Daylight 25 Degree Rule of Thumb
(Building Research Establishment (BRE) document ‘Site Layout Planning for Daylight and Sunlight: A guide to good practice (2011)
The BRE 25° test is used where the development is opposite the window of adjacent dwellings. The centre of the lowest habitable room window should be used as the reference point and if the whole of the proposed development falls beneath a line drawn at 25° from the horizontal, then there is unlikely to be a substantial effect on daylight and sunlight.
However, if the proposed development goes above the 25° line, a developer should undertake further checks on daylight and sunlight to determine the impact of the development.
As can be seen from example below; based upon the approximate centre of the plane of a ground floor window; most of the proposed building would be above the 25°angle.
The BRE guide gives the following advice on window design.
If the obstruction angle is:
- less than 25°, conventional window design will usually give reasonable results.
- between 25° and 45°, special measures (enlarged windows, changes to room layout) are usually needed to provide adequate daylight.
- between 45° and 65°, it is very difficult to provide adequate daylight, unless very large.
This may require modelling to show no impact or in extreme cases a re-design of the proposal to reduce the impact on the adjacent dwelling to secure planning approval.
Energy Evaluation Services Comments
We can provide further detailed analysis including:
- Vertical Sky Component
- Daylight Distribution / No Sky Line (where room layouts are known)
- Average Daylight Factor (where BRE Appendix F criteria apply)
- Annual Probable Sunlight Hours
To aid the design and limit the impact on the development