The "45-degree rule" is the test neighbours and planning officers reach for first when an extension might block a window's light: draw a 45° line from the affected window — if the new building crosses it, expect trouble. This checker does the geometry for you, and adds the number the professionals use: BRE BR 209's 25° screening angle.
Check an extension against the angles
The checker needs JavaScript. The BRE method behind it is in our daylight targets reference.
What the two angles mean
- The 45° test — an informal rule of thumb found in many councils' householder design guides, drawn on plan, elevation or both. Each council words its own version, so treat it as a first screen, not policy.
- The BRE 25° screen — from BR 209 (2022): if a new obstruction subtends less than 25° from the centre of a window, daylight to that window is unlikely to be noticeably affected, and detailed testing is normally unnecessary. Above 25°, a proper VSC check is the next step.
If the checker flags a problem
An angle over 45° does not automatically mean refusal — and under 45° does not guarantee approval. What decides marginal cases is the actual daylight arithmetic: the Vertical Sky Component and the 0.8-times rule. Run the numbers in our VSC and APSH calculator, and see every target in the BRE 2022 tables. For gardens rather than windows, the question becomes overshadowing — try the 21 March shadow calculator.
Honest limits
This tool models one window and one obstruction on flat ground. Real assessments handle multiple windows, room uses, orientation and the neighbour's own massing — which is why councils ask for a professional daylight and sunlight report when objections land.
How Fortress Associates can help
Whether you are the applicant defending a scheme or the neighbour objecting to one, our BRE daylight and sunlight reports test VSC, NSL, APSH and BS EN 17037 compliance to BR 209 (2022) — typically delivered in 4–5 days with no advance payment. All our free tools are at /free-tools; contact us for a quote.
Sources & further reading
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