BRE Daylight & Sunlight Assessments: A Vital Component to Planning Applications

In short, BRE Daylight & Sunlight Assessments advise on planning developments for good access to daylight and sunlight. These assessments are widely used by design teams and local authorities for planning applications to help determine the impacts that a new development may have on itself and the surrounding environment.

If you’re a property developer, architect, or Irish planning consultant, you should be aware that it is now industry standard to include daylight & sunlight reports as part of a planning application to local authorities or An Bord Pleanála.

If you’re working on large-scale schemes, such as Strategic Housing Developments (SHD), Private Rental Schemes (PRS), and Built to Rent developments (BTR), BRE Daylight & Sunlight Assessments are now considered a requirement along with other 3D architecture documents. It is also a vital tool in the early design stages of a project to help determine the mass and scale a proposed scheme can be designed.

As there is no specific guidance for daylight and sunlight in Ireland, all target values of daylight and sunlight are taken from the 2011 BRE guidelines as set out in “Site layout planning for daylight and sunlight”.

While most planning authorities now require these assessments, it is noted in the BRE Guidelines that they should be treated as guidelines as opposed to rules:

“The advice given here is not mandatory and the guide should not be seen as an instrument of planning policy; it aims is to help rather than constrain the designer. Although it gives numerical guidelines, these should be interpreted flexibly since natural lighting is only one of many factors in site layout design. In special circumstances, the developer or planning authority may wish to use different target values. For example, in a historic city centre, or an area with modern high rise buildings, a higher degree of obstruction may be unavoidable if new developments are to match the height and proportions of existing buildings.”

However, it should be noted that all planning authorities and An Bord Pleanála do consider the guidelines as extremely important and look for developments to reach a high level of compliance with them. Furthermore, there are likely changes to these guidelines before the end of 2021.

What is a BRE Daylight & Sunlight Assessment?

A BRE Daylight & Sunlight Assessment is a detailed technical analysis and report that evaluates the potential environmental impact on daylight, sunlight and overshadowing by a proposed development. BRE Daylight and sunlight studies focus on impact analysis (the proposed development on its surrounding environment) and internal analysis (the proposed development on itself).

Under the BRE guidelines, property developments are assessed for impacts on sun lighting, vertical sky component (VSC), annual probable sunlight hours (APSH) and average daylight factor (ADF). Depending on the results generated, design changes may be required to mitigate impacts a development may have on its environment and on itself.

Carrying out BRE daylight and sunlight analysis at a very early design stage is highly advisable for many proposed developments. This helps assess and determine if the early massing and scale design of a scheme will be suitable for a given site.

Impacts That Are Assessed in BRE Daylight & Sunlight Reports:

1. Impact to Vertical Sky Component (VSC)

Obstructions can limit access to light from the sky. This can be checked by measuring or calculating the Vertical Sky Component (VSC) at the centre of a window where daylight is required.

If VSC is:

– At least 27%, then conventional window design will usually give reasonable results.
– Between 15% and 27%, then special measures (larger windows, changes to room layout) are usually
needed to provide adequate daylight.
– Between 5% and 15%, then it is very difficult to provide adequate daylight unless very large windows are used.
– Less than 5%, then it is often impossible to achieve reasonable daylight, even if the whole window wall is glazed

When measuring the effect a proposed development will have on the VSC of an existing window, if the value drops below
the 27% guideline and is less than 0.8 times the existing value, the proposed development could possibly have a noticeable impact on the amount of daylight received.

The VSC of the windows of the assessed properties as listed above will be calculated both in the baseline state and as they would appear should the proposed development be constructed as proposed.
A comparison between these values will determine the level of impact.

2. Impact to Annual Probable Sunlight Hours (APSH)

Annual probable sunlight hours (APSH) is a measure of sunlight that a given window may expect over a year period. The BRE guidance recognises that sunlight is less important than daylight in the amenity of a room and is heavily influenced by orientation. North facing windows may receive sunlight on only a handful of occasions in a year, and windows facing eastwards or westwards will only receive sunlight for some of the day. Therefore, BRE guidance states that only windows with an orientation within 90 degrees of due south need be assessed.

If the assessment point of a window can receive more than 25% of APSH, including at least 5% of APSH in the winter months, then the room should receive enough sunlight.

When measuring the effect a proposed development will have on the APSH of an existing window, if the APSH value drops below the annual (25%) or winter (5%) guidelines and is both less than 0.8 times the baseline value and there is a reduction of more than 4% of the annual APSH, the proposed development could possibly have a noticeable impact on sunlight.

3. Average Daylight Factor (ADF).

ADF is the ratio of total daylight flux incident on the working plane to the area of the working plane, expressed as a percentage of the outdoor illuminance on a horizontal plane due to an unobstructed CIE standard overcast sky.

The BS 8206-2 Code of practice for daylighting, recommends an ADF of 5% for a well day lit space and 2% for a partly daylight space. A room with below 2% will look dull and electric lighting is likely to be turned on. In terms of housing, BS 8206-2 also gives minimum values of ADF:
2% for Kitchens, 1.5% for living rooms and 1% for bedrooms.

4. Impact to Sunlighting in Existing Gardens:

The BRE guidelines recommend that for a garden or amenity area to appear adequately sunlit throughout the year, at least half of it should receive at least two hours of sunlight on March 21st. If as a result of a new development, an existing garden does not meet the above, and the area which can receive two hours of sun on March 21st is less than 0.8 times its former value, then the loss of sunlight is likely to be noticeable.

To calculate this, the percentage of assessed areas which can receive two hours or more of direct sunlight on March 21st will are calculated in both the baseline and proposed states. A comparison between these values will determine the level of impact.

A visual representation of these readings is also produced as 2 hour false colour plans on and in the hourly shadow diagrams for March 21st.

5. Sunlighting in Proposed Outdoor Amenity Areas:

As per impact to existing gardens, the BRE guidelines recommend that for a garden or amenity area to appear adequately sunlit throughout the year, at least half of it should be capable of receiving two hours or more of direct sunlight on March 21st.

Assessments are carried out on the proposed private and public external amenity areas within a scheme, as defined by the architect, to determine what portion of it can receive at least 2 hours of sunlight on March 21st. The portion of each space capable of receiving 2 hours of direct sunlight on March 21st will be calculated.

A visual representation of these readings is also produced as 2 hour false colour plans on and in the hourly shadow diagrams for March 21st.

3D Design Bureau’s Methodology for Producing BRE Daylight & Sunlight Reports:

1. Building the Proposed and Existing Digital 3D Models for Analysis.

In order to obtain the results of this analysis, 3D Design Bureau models the proposed development from a set of architectural drawings issued by the project architect or optimises supplied 3D models. This is done within our chosen software – Autodesk Revit.

A combination of 3rd Party resources such as survey information, aerial photography, available on-line photography & ordnance survey information are used to model the any existing buildings on the proposed site along with the surrounding context.

Note: As the information gathered from on-line sources is not as accurate as surveyed information, some tolerance should be allowed to the results generated. Ideally, 3D Design Bureau would be furnished with accurate survey information of the existing site and surrounding context

2. Inclusion / Exclusion of Trees: Normally trees and shrubs do not need to be included, partly because their shapes are almost impossible to predict, and partly because the dappled shade of a tree is more pleasant than the deep shadow of a building (this applies especially to deciduous trees).

Where a dense belt or group of evergreens is specifically planned as a windbreak or for privacy purposes, it is better to include their shadow in the calculation of shaded area. No such dense belt of coniferous trees occurs in the current context.

3. Defining Areas.

Where living spaces in a proposed development are connected to a kitchen but there is a partition wall between the two spaces, the kitchens can be considered completely internal. As such the kitchen areas will not be calculated but can be assumed to have an ADF of lower than the target of 2.0%. Living spaces will have a target ADF values of 1.5%.

A target value of 2% is generally applied to studios with circulation areas being removed from the analysis area.

Bedrooms will have a target value of 1%, with circulation areas being removed from the analysis area.

Circulation spaces, corridors, bathrooms etc are generally not analysed as they have no target values for daylight.

4. Work plane.

The calculation of ADF is carried out on a hypothetical work plane which lies 850mm from the finished floor level and is offset 500mm from the room boundaries. Room boundaries are taken from the inside face of the interior walls. Daylight Factor has been calculated on the work plane across a series of points on a grid of approximately 200mm. The average of these figures determines the Average Daylight Factor (ADF)

5. Assessment Points.

Assessment points, when measuring VSC or APSH of a window, are taken from the centre point of a standard window. If the window being assessed is a full height window the assessment point is taken at 1600mm above the finished floor level.

If it can be determined that multiple windows are servicing the same room, each window will be assessed, and the average value will be taken.

Only habitable rooms need to be assessed for impact to daylight and sunlight.

6. Generating & Production of Results.

All results generated in a study are calculated using the purpose built digital 3D models as stated above, using daylight and sunlight analysis techniques within the Autodesk AEC Suite & the Vray render engine.

All target values are obtained from the 2011 BRE guidelines as set out in “Site layout planning for daylight and sunlight

7. Shadow Study

The shadow study renderings are produced in order to give a visual representation to the results set out in the sunlight analysis section of a report.

Hourly renderings have been shown from sunrise to sunset on the following dates:

  • Spring Equinox: March 21st. Sunrise 6:25 | Sunset 18:40.
  • Summer Solstice: June 21st. Sunrise 4:57 | Sunset 21:57.
  • Winter Solstice: December 21st. Sunrise 8:38 | Sunset 16:08.

Note: The Spring and Autumn Equinox yield similar results.

8. Handover

3D Design Bureau then produce a PDF and printed document of BRE Daylight and Sunlight Assessments that is ready to hand over to the planning authorities as part of the planning application.

9. Expert Witness Service:

As BRE is considered a guideline, CGI consultants, 3D Design Bureau can provide an expert witness service that provides evidence at planning hearings and public inquiries.

We’d love to hear about your project: