference building’s superstructure, along with the application of the selected circular and low-carbon solutions. The table provides the whole-of-life carbon reduction percentage for each solution compared to the reference building and the cumulative carbon reduction.

As shown in Table 7, to achieve net zero, the additional solution of using rocking dissipative concrete shear walls related to the circular strategy of Design for longevity results in a 9% reduction, and employing steel–timber hybrid flooring systems related to the low-carbon strategy of Lower Carbon Intensity leads to a 44% reduction in whole life cycle carbon. Thus, the additional solutions result in a 53% carbon reduction in addition to the previous 50% carbon reduction. Therefore, the total carbon reduction of 103% has been achieved, which means the whole-of-life carbon emissions of this building are −3% of the reference building.

While the lateral and gravity-resisting systems remain unchanged, replacing steel–concrete composite with steel–timber hybrid flooring systems can affect the acoustic and structural fire performance of the building. Addressing these issues may increase the carbon emissions of the building. However, using steel–timber hybrid flooring systems reduces the gravity and seismic weight of the buildings, leading to a reduction in structural materials for the substructure and superstructure, and consequently, further carbon reduction in the building. It is estimated that the further carbon reduction due to reduced building weight is greater than the carbon emissions due to addressing fire and acoustic issues in the building with steel–timber hybrid flooring systems. Future studies should delve into this matter and investigate these effects on the whole-of-life cycle carbon of the building.

10. Future Research

Novel strategies and solutions for circular and low-carbon construction represent significant areas for future research. Emerging generations of construction, such as Construction 4.0 and beyond, will provide innovative platforms to develop and integrate these solutions through multivariate design optimization tools. These technologies can support a transition to more sustainable practices by enabling advanced decision making and performance evaluation during the design phase.

In addition to design innovation, it is essential to investigate the impact of circular and low-carbon strategies on end-of-life scenarios and the generation of construction and demolition waste. This includes evaluating how such strategies influence material recovery, reuse, and disposal processes. Future research should also prioritize the development of efficient optimization and multi-criteria decision-making (MCDM) methodologies to support complex decision making in circular and low-carbon building projects.

Cost remains a critical factor in the adoption of these solutions. Accordingly, future work should focus on creating robust cost assessment frameworks that account for the economic implications of implementing circular and low-carbon design principles throughout a building’s life cycle. Complementing this, more advanced and comprehensive life cycle assessment techniques—such as dynamic LCA—should be employed to understand how the inclusion of circular strategies affects the carbon footprint and waste outputs over time.

However, the implementation of these solutions may encounter formal challenges, including regulatory and certification barriers, as well as integration difficulties within current construction practices. Additionally, potential upfront costs and financial risks could deter entrepreneurs and investors. Future research should therefore also investigate ways to overcome these obstacles to enable broader adoption of circular and low-carbon approaches.

As a previous study [20] has noted, a key limitation in low-carbon building design lies in the inability to optimize design variables based on real-time carbon emissions during the early design stages. Addressing this issue requires the development of integrated methodologies capable of assessing and responding to live carbon data within the design workflow.

Moreover, the global lack of comprehensive deconstruction guidelines remains a critical gap [38]. Future research should explore the role of Construction 4.0 not only as a means of enabling circular and low-carbon solutions but also as a bridging framework to support sustainable deconstruction and promote long-term building sustainability.

Finally, to improve the generalisability of findings and support practical implementation, it is recommended that future studies expand the development of case studies across diverse building typologies, including residential, commercial, and public buildings. This will help ensure that circular and low-carbon solutions are both scalable and adaptable to varied construction contexts.

11. Conclusions

This research conducted a comprehensive review to identify existing circular and low-carbon strategies and associated solutions. Additionally, a survey was performed to identify barriers and solutions to low-carbon design in Aotearoa New Zealand. Subsequently, a design guidance framework and specific guidance (a pilot to the Framework) were developed to address the identified research gaps. The effectiveness of the Framework and specific guidance was investigated through case studies. Finally, a low-carbon design methodology was proposed, utilizing the developed framework to design low-carbon buildings. The key conclusions are summarized as follows:

• The Framework unified existing circular and low-carbon strategies into a simple, hierarchical chart. Additionally, it introduced a novel Construction 4.0 strategy that also serves as a bridge between greenfield and brownfield strategies, linking them together seamlessly.
• While Construction 4.0 offers valuable solutions for circular and low-carbon design, its greatest potential lies in bridging greenfield and brownfield strategies. This integration has the potential to greatly enhance sustainable building design, construction, and operation by creating a platform for future innovation and implementation of novel sustainable building solutions;
• In addition to circular and low-carbon design strategies, the Framework encompasses solutions related to these strategies, along with practical LCA to enhance awareness and deliver basic LCA knowledge.
• A specific guidance for steel, steel–concrete and steel–timber hybrid low-rise buildings, serving as a pilot to the Framework, was developed. It will be used by industry experts, researchers, and building designers. Feedback from users will guide revisions to both the Framework and the specific guidance, addressing identified gaps and improving their efficiency for sustainable building design;
• A low-carbon and circular building design Hierarchy is proposed to design low-carbon buildings using the Framework and specific guidance.
• The results of the reference building and low-carbon solutions demonstrate that it is feasible to achieve a 57% reduction in carbon without changing materials or compromising structural or building performance.
• To achieve a net-zero building, one of the employed solutions was replacing the steel–concrete composite flooring system with a steel–timber hybrid flooring system, which may affect the structural and building performance compared to the reference building. However, the carbon reduction resulting from the reduced weight of the building and subsequently reduced building material offsets the carbon emissions required to address the structural and building performance.