Overshadowing assessments in UK planning hinge on one date: 21 March. BRE BR 209 asks whether at least half of a garden or amenity area can still receive two or more hours of direct sunlight on the spring equinox. This calculator uses real solar geometry to show how long a shadow any building, fence or extension casts that day — at any UK latitude, at any hour.
Calculate the shadow
The calculator needs JavaScript. The two-hour amenity test it supports is in our BRE targets reference.
Why 21 March?
The equinox is the fair test day: the sun's path is exactly halfway between winter gloom and summer height, so a garden that gets decent sun on 21 March will do better for most of the growing season. At noon on the equinox the sun's altitude is simply 90° minus your latitude — about 38.5° in London — so a 6 m high extension throws a shadow of roughly 7.5 m even at midday, and far longer in morning and afternoon.
Reading the result like an assessor
- Shadows point away from the sun — due north at solar noon, swinging from west-of-north in the morning to east-of-north in the afternoon. North-facing gardens behind tall rear extensions are the classic losers.
- The 0.8-times rule applies here too — if the well-sunlit area of a neighbour's garden drops below 0.8 times its former value, BR 209 treats the loss as noticeable.
- Screening vs assessment — this tool shows a single object on flat ground; a formal overshadowing study models the whole 3D scene hour by hour and plots the two-hour contour on 21 March.
How Fortress Associates can help
If a neighbour's garden — or your own scheme's amenity space — is in question, our daylight, sunlight and overshadowing reports include the 21 March two-hour assessment to BR 209 (2022), delivered in 4–5 days with no advance payment. Screen windows first with the 45-degree checker and the VSC calculator, or see all tools at /free-tools. Questions? Get in touch.
Sources & further reading
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