A daylight and sunlight assessment is a technical study used to understand how a proposed development will perform in terms of natural light, both within the development itself and in relation to neighbouring properties and external spaces.
There are two main components to a daylight and sunlight assessment: impact assessment and scheme performance. Impact assessment considers how a proposed development may affect daylight and sunlight received by surrounding properties and amenity spaces, while scheme performance looks at the levels of daylight and sunlight achieved within the proposed development itself.
The way daylight and sunlight are assessed within proposed developments has changed over time, particularly with the introduction of EN 17037 and the third edition of BRE 209. This article provides an overview of the main metrics and methods now used, while also explaining the shift from average daylight factor (ADF) to the current approaches for assessing internal daylight.
A detailed, comprehensive explanation of daylight and sunlight assessments and their components can be found here.
Daylight and sunlight standards
A number of guidance documents and standards are used when assessing daylight and sunlight, and the relationship between them has evolved over time.
Historically, two documents were widely referenced in Ireland and the UK: Site Layout Planning for Daylight and Sunlight: A Guide to Good Practice by BRE, commonly referred to as BRE 209, and BS 8206-2, Lighting for buildings – Part 2: Code of practice for daylighting.
In 2018, the European standard EN 17037 – Daylight in buildings was published, followed by its national implementations, including BS EN 17037 in the UK and I.S. EN 17037 in Ireland. These introduced a different approach to assessing daylight provision within buildings, based on illuminance rather than the average daylight factor (ADF) method previously referenced by BRE 209 and BS 8206-2.
The third edition of BRE 209, published in 2022, subsequently replaced the 2011 edition and refers to BS EN 17037 for recommendations on daylight within new buildings. BS 8206-2 has also been superseded by BS EN 17037.
In Ireland, the applicable methodology should also be considered alongside current national planning guidance and any requirements of the relevant local planning authority.
Impact Assessments
Of the aforementioned guidance documents, BRE 209 sets out the methodology for assessing the impact of a proposed development on surrounding properties and external spaces using vertical sky component (VSC), annual and winter probable sunlight hours (APSH/WPSH), and sun on ground (SOG). Detailed descriptions of these studies can be found in our guide to impact assessments.
The third edition of BRE 209 retained the established methodology used to assess impacts on neighbouring properties, while introducing additional guidance and refinements. EN 17037 and BS EN 17037 focus primarily on daylight and sunlight provision within buildings and do not set out methods for assessing impacts on neighbouring properties.
Scheme performance
When assessing scheme performance, three studies are of relevance: sunlight access within proposed amenity spaces and gardens, internal daylighting within proposed buildings, and sunlight exposure within proposed dwellings. These are outlined below, with more detailed explanations available in our guide to scheme performance in daylight and sunlight assessments.
1. Sunlighting (sun on ground)
The study used to assess proposed amenity spaces or gardens for sunlight access is sun on ground (SOG). BRE 209 recommends that, for a garden or amenity area to appear adequately sunlit throughout the year, at least 50% of the space should receive at least two hours of direct sunlight on 21 March.
The sunlight levels received on 21 March are considered to provide a representative indication of sunlight access throughout the year.
2. Internal daylighting
Background on ADF
Under the previous edition of BRE 209 (2011), internal daylight provision within proposed dwellings was commonly assessed using average daylight factor (ADF), with target values taken from BS 8206-2.
To calculate ADF, a reference plane was positioned within a room and divided into a calculation grid. The daylight factor was calculated at each point, with the average of these values providing the ADF for the room. Recommended minimum values were 2% for kitchens, 1.5% for living rooms and 1% for bedrooms.
With the third edition of BRE 209 (2022), ADF is no longer used as a measure of daylight provision within new dwellings. Instead, the guidance provides a choice between a target illuminance method and a target daylight factor method.
Targets
The European standard EN 17037 sets general recommendations for daylight provision within buildings. Its Irish adoption, I.S. EN 17037, follows these European criteria.
Under this approach, a target illuminance of 300 lux should be achieved across at least 50% of the reference plane for at least half of the daylight hours. For spaces with vertical or inclined daylight openings, a minimum illuminance of 100 lux should also be achieved across 95% of the reference plane.
In the UK, BS EN 17037 includes a UK National Annex which provides an alternative set of room-specific recommendations for dwellings:
- 100 lux for bedrooms;
- 150 lux for living rooms; and
- 200 lux for kitchens.
These target values should be achieved across at least 50% of the reference plane for at least half of the daylight hours. The additional minimum illuminance target across 95% of the reference plane does not apply when using the UK National Annex recommendations for dwellings.
The appropriate criteria will depend on the project location and applicable planning requirements. In Ireland, national planning guidance identifies I.S. EN 17037, the UK National Annex to BS EN 17037 and BRE 209 as relevant quantitative approaches where a technical daylight assessment is required.
Methods of calculation
There are two methods of calculation for assessing daylight provision set out in EN 17037.
The first method uses daylight factor calculations. Under this approach, target and minimum target daylight factors are derived from the corresponding illuminance targets and the daylight availability associated with the geographical location of the proposed development.
The second method uses detailed climate-based calculations to determine the internal daylight illuminance achieved throughout a typical year. This method uses appropriate climate and sky data for the project location and can be carried out using specialist software.
Either method can be used to assess daylight provision.
The geographical location of the proposed development is relevant to both methods, but in different ways. Under the first method, location influences the applicable target daylight factor values, while the daylight factor calculation itself uses a standard overcast sky and is therefore not directly affected by orientation.
Under the second method, the assessment uses climate data appropriate to the project location and takes the orientation of the room into account. This provides a more detailed picture of how daylight levels can vary throughout the year.
3. Sunlight exposure (SE)
Sunlight exposure (SE) is used to assess direct sunlight within proposed dwellings. Under BRE 209 third edition, this replaces the annual probable sunlight hours (APSH) approach previously used to assess sunlight provision within new dwellings.
BRE 209 recommends that a habitable room should receive a minimum of 1.5 hours of direct sunlight on a selected date between 1 February and 21 March, under cloudless conditions. The guidance suggests using 21 March as the assessment date.
Three levels of sunlight exposure are identified:
- 1.5 hours – minimum;
- 3 hours – medium; and
- 4 hours – high.
For dwellings, at least one habitable room should achieve the minimum recommendation, preferably a main living room.
APSH and winter probable sunlight hours (WPSH) continue to be used when assessing the potential impact of a proposed development on sunlight received by existing neighbouring properties.
Daylight and sunlight: design and planning considerations
Daylight performance within a proposed development is influenced by a combination of factors, including room depth and layout, orientation, window size, glazing specification and the reflectance of internal and external surfaces.
Where daylight and sunlight performance is not properly considered during the design process, it can create additional planning risk. Poor results or an assessment that does not adequately address the relevant guidance may contribute to objections, requests for further information, planning refusals or appeals. In some cases, daylight and sunlight matters may also form part of the grounds raised in a judicial review of a planning decision.
This is why early assessment is important. As daylight and sunlight consultants, part of 3DDB’s role is not simply to report the final results, but to work with the design team throughout the process. Where areas of lower performance are identified, we can advise on potential design changes, such as adjustments to room layouts, building form, openings, materials or other elements, to improve the results before the proposal is finalised.
The appropriate assessment methodology and criteria should always be considered in relation to the project location, development type and applicable planning requirements.
