45-Degree Rule Calculator
The 45-degree rule is a planning policy test used by most councils to check whether an extension will cause unacceptable loss of light to neighbouring properties. If you need planning permission for your extension, this is one of the first things a planning officer will check. Enter three measurements to see whether your proposal is likely to pass.
How the 45-degree rule works
Imagine standing at the centre of your neighbour’s nearest habitable room window and looking along their boundary. Now draw an imaginary line at 45 degrees from that window across your garden. If your extension crosses that line, a planning officer is likely to consider it harmful to your neighbour’s daylight.
In practice, the extension depth past the window must not exceed the distance from the window centre to the boundary. If it does, the 45-degree line is breached.
Check your extension
You need two measurements: (1) the distance from your neighbour’s nearest habitable room window to your shared boundary, and (2) how far your extension projects past the point level with that window. You can estimate these from your garden without needing access to the neighbour’s property.
Measure (or estimate) from the centre of their nearest habitable room window to the shared boundary line, at right angles to the boundary.
How far your extension projects beyond the point level with the neighbour’s window. If the neighbour’s window is 2m back from the boundary and your extension projects 5m total, the depth past the window is 3m.
Two-storey extensions are also checked in vertical elevation, which this calculator does not cover.
Important things to know
- Plan view only: This calculator checks the horizontal (plan) view. Two-storey extensions require a separate vertical elevation check using the same 45-degree principle from the window head height.
- Council variation: Not all councils apply the rule identically. Some measure from the corner of the extension, others from the nearest point. Some use 45 degrees, others use 25 degrees for two-storey extensions. Check your council’s Supplementary Planning Document.
- Policy, not law: The 45-degree rule is a planning policy consideration, not a legal requirement. Officers weigh it alongside other factors such as orientation, existing screening, and the nature of the affected room.
- Not relevant to PD: If your extension qualifies as permitted development, the 45-degree rule does not apply. It only matters for full planning applications.
- Both neighbours: Check against all affected neighbouring properties, not just one side. The rule applies to every neighbouring window that could be impacted.