Want to know whether a neighbouring extension — or your own scheme — passes the BRE daylight tests before paying for a full assessment? This free calculator applies the two headline screening tests from BRE BR 209 (2022): the 27% VSC / 0.8-times rule for daylight, and the 25% annual / 5% winter APSH tests for sunlight.
Daylight & sunlight calculator (BR 209 screening)
The calculator needs JavaScript enabled. The tests it applies are set out in our BRE 2022 daylight targets reference tables.
How to use it
- VSC test — enter the existing and proposed Vertical Sky Component for the affected window. VSC is measured at the outside centre of the window; a full report calculates it from 3D modelling, but architects often have indicative values from early testing.
- APSH test — for windows facing within 90° of due south, enter existing and proposed Annual Probable Sunlight Hours (annual and winter figures).
What the results mean
The calculator applies BR 209's logic in order. A window that keeps 27% VSC or better is treated as well daylit whatever the loss. Below that, the 0.8-times rule takes over: keep at least 80% of the former value and the change is unlikely to be noticed; fall below 0.8 times and the window flags as an adverse effect. The same ratio logic applies to sunlight when the 25%/5% APSH targets are missed. For the full numbers — including the BS EN 17037 targets for rooms inside new dwellings — see our daylight targets tables.
What this calculator cannot tell you
Honest limits: BR 209 screening at a single window is the start, not the whole assessment. A full daylight and sunlight report also tests NSL (daylight distribution) inside affected rooms, room use, overshadowing of gardens, and — in dense urban or conservation contexts — whether the NPPF's flexible approach justifies alternative targets. The calculator flags risk; it does not replace the report your planning officer will ask for.
How Fortress Associates can help
If the calculator flags a problem — or a neighbour's objection has raised one — our daylight and sunlight reports assess VSC, NSL, APSH and BS EN 17037 compliance to BRE BR 209 (2022), typically delivered in 4–5 days with no advance payment. Explore our other free planning tools, browse all services or get in touch.
Sources & further reading
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