Building Shadow Calculator
How long a building’s shadow is at a given sun altitude, how high that shadow reaches on a neighbouring building at a given distance, and whether a neighbour’s window is overshadowed.
A vertical object of height h casts a shadow h ÷ tan(sun altitude) long on flat ground.
How the building shadow calculator works
A vertical object of height h casts a shadow h ÷ tan(sun altitude) long on flat ground. At a neighbour standing d away, the shadow reaches a height of h − d × tan(altitude) on its facade; a window whose sill is above that is in sun. Planning controls typically test the winter solstice at 9 am, noon and 3 pm, when the sun is lowest; enter the altitude for the time and latitude you are assessing.
Formula: shadow length = h ÷ tan α; shadow height on a facade at distance d = h − d tan α
Worked examples
| Inputs | Shadow length on the ground | Note |
|---|---|---|
| 24 m building, 33° sun, neighbour 20 m away | 37 m | 37 m shadow; facade shaded to 11 m — window overshadowed |
| Same building at 15° (London midwinter noon) | 89.6 m | 90 m shadow |
| A 9 m house, 60° sun | 5.2 m | 5.2 m shadow — the neighbour is in sun |
FAQFrequently asked questions
What sun altitude should I test?
Planning codes test the worst case: the winter solstice, usually at 9 am, noon and 3 pm. Noon altitude on that day is 90° − latitude − 23.4°: about 33° at 34° latitude (Los Angeles, Cape Town), 15° at 51° (London, Vancouver). Morning and afternoon are lower and the shadow swings sideways.
Does the direction matter?
Yes — the calculator gives the length along the sun’s direction; a neighbour off that line receives less. Draw the shadow on a plan at the sun’s azimuth for the hour to see who is affected.
How much sun is a neighbour entitled to?
Typical controls ask that living rooms and private open space keep two or three hours of direct sun between 9 am and 3 pm in midwinter; the exact rule is local. The window row tells you whether a given hour is lost.
Can I reduce the shadow?
Setbacks at the upper levels, a lower building on the sunward side, or orientation that puts the long axis east–west. The shadow-to-height ratio shows how the geometry scales — at 15° sun altitude every metre of height adds 3.7 m of shadow.
Where these figures come from
- UN-Habitat — Urban Indicators Guidelines — density, green space and public space benchmarks
- World Health Organization — Urban green spaces — the 9 m² per person minimum
- ITE — Parking Generation Manual — parking demand rates by land use
- Office for National Statistics — Population estimates — the UK's national statistics agency
Last checked: September 2026. Default per-capita rates are mid-range figures from national statistical agencies and utility reports; every one is an input you should replace with the local figure.