Assessment of Criteria for Residential Buildings’ Insolation: A Comparative Review of European Standards

Abstract

Sunlight exposure is among the key factors in architectural and urban design. People spend more than half of the day in residential buildings, where sunlight exposure positively affects comfort, well-being, and some health problems. Insolation regulations and recommendations in many countries include criteria for minimum sunlight exposure of interior spaces to meet hygiene and technical standards and to enhance users’ comfort. This research shows a decrease in mandatory insolation criteria for residential spaces in Croatia since the end of the 20th century, which was the motivation for the assessment of European insolation standards. It provides a comparative review of current European recommendations and regulations and addresses the comprehensive issue of residential building insolation through the assessment of planning, urban and architectural design parameters. Research results show that regulatory traditions, climatic conditions, and planning cultures jointly shape national approaches to residential insolation. The research also shows that in the European countries analysed, insolation criteria are mostly mandatory at the national level. The insolation criteria assessed by this research range from urban standards, which refer to buildings, to apartment standards, which refer to rooms. The minimum window-to-floor ratio, with values ranging from 10 to 20%, and the minimum duration of sunlight exposure, with values from 1 to 4 h, were identified as dominant insolation criteria. Sunlight exposure criteria are necessary to protect residents’ hygiene rights, but should be adaptive to socio-economical, cultural and climatic contexts to support sustainable urban development.

1. Introduction

Sunlight exposure significantly affects living and working conditions and is therefore considered one of the fundamental elements of architectural and urban design [1]. Insolation of interior spaces depends on the climatic conditions at a building’s geographical location, the time of day and year, and the degree of shading caused by the building’s surroundings [2,3,4,5]. A building’s openings through which sunlight passes can be shaded by the surrounding topography, other buildings, vegetation, the geometry of the building’s façade, or sun protection systems. The literature provides data on the negative impact of increased building density and reduced distances between buildings on the sunlight exposure of interior spaces [6,7,8]. The development of buildings in terms of maximum allowable dimensions results in increased shading of surrounding interior spaces [9] and, consequently, in insufficient sunlight exposure of living areas. The characteristics of the openings through which direct sunlight passes into interior spaces also strongly affect insolation of interior spaces. The size, shape, position, orientation of the opening, as well as opening element materials affect the passage of solar radiation into the interior space [1,10].

Sunlight exposure of living spaces is an important factor in achieving residents’ comfort and housing hygiene [5]. Living (habitable) spaces of apartments include all rooms used for rest, sleep, and dining activities: bedrooms, living rooms, studies, dining rooms, and residential kitchens (including not only food preparation but also dining or general daily activities). Guidelines and regulations define various criteria and their minimum values for sunlight exposure of interior spaces to fulfil hygiene and technical requirements, as well as to influence comfort in the indoor space. In addition, exposure to sunlight enables passive heating and daylighting of rooms, which consequently contributes to energy savings, thereby positively affecting the energy balance of the apartment and the building [11]. Excessive insolation can be associated with reduced visual and thermal comfort in interior spaces, due to glare and overheating. The complexity of sunlight exposure of living spaces arises from the need to balance opposing requirements, which makes this topic both challenging and interesting to research.

Europeans spend approximately 56 to 66% of their daily time inside residential buildings [12,13]. Solar radiation exposure affects indoor living comfort, positively impacts human well-being, and beneficially influences certain health issues [3,5,8,14,15,16]. In the second half of the 20th century, many countries established the concept of solar rights to guarantee apartments’ direct sunlight access. Ongoing urban development has created a constant tension between solar rights and landowner rights, making the protection of solar access in urban environments highly complex issue [7]. The sunlight exposure of residential living spaces is often prescribed as a mandatory requirement to avoid the poor quality of single-oriented (often north-facing) apartments or living spaces without external openings, as seen in examples of contemporary residential architecture [17]. Studies show that housing quality has a significant impact on the physical and mental health, as well as the social well-being of its residents [18,19]. Apartment buildings, as a form of collective housing, represent a socially organized method of addressing housing needs and, as such, are subject to investigation through analysis of the context in which they arise [20]. Apartment buildings are the most prevalent housing type in urban areas, and their characteristics affect many people. In the European Union, residential buildings make up nearly 75% of the building stock [21].

This review explores the multifaceted issue of residential buildings’ insolation, with a particular focus on architectural design and urban planning parameters. These parameters affect the residential built environment and impact residents’ comfort and well-being. The research addresses the assessment of insolation criteria for residential buildings by comparing current European spatial planning, urban planning, and architectural regulations and guidelines. The motivation for the assessment of European insolation standards is the reduction of mandatory criteria for the insolation of residential buildings in Croatia since the end of the 20th century, as shown in the paper. It is assumed that this deregulation affects the quality of habitable spaces in recent apartments, which will be addressed in future studies. This review identifies common insolation criteria and their prescribed or recommended values, which ensure adequate sunlight exposure in the habitable spaces of apartment buildings. Differences and potential gaps in residential insolation standards are identified and discussed.

1.1. Sunlight Exposure and Compliance with Hygiene and Technical Standards in Housing

Solar radiation reaches the Earth’s surface directly as insolation, as well as through diffuse and reflected pathways. Sunrays enter interior spaces by passing through the transparent elements of openings in the building’s external envelope. Indoor spaces can be exposed to direct, diffuse, and reflected solar radiation (Figure 1). Sunlight is defined as the part of direct solar radiation capable of causing a visual sensation. Sunlight exposure is defined as the sum of the time expressed in hours on a selected cloudless day, within a given period during which the sun is above the actual horizon, which may be limited by permanent obstructions (mountains, buildings, trees or sun protection) [4] (p. 9).

Solar radiation reaching the Earth’s surface mainly includes ultraviolet radiation (UV radiation, wavelengths 280 to 380 nm), visible radiation (daylight, wavelengths 380 to 780 nm), and infrared radiation (thermal radiation, wavelengths 780 to 2500 nm). The ultraviolet part of the solar radiation spectrum is characterized by antibacterial effects; therefore, exposure to direct solar radiation can have a positive impact on meeting hygiene and technical requirements in living spaces [5,22,23,24]. The residential hygiene minimum defines that living spaces must be sunlit for two hours daily during approximately 250 days a year [23,25,26]. The disinfecting effect of sunlight largely depends on the material characteristics of the transparent part of the opening element (e.g., glass) [10,22]. Glazed parts of the external building envelope largely absorb solar radiation with wavelengths shorter than 320 μm, which results in a reduced hygiene effect of sunlight exposure. Research estimates that 30–40% of UV radiation passes from the outside to the inside surface of glass. Exposure to UV radiation of longer wavelengths can compensate for this shortcoming, so a daily sunlight duration of at least 3 h is recommended [22] (p. 55).

1.2. Historical Overview of Sunlight Exposure Criteria for Residential Buildings

Since the very beginnings of architectural history, sunlight exposure has influenced the choice of location and orientation of settlements and buildings, as well as the building form, construction, and selection of building materials. Along with other climatic characteristics, it determined the development of regional building styles and building types. Throughout history, the planning of settlements and cities has consistently been guided by regulations, with their implementation being the responsibility of sovereigns and builders [27] (p. 39). One of the earliest examples is Vitruvius’ text De Architectura (dated in the 1st century BC, published as Ten Books on Architecture), where numerous guidelines for the proper planning, design and orientation of buildings were listed to ensure sunlight exposure of interior spaces [28]. Before Vitruvius, in the 4th century BC, Hippocrates and Socrates were also involved in sun exposure research, especially according to facade orientation [29] (pp. 11, 13).

It was only after 1850 that urban planning standards similar to contemporary standards appeared, referring to the influence of the Industrial Revolution on lifestyle and settlement (or city) planning. The period of industrialization (1750–1850) was marked by uncontrolled construction of new parts of cities, which led to poor housing conditions, especially in terms of hygiene. This was the result of the rapid growth in industrial production, during which many people moved to cities. Many of the apartments built at that time were substandard, particularly regarding sunlight exposure and natural ventilation [27] (p. 41). Therefore, after 1850, urban planning legislation developed. By prescribing criteria for site coverage ratio and building height, the urban legal framework addressed the problems of the previous period. In Europe, and in Croatia as well, these criteria were usually initially regulated by building codes at the country level, and later by city-level regulations concerning zoning and building construction regulation [27] (p. 46).

In the post-Industrial Revolution period, cities and their built environments transformed to enhance the hygiene conditions of public areas and building interiors. In the second half of the 19th century, the historic core of Paris was transformed under the direction of Baron Haussmann. By the mid-19th century, it was characterized by a medieval structure with numerous narrow and crowded streets, which resulted in poor hygiene conditions. In 1852, Napoleon III entrusted Haussmann with the modernization of Paris through the construction of safer streets, wide avenues, and boulevards. The new streets improved traffic flow and enabled the movement of military units. However, the broader streets also brought progress in terms of hygiene in an overcrowded Paris exposed to epidemics. Greater air circulation, as well as the exposure of streets and buildings to sunlight, contributed to improved health conditions in the central part of the city [30] (p. 31). Access to sunlight in a densely built-up city centre was first addressed by the 1916 Zoning Resolution in New York City, which caused many skyscrapers built in the early 20th century to taper towards the top [15] (p. 502).

1.3. Sunlight Exposure in Modern Architecture

In the first half of the 20th century, modernist architecture defined standards for cities that were universally beneficial. At the International Tuberculosis Congress held in Washington in 1908, it was stated that orientation towards the sun must be one of the basic requirements of city planning and that every apartment or house must be exposed to direct solar radiation. Modernist architects often referred to these conclusions in their projects [31] (p. 119). CIAM (fr. Congrès internationaux d’architecture moderne), an international organization of architects founded in 1928 in La Sarraz, Switzerland, promoted the principles of modern functionalist architecture. The topic of the CIAM congress in 1933 was Functional City, with the Athens Charter (fr. Charte d’Athènes) as a result. The city was zoned into basic functional groups (housing, work, leisure, transport), and minimum hygiene requirements for sunlight exposure and ventilation were defined. Point 26 of the Athens Charter specified that each apartment should be sunlit for a minimum of two hours daily during the winter solstice, on 21 December [32] (p. 47). This provision was considered a prerequisite for obtaining a building permit, aimed at preventing the design of north-facing apartments [11] (p. 2). During this period, excessive housing densities were observed in some cities, and the construction of apartment buildings at greater distances was prescribed to meet minimum sunlight requirements. Many modernist architects studied the topic of sunlight exposure to meet hygiene requirements. Their work was significantly influenced by the architecture of sanatoriums and heliotherapy [24,31].

2. Sunlight Exposure Criteria for Residential Buildings in Croatia

Since the end of the 20th century, the criteria for insolation of residential buildings in Croatia have been significantly reduced, as will be shown in this section. This deregulation was the motivation for the assessment of European insolation standards, as it probably affects the quality of living spaces in recently built apartments, which will be verified by further research. This section is mainly focused on the territory of Croatia’s capital, Zagreb, due to its high population density and status as the largest Croatian urban centre. An understanding of Croatia’s residential insolation criteria and current legislative system requires a brief overview of the historical context. Croatia was part of the Habsburg Empire until 1918 and part of the Kingdom of Yugoslavia from 1918. In 1945 it became one of the six republics of the former Yugoslavia, a one-party communist state. In 1990, Croatia experienced profound socio-political changes that transformed the structure of society and the state. The one-party communist system, which had dominated political life for decades, came to an end with the introduction of pluralist democracy. The new political era redefined Croatia as a sovereign and democratic state, replacing the socialist framework that had previously shaped its political order.

The changes in the political and social system during the 1990s led to transitions in housing policies, which became almost entirely subject to free market conditions. A comprehensive spatial planning system failed to transform according to new conditions, which is evident in the discontinuation of prior research and the development of urban planning standards at the national level. The housing programs of the new Croatian democratic state aimed at organizing social housing construction: the Housing Care Program for Independence War Victims from 1997 and the Socially Subsidized Housing Program from 2000. These programs focused only on the design of individual apartments rather than the planning of residential neighbourhoods [27] (p. 81). The lack of a systematic housing policy negatively impacts the quality of residential construction [33] (p. 191). The new legislative framework did not adopt the previously recommended housing standards.

In the period preceding the 1990s, informal DUSI standards were particularly influential. Socially Directed Housing Construction (cro. Društveno usmjerena stambena izgradnja—DUSI) was a housing model in the Socialist Republic of Croatia (part of the Socialist Federal Republic of Yugoslavia) that emerged in the late 1970s. The DUSI standard (1983) was the result of a complex and long-term research effort aimed at optimizing costs and constructing the maximum number of apartments with the minimum possible expenses [34]. In terms of residential buildings’ insolation, the DUSI standard recommendations included building distance standards, sunlight exposure duration standards, building orientation and room orientation guidelines, sun protection standards, and windows’ size, shape and position guidelines. Also, sunlight exposure standards had to be shown in buildings’ project documentation, as they were among the indicators for evaluating apartment and building usability.

The Republic of Croatia has been a member of the European Union since 2013. The majority of Croatian regulations are currently aligned with the European Union’s guidelines; however, recommendations such as solar access rights and the proper orientation of living spaces are not mandatory. In Croatia, there is no specific regulation at the national level addressing building insolation standards. The Building Act [35] defines the fundamental requirements for a building, including hygiene, health, and environmental requirements. This requirement refers to the impact of the building on its surroundings and is not aimed at providing sunlight access to the building users.

The standard HRN EN 17037:2022 (Daylight in Buildings) represents a European standard adopted in Croatia in 2019 [4]. Unlike traditional daylighting standards, which mainly rely on static measures such as minimum duration of sunlight exposure, EN 17037 introduces a holistic approach evaluating daylight performance through four dimensions: daylight availability, access to outdoor views, exposure to sunlight and glare protection. The EN 17037 standard represents a fundamental shift in daylighting assessment toward a dynamic, performance-based framework. Earlier approaches typically relied on fixed thresholds of direct sunlight exposure, often neglecting variations in climate, orientation, and occupant perception. In contrast, EN 17037 introduces a multidimensional evaluation model assessed through climate-based simulations. This shift reflects a broader understanding of visual comfort and occupant well-being. The application of the HRN EN 17037:2022 standard is not mandatory in Croatia.

In Croatia, there are no spatial plan provisions regarding the limitation of the height of new construction for the purpose of preventing excessive shading of existing buildings. Spatial plans usually prescribe minimum permissible distances of a building from its building plot boundary, primarily concerning potential building collapse areas or fire emergency routes. Current Croatian guidelines and regulations regarding residential buildings’ insolation criteria are presented in

Table 1. Current Croatian guidelines and regulations affecting residential buildings’ insolation criteria and recommended values [4,36,37]. Non-mandatory guidelines are written in italics. Town (municipality) level requirements are marked in grey.

Year	Guideline/Regulation Name	Criteria	Value
2004	Regulation of Min. Tech. Req. for the Design and Construction of Apartments from the Socially Subsidized Housing Program	min. glazed WFR *	glazed surface area min. 1/7 of the respective room floor area; glazed surfaces up to 0.50 m above the finished floor level are not considered
		sun protection	mandatory
2013	2007 General Urban Plan for the City of Zagreb, Amendments	min. sunlight exposure of new apartments	min. 2 h of sunlight exposure per day during the summer solstice for each apartment
2022	HRN EN 17037:2019 Daylight in Buildings updated in 2021 and 2022 **	min. sunlight exposure (1 February to 21 March)	min. one habitable space of apartment
			minimum medium high	1.5 h 3.0 h 4.0 h

* Window (glazed) area-to-floor area ratio. ** The application of HRN EN 17037:2019 is not mandatory in Croatia.

.

In Croatia, the minimum WFR value (window-to-floor area ratio) is defined, but only for buildings included in Socially Subsidized Housing Program. The minimum sunshine duration value is only prescribed for residential buildings in the country’s capital, Zagreb. The adopted European Union standard HRN EN 17037:2022 defines a minimum time of sunlight exposure, but its application is not mandatory. The deregulation of the Croatian case is also evident in terms of the application of standards, considering that the previously applicable Yugoslav standard JUS U.C9.100 (Daylight and Electric Lighting of Rooms in Buildings) [38] had mandatory provisions.

At the local level in Croatia, the General Urban Plan of the City of Zagreb regulates the minimum sunlight exposure of living spaces in multi-residential buildings. Croatia’s capital, Zagreb, faces intensified urban challenges, which require planning guidelines and regulations to ensure compliance with standards for sunlight exposure and hygiene. Increasing urban density as a strategy for sustainable land use may adversely affect the sunlight exposure of interior spaces [9]. The Zagreb General Urban Plan defines minimum sunlight exposure for each apartment, but this criterion must be fulfilled only on the summer solstice day. This criterion effectively excludes the possibility of single, north-oriented apartments due to the angle of solar radiation to which Zagreb is exposed throughout the year. Unfortunately, an examination of examples of contemporary residential architecture reveals single, north-oriented apartments in Zagreb that do not comply with the provisions of the general urban development plan [17]. Also, if the sunlight exposure criterion is prescribed for the summer period, it probably will not be accomplished during other parts of the year, so it represents a very loose standard that may not ensure satisfactory hygiene and technical conditions in residential spaces throughout the year.

3. Method

To provide guidelines for the improvement of insolation standards in residential buildings, this research focuses on a criteria framework within European sunlight standards. This paper examines and compares national regulations, guidelines and selected scholarly references related to residential insolation across Europe. The research method concerning recommendations and regulations for sunlight exposure in residential spaces is divided into two parts:

• a comparative analysis of current standards in European countries (including Croatia) and
• an assessment of planning, urban and architectural design criteria framework that ensures insolation in residential spaces and thereby enhances the overall quality of housing.

The research started with the identification and systematization of current European recommendations and regulations concerning sunlight exposure in residential spaces. The insolation criteria and their values, as well as implementation models, were identified and compared. In addition, an assessment model for insolation standard criteria was developed. An analytical framework grounded in the insolation criteria implementation features exposes regulatory divergences and facilitates an assessment of their causes and consequences.

4. Sunlight Exposure Criteria for Residential Buildings in Europe

This part of the research is focused on identifying urban and architectural sunlight exposure design criteria through a comparative analysis of currently available European insolation recommendations and regulations. An analysis of criteria for sunlight exposure of residential buildings covered 22 European countries, including Croatia. Sunlight exposure of residential buildings is considered differently in architectural design depending on the climatic, social and economic factors of each country [3]. Countries outside the European context were excluded to ensure greater comparability, as European countries share broadly similar cultural and institutional characteristics. Within Europe, countries from diverse cultural, socio-economic, and climatic contexts were analysed to achieve a more comprehensive understanding of the recommended criteria. This research aims to assess the residential insolation criteria, implementation models and differences in criteria values. This approach is adopted to clarify the origins and impacts of observed differences.

Table 2. Guidelines and regulations for sunlight exposure of residential buildings in different European countries according to mandatory or non-mandatory implementation [3,4,7,9,11,36,37,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67]. Town (municipality) level requirements are marked in grey.

Country	Guideline/Regulation Name	Sunlight Exposure Recommendation/Requirement
mandatory implementation
Austria	Richtlinien OIB-RL 3: Hygiene, Gesundheit, Umweltschutz, 2023	min. WFR 12%
		shading/obstruction angle 45°
	Vienna Building Code 2012	defined max. height of new buildings
Belgium	Brussels Decree of 21 November 2006	min. WFR 20%
Bulgaria	Law on Territorial Planning, 2001; Ordinance No. 7 of 2003 on rules and regulations	min. distance between buildings equals the building height
		min. WFR 14%
Croatia	Socially Subsidized Housing Program	min. WFR 14% *
	Zagreb General Urban Plan	min. 2 h of sunlight daily on summer solstice (each apartment)
Czech Republic	Prague Building Regulations	min. 1/3 of the total floor area of all habitable rooms in each apartment must receive direct sunlight
		min. WFR 10%
Finland	Decree on housing, accommodation and work premises 127/2018	min. WFR 10%
		shading/obstruction angle 45°
France	Réglementation environnementale RE2020	min. WFR 17% **
		at distance > 1 m from the facade: in at least one main room, the occupant has a view of the exterior allowing them to see both the horizon and the sky
Germany	Berlin Building Code	min. WFR 12.5%
Iceland	Amendment 360/2016 Regulation on the (4th) Amend. to the Build. Reg., 112/2012	min. WFR 10%
Italy	Ministerial Decree of Health, 1975	min. WFR 12.5%
	Build. reg. of the mun. of Milan, 2016	shading/obstruction angle of 60°
		min. WFR 10% (12.5% in specific cases)
Poland	Reg. No. 690/2002 of the Ministry of Infrastructure on the technical requirements to be met by buildings and their placement	min. 3 h of sunlight between 7:00 and 17:00 on equinox, for at least one room min. 1.5 h for town apartments (not applicable for one-bedroom apartments)
		shading/obstruction angle 60°
		min. WFR 12.5%
Romania	Order of the Minister of Health no. 119/2014	min. 1.5 h of sunlight on 21 December for living units in the building and in neighbouring dwellings, as proven by the sun study if the distance between neighbouring buildings is less than or equal to the height of the highest building
Serbia	Rulebook on General Rules for Parcelation, Regulation and Construction, 2015	building must not block direct sunlight to another building for more than half of the duration of direct sunlight exposure
Slovakia	Reg. No. 532/2002 of the Min. of Env. of the Slovak Republic; Standard: STN 73 4301: Dwel. buildings	min. 1.5 h of sunlight (21 March to 22 September)
		min. WFR 10%
Slovenia	Rules on Min. Tech. Req. for the Construction of Apart. Buildings and Apartments, 2011	min. WFR 20%
Spain	Urban Planning Regulations of the General Urban Development Plan of Madrid	min. WFR 12%
Switzerland	Canton of Zurich, Building Department, Planning and Building Act (PBG) ***	no north-facing orientation (NW–NE)
		min. WFR 10%
		high buildings must not cause significant shading in residential zones
non-mandatory implementation
Croatia	HRN EN 17037	min. 1.5 h of sunlight for one habitable space of apartment from 1 February to 21 March
Czech Republic	Standard ČSN 73 4301:2004	min. 1.5 h of sunlight on 1 March and 21 June or balance of sunlight duration in the period from 10 February to 21 March is min. 1.5 h daily (new and existing buildings)
		min. window dimension ≥ 90 cm (for roof windows ≥ 70 cm)
		min. solar altitude 5°
Denmark	Bygningsreglementet 2018 (BR18)	min. WFR 10%
Germany	2007 Urban planning—assessing the impact of the planned construction on daylighting of adjacent rooms; Standard: DIN 5034-1: 2011-7	min. one room in the apartment: min. 4 h on 21 March; 1 h possible insolation on 17 January (additional winter criterion)
		min. 2 h of daylight per day for 250 days in a year
		min. distance between buildings is twice the building height
Greece	TOTEE 20701-7:2021	min. 1 h of sunlight on 21 February
Netherlands	‘TNO standard’ in some big cities	min. 2 h of daily sunlight (19 February to 21 October) min. 3 h of daily sunlight (21 January to 22 November)
		min. WFR 10%
Romania	Normative regarding the design of housing buildings NP 057-02	min. 2 h for at least one of the living units, on 21 February or 21 October
Sweden	Boverket Building Regulation BBR	min. WFR 10%
U.K.	London housing supp. plan. guid. 2016 ****	sunlight exposure of min. 1 habitable room for part of the day
	BS 8206-2:2008; BS EN 17037; Site layout planning for daylight and sunlight: a guide to good practice (BR209), 2022; The “45-Degree Rule” in urban planning approval;	min. 1.5 h of sunlight exposure on the assessment date
		if a new building causes the daylight duration of adjacent buildings to be lower than this standard and less than 80% of that before development, it is an obstruction to daylight
		min. WFR 20%
		min. 25% of probable annual sunlight h, min. 5% of probable sunlight from 21 September to 21 March

* Only buildings included in Socially Subsidized Housing Program. ** Min. residential WFR is waived in protected areas, heritage zones, and other sites designated by local or national preservation rules. *** Exception: city centre or protected buildings. **** The guidelines also specify possible exceptions to the strict application of sunlight recommendations, such as central areas of the city with high building density.

presents mandatory or non-mandatory insolation standards and identified sunlight exposure criteria in different European countries.

The data presented in Table 2 reveal notable differences in the use of various criteria for ensuring adequate solar access across European countries. At first glance, distinctions are evident between the mandatory and non-mandatory implementation of these criteria. Furthermore, variations can be observed in the level at which these criteria are implemented, whether at the national or local (municipal) level. Additional differences emerge among the identified criterion types, as well as in the recommended threshold values that serve as architectural design parameters. Insolation criteria differ according to the implementation range, as the research identifies urban (building) criteria as well as apartment (room) criteria. Collectively, these variations reflect a diverse insolation criteria framework encompassed by regulatory approaches and planning practices across Europe. A detailed analysis of these aspects is presented in the following section.

5. Results

Assessment of Residential Insolation Criteria in European Countries

Criteria for adequate sunlight exposure of apartment buildings in urban planning and architectural design are approached differently in each European country, as shown in Table 2. This can be explained by different socio-economic and climate conditions, even within one country. The differences in criteria can be classified into four categories:

• implementation model (mandatory or non-mandatory),
• implementation level (national or local),
• implementation range (urban/building or apartment/room),
• criterion type and value.

In 17 European countries (77%), out of 22 analysed, residential insolation criteria are mandatory, as shown in Figure 2. In five countries (23%) residential insolation criteria are recommended. In four countries (18%) both implementation models are detected, depending on the type of criterion. Considering the criteria implementation level, in 55% of the European countries analysed, insolation parameters are identified at the national level, in 18% at the local level, and in 27% at both levels. Considering the implementation range of insolation criteria, urban (building) criteria are not applied independently. They are applied in combination with the apartment (room) insolation criteria in 55% of the analysed countries. The apartment (room) insolation criteria are applied independently in 45% of the analysed countries. The comparative analysis of residential insolation criteria frameworks in 22 European countries revealed seven types of sunlight exposure criteria:

• minimum WFR (or minimum window dimensions),
• minimum daily duration of sunlight exposure,
• minimum sun altitude (considered when assessing building sunlight exposure),
• four criteria related to the shading impact factor of surrounding buildings (maximum permissible height of new buildings, minimum distance between buildings, maximum shading angle, limited duration of shading of neighbouring buildings or plots).

These criteria represent the basic tools used to ensure sufficient insolation of residential living spaces. They can be classified into two categories corresponding to different ranges of implementation:

• urban/building (minimum sun altitude, and criteria covered by the shading impact of surrounding buildings: maximum shading angle, limited shading of neighbouring buildings or plots, minimum distance between buildings, maximum height of buildings)
• apartment/room (minimum daily sunlight exposure of habitable spaces and minimum WFR).

Table 3. Prescribed or recommended values of sunlight exposure criteria for living spaces in multi-residential buildings in different European countries or cities [3,4,7,9,11,36,37,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67]. Non-mandatory guidelines are written in italics.

Criteria	Criteria Value		Town or Country
apartment/room criteria
minimum WFR	10%		Prague, Finland, Iceland, Milan, Slovakia, Zurich, Denmark, Dutch bigger cities, Sweden
	12%		Austria, Madrid
	12.5%		Berlin, Italy, Poland
	14%		Bulgaria, Croatia *
	17%		France
	20%		Brussels, Slovenia, U.K.
min. window dimension	≥ 90 cm (for roof windows ≥ 70 cm)		Czech Republic
minimum daily duration of sunlight exposure	1 h	on 17 January for min. one room in the apartment	Germany
		on 21 February	Greece
	1.5 h	1 February to 21 March, for one habitable space of apartment	Croatia
		on 1 March and 21 June or balance of sunlight duration in the period from 10 February to 21 March (new and existing buildings); min. 1/3 of the total floor area of all habitable rooms in each apartment must receive direct sunlight	Czech Republic Prague
		on 21 December, if the distance between neighbouring buildings is ≤ height of the highest building	Romania
		21 March to 22 September	Slovakia
		town apartments (not applicable for one-bedroom apartments)	Poland
		on the assessment date	U.K.
	2 h	during the summer solstice for each apartment	Zagreb
		19 February to 21 October	Dutch bigger cities
		daylight per day for 250 days in a year	Germany
		on 21 February or 21 October, for at least one of the living units	Romania
	3 h	21 January to 22 November	Dutch bigger cities
		between 7:00 and 17:00 on equinox, at least one room (not applicable for apartments in central town area)	Poland
	4 h	min. one room in the apartment on 21 March	Germany
	-	min. one habitable room for part of the day; min. 25% of probable annual sunlight h, min. 5% of probable sunlight from 21 September to 21 March	London U.K.
		no north-facing orientation (NW–NE)	Zurich
urban/building criteria
maximum shading angle	45°		Austria, Finland
	60°		Milan, Poland
limited shading of neighbouring buildings or plots	building must not block direct sunlight to another building for more than half of the duration of direct sunlight exposure		Serbia
	high buildings must not cause significant shading in residential zones		Zürich
	if a new building causes the daylight duration of adjacent buildings to be lower than this standard and less than 80% of that before development, it is an obstruction to daylight		U.K.
minimum distance between buildings **	equals the building height		Bulgaria
	usually twice the building height		Germany
maximum height of buildings **	defined in terms of insolation		Vienna
	at distance > 1m from the facade: in at least one main room, the occupant has a view of the exterior allowing them to see both the horizon and the sky		France
minimum sun altitude ***	5°		Czech Republic

* In Croatia, min. WFR is prescribed only for Socially Subsidized Housing Program buildings. ** Criteria defined in relation to sunlight exposure. *** Which is considered for the assessment of building sunlight exposure.

presents analysed data according to detected insolation criterion type and values, as well as the implementation model and range. The data are presented by identifying the most prevalent types of criteria (separately by implementation range) and arranging them from lower to higher values.

Analysis of European guidelines and regulations covered 22 countries from different socio-economic and climate contexts to achieve a better understanding of the criteria that ensure sunlight exposure of residential buildings. The framework for selecting the analysed European countries was based on ensuring comparability through shared cultural and institutional characteristics while also capturing diversity in socio-economic and climatic contexts. This study showed that sunlight exposure criteria for residential buildings are applied differently in European countries. The comparison showed that just over half of the 22 analysed countries have recommended or prescribed multiple criteria for sunlight exposure (59% of countries). The most common criterion for the sunlight exposure of living spaces in multi-residential buildings is the minimum WFR, i.e., window (glazed) area-to-floor area ratio. This criterion is included in the standards of 86% of the analysed countries. The next most frequently used criterion is the minimum daily duration of sunlight exposure. This criterion is included in the standards of 45% of the analysed countries. In a smaller number of analysed countries, criteria related to the impact of shading from surrounding buildings are included: maximum shading angle (in 18% of countries), limited duration of shading from neighbouring buildings or plots (in 14% of countries), as well as the maximum permissible height of new buildings and minimum distance between buildings (in 9% of countries). The criterion of minimum sun altitude (considered in assessing building sunlight exposure) is defined in 5% of analysed countries. The distribution of sunlight exposure criterion types in the analysed countries is shown in Figure 3.

The values of prescribed or recommended criteria for sunlight exposure vary greatly between analysed countries. The criterion of minimum WFR (the ratio of the window area to the floor area of the corresponding room) is defined with values ranging from 10% to 20%. It should be emphasized here that this criterion represents a tool that is also related to the daylight exposure of interior rooms, so it does not refer exclusively to the direct sun exposure required to meet hygiene and technical conditions in habitable spaces.

For the second most frequently applied criterion (minimum daily duration of sunlight exposure), the values are determined depending on the following circumstances of criterion value implementation:

• the specific date or period of the year during which the value applies,
• the specific part of the day during which the value applies,
• the level of population or built density in the area where the value applies,
• the number of living spaces in an apartment to which the value applies,
• the share of floor area of habitable room(s) to which the value applies.

The recommended values differ from 1 to 4 h daily, on different dates or date periods, during different daytime periods. These values show a strong emphasis across Europe on requiring a minimum number of sunlight hours during key calendar dates, usually tied to equinoxes and solstices. In some countries, sunlight exposure recommendations differ depending on the built area density and location latitude, whether the building is new or existing, as well as the status of heritage protection. In certain European countries, several values of the minimum daily duration of sunlight exposure are defined, depending on different circumstances of application.

The group of criteria related to the shading impact factor of surrounding buildings is defined by values of the maximum shading angle (45° or 60°), the minimum distance between buildings (equal to the building height or twice the building height), the maximum permissible height of new buildings (to prevent excessive shading of the ground-floor rooms of existing buildings or to enable a view of the exterior), and the different provisions for limited duration of shading of neighbouring buildings or plots. Minimum sun altitude, which is considered in the assessment of building sunlight exposure, is defined only in one country and is set at 5°.

Mandatory implementation contributes to on-site insolation criteria implementation. However, informal interviews with architects from various European countries indicate that numerous recommendations, while not legally binding, are widely implemented in practice to achieve technically sound and high-quality design outcomes. This discrepancy highlights the gap between regulatory frameworks and on-site application. According to literature [56], compliance procedures in European countries primarily concentrate on the structural integrity and energy efficiency of new buildings, including factors such as U-values, proper installation of heating systems, airtightness, and the availability of energy performance certificates. In contrast, compliance with indoor air quality, thermal comfort or insolation standards is seldom assessed by authorized inspection bodies and, when it is, the evaluation usually takes place in the design stage rather than by on-site observation.

Although the analysis shows that insolation criteria are mostly applied at the national level, some standards are applied at the municipal or town level. This feature is important, as it enables the implementation of standards adapted to various local contexts. Regional sunlight standards allow adaptation to the specific conditions of each location’s climate, residential density, shading, and limited distances between buildings relative to their height, which is of particular concern in urban areas. Local standards can be a powerful tool for fine-tuning general (national) insolation standards to the local context and the specific characteristics of individual areas. Given that building insolation is significantly influenced by geographical location and local climatic characteristics, it is important to illustrate the geographical distribution of insolation criteria patterns (Figure 4). Several spatial and regulatory patterns can be identified in the presented maps.

First, in terms of the implementation model, countries in Central and Southern Europe tend to rely more on mandatory national regulations, while Western and parts of Northern Europe more frequently apply non-mandatory or mixed approaches, often supported by local guidelines. This suggests a stronger tradition of centralized regulation in southern and post-socialist countries compared to more decentralized planning systems in North-western Europe.

Second, regarding the mandatory criteria implementation range, two dominant approaches can be observed. Some countries regulate primarily urban/building insolation criteria, while others focus more on apartment/room criteria. A number of countries apply combined approaches, indicating attempts to balance urban morphology control with indoor environmental quality requirements.

Third, the map illustrating mandatory minimum window-to-floor ratios (WFRs) reveals considerable variation across Europe. Lower minimum values are more common in northern regions, while higher thresholds are more frequently found in southern and central countries, reflecting differences in climatic conditions, daylight availability, and traditional building typologies.

Fourth, the distribution of mandatory minimum daily sunlight duration shows that explicit time-based requirements are relatively limited to a smaller group of countries, mainly in Central and Eastern Europe. In many Western European countries, such quantitative criteria are either absent or replaced by qualitative performance-based standards.

6. Discussion

6.1. Study Limitations

While efforts were made to ensure diversity within the research sample, the analysis was limited to 22 European countries due to data constraints. However, the analysed countries account for approximately 82% of Europe’s population (excluding Russia) [68], which highlights the importance of the results of this study.

This research has examined in detail the obligation to apply European recommendations for the sunlight exposure of residential spaces. Although the regulatory status of the analysed standards has been clarified, these data should be interpreted with caution, as formal mandatory requirements do not necessarily ensure consistent implementation on-site. Also, professional experience suggests that enforcement depends on implementation circumstances, since voluntary guidelines are often applied alongside or instead of mandatory provisions. Compliance procedures in European countries rarely focus on interior insolation and its impact on indoor comfort, which is usually evaluated only during the design phase.

It is advisable to conduct research on sunlight exposure criteria as part of broader studies on daylighting, thermal protection and rational energy use since the direct component of solar radiation (insolation) is inseparable from UV, daylight and thermal radiation. Sunlight exposure is part of a complex system of external factors that contribute to indoor comfort, and research into sunlight exposure should therefore be approached accordingly. The dynamic methodology established within the European standard EN 17037, unlike the traditional static approach, enables a more comprehensive and realistic representation of visual comfort and environmental quality throughout the year. Consequently, the implementation of the EN 17037 standard reflects a broader paradigm shift in architectural solar radiation research from simplified, time-based metrics to holistic, user-centred performance indicators [69]. This review encourages adoption of the mentioned framework, highlighting its relevance for contemporary building design and evaluation. In support of the above, according to the presented analysis of European standards, the most used criterion for ensuring the sunlight exposure of residential buildings is the min. WFR. This insolation criterion supports EN 17037 methodology, as it affects all four components of the evaluation model described in that standard (daylight provision, quality of view, access to sunlight, and glare protection).

6.2. Future Research Directions

At present, there are no comprehensive empirical studies or publicly available datasets that quantify the impact of deregulation on architectural design, daylight and sunlight conditions and their effects on housing quality and interior comfort. Existing research and regulatory practice often consider daylight and solar exposure as input parameters for energy performance assessments rather than as indicators of residential quality and occupant well-being. Therefore, a detailed quantitative evaluation of these impacts is currently not feasible. This study represents an initial step towards identifying regulatory gaps and methodological limitations in this field. The systematic assessment of the consequences of deregulation on housing quality is an important topic for future research. Future research directions could include the use of field studies and in situ measurements of sunlight exposure within residential rooms in buildings, comparing them to insolation criteria applied in the design stage. This future research would combine on-site sunlight measurements using appropriate instruments with computational modelling of sunlight exposure in specialized software tools. The results of such future research could then be compared with sunlight exposure recommendations and regulations, providing insights into the most effective model of insolation criteria implementation.

7. Conclusions

Sunlight exposure has played a key role in settlement and building design throughout architectural history. The lack of a systematic housing policy has a negative impact on the quality of residential construction [17,33], which is also reflected in the insolation of residential buildings. Studies indicate that housing quality has a significant impact on residents’ physical and mental health, as well as their social well-being [18,19]. Croatia was examined separately to assess the scope of deregulation. This part of the research showed that in Croatia,

• there are no obligatory national insolation standards,
• obligatory insolation standards are prescribed just locally (Zagreb),
• the only national-level standard is non-mandatory (HRN EN 17037:2022),
• just apartment/room insolation criteria are detected,
• detected insolation criteria are min. WFR (14%) and sun protection (both mandatory, but just for buildings included in the Socially Subsidized Housing Program), and min. sunlight duration (2 h, mandatory just in Zagreb).

The comparison of current sunlight standards in Croatia with those prior to the 1990s socio-political changes showed that a significantly larger number of criteria for sunlight exposure was encompassed by the former DUSI standard. The criteria were more comprehensive, including urban/building criteria as well as apartment/room sunlight criteria. DUSI recommendations were not mandatory, but at that time professional control mechanisms were in place to ensure that spatial planning and construction complied with the specified criteria [27] (p. 75). Identified changes reflect a broader process of deregulation in the Croatian housing sector following the socio-political transition of the 1990s, marked by reduced state involvement and increased reliance on voluntary and local-level regulation. Nowadays, obligatory country-level sunlight exposure criteria (WFR and sun protection) refer only to the Socially Subsidized Housing Program. This creates a paradox where socially subsidized housing is subject to more precise and stricter insolation standards than usual, market-oriented housing [17] (p. 170). The consequences of this anomaly could be mitigated by implementing at least some elements of socially supported housing programs into the binding framework of minimum hygiene-technical requirements for residential spaces (including criteria for sunlight exposure). This is especially important in collective housing, where users are mostly unknown, so such spaces need to be universally desirable.

The assessment of insolation criteria in European standards offers an overview of possible tools that could be adapted to local socio-economic and climate circumstances. This review includes a comparative analysis of European spatial planning, urban planning, and architectural design criteria that influence sunlight exposure of habitable spaces in apartment buildings. Insolation requirements in guidelines and regulations are defined both by criteria and their threshold values. Based on the comparison of residential insolation standards among 22 European countries, this study showed that

• the implementation of insolation standards is mostly mandatory (77%),
• implementation mostly refers to national (country) level (55%),
• the combined implementation of urban/building and apartment/room insolation criteria is most common (55%), followed by implementation of only apartment/room insolation criteria (45%),
• the most common insolation criterion types are min. WFR (86%) and min. sunlight duration (45%), both referring to apartment/room criteria,
• insolation criteria values differ from 10 to 20% for min. WFR, and from 1 to 4 h for min. sunlight duration.

Overall, this study indicates geographical differentiation in insolation regulatory approaches. Northern and Western European countries tend to favour decentralized, flexible, guideline-based and performance-oriented frameworks, whereas Central, Southern, and Eastern European countries more often rely on prescriptive, quantitatively defined standards. This may reflect a stronger tradition of state-led planning. These patterns suggest that regulatory traditions, climatic conditions, and planning cultures jointly shape national approaches to residential insolation.

The mandatory implementation of sunlight exposure regulations should remain sufficiently flexible to allow fulfilment of other important requirements in interior spaces, while at the same time avoiding adverse effects on living comfort such as glare and overheating. The analysis of European insolation standards shows that in certain densely populated or historic areas of the city, it may be necessary to apply the guidelines with flexibility to accommodate urban development, while safeguarding the hygiene and technical standards of residential spaces. The insolation recommendations have to be aligned with the climatic, social and economic context of a specific country and location. Determining sunlight criteria values at a local level (e.g., for a specific city) can have a positive effect in countries with variable climatic conditions. Therefore, sunlight recommendations must be adapted to local conditions [6,69].

The non-mandatory insolation recommendation approach allows planners and designers to adapt to diverse urban contexts and specific project conditions, encouraging better solutions without imposing rigid constraints. Considering the fact that compliance procedures rarely concentrate on ensuring adherence to recommended or prescribed insolation standards, it seems extremely important to pay special attention to the implementation of these standards during the planning and design of residential buildings. In cases where the implementation of insolation criteria is not mandatory, it would be useful to examine developers’ inclination to comply with non-mandatory insolation standards if they gain certain benefits by doing so.

The comparative analysis reveals that minimum daily duration of sunlight exposure values are determined depending on the specific date or period of the year during which the value applies. Due to energy savings contributions, it would be ideal to set sunlight criteria according to the winter season to maximize passive heating and daylighting of interior space. However, in urban areas, this can create challenges for building orientation and city development, as the low winter sun angle could limit building heights or require greater spacing between them. A compromise shown by this research is to establish sunlight exposure criteria based on the sun’s position during spring or autumn, as adopted in several national standards and guidelines. This approach supports urban growth while allowing better use of solar energy.

This review examined European insolation criteria to reveal differences in priorities, legal frameworks, and approaches to key planning and architectural issues such as sunlight exposure, housing quality, and urban density. By understanding these differences, researchers and policymakers could identify best practices, fill regulatory gaps, and propose improvements sensitive to local conditions. The insolation criteria and correlated implementation determined by this research could represent a framework for evaluating and improving the existing tools referring to the insolation of residential spaces.