Is the 45-degree rule actually a rule?
No. It appears in no Act, no regulation and not in the permitted development order. It is a daylight test councils publish in their own supplementary planning documents, and each borough draws it slightly differently — which is why the same extension passes in one borough and fails next door.
| Test | Where it comes from | Status |
|---|
| 45-degree rule | borough supplementary planning documents | policy guidance, not law |
| 25-degree rule for the vertical section | borough SPDs, drawn from BRE practice | policy guidance |
| Vertical sky component, 27% | BRE BR 209, 3rd edition (2022) | advisory good practice |
| Annual probable sunlight hours, 25% and 5% | BRE BR 209 | advisory good practice |
| Permitted development depth, 3m or 6m | GPDO 2015, Schedule 2, Part 1, Class A | law |
| Right of light, 20 years | Prescription Act 1832 | law |
Sources: BRE BR 209, Site Layout Planning for Daylight and Sunlight, 3rd edition (2022); the Town and Country Planning (General Permitted Development) (England) Order 2015; the Prescription Act 1832. Read at 1 September 2026. Because the 45-degree test is local policy, the only reliable source for the version that applies to your site is the council’s own adopted design guidance.
Where does the rule come from?
It derives from BRE daylight guidance and is adopted, with local variations, in borough supplementary planning documents — it is not in the GPDO or the Planning Acts.
Because it is local guidance, the detail differs: some boroughs apply 45 degrees in plan and 25 degrees in section, some use both at 45, some measure from the window's midpoint and some from its nearest edge. Read your own borough's householder SPD before you design to a number you read online.
What does not differ is the underlying concern: whether your extension takes a neighbour's daylight, outlook or sense of enclosure to an unacceptable degree.
How is the line actually drawn?
In plan
From the midpoint of the neighbour's nearest habitable-room window, a line is drawn at 45 degrees away from the shared boundary. If the extension's footprint crosses it, the scheme is flagged.
In elevation
The same test is applied vertically from the same window: a 45-degree line rising from the window head or midpoint, depending on the borough, should clear the extension's roofline.
A single-storey rear extension frequently passes in plan and fails in elevation because of the roof pitch — which is why the fix is often a change of roof form rather than a change of depth.
Which windows count?
Habitable rooms — living rooms, dining rooms and bedrooms. Kitchens, bathrooms, hallways and utility rooms are usually excluded.
The classification is worth checking before you concede a point. A neighbour objecting about a kitchen window is objecting about a window most boroughs do not protect under this test — though the officer will still weigh general amenity.
Conversely, an unnoticed rear bedroom window on the flank wall can fail a scheme that looked fine from the garden.
