45-Degree Rule Checker

Check in about two minutes whether your extension is likely to pass the 45-degree test councils use to protect a neighbour's daylight.

• Free to use
• Headline result with no sign-up
• Based on published council 45-degree guidance
• Rules last checked 12th November 2026

45-degree rule checker: will your extension pass the 45-degree test?

The 45-degree rule is a rule of thumb many councils use to judge whether an extension would take too much daylight from a neighbour's window. A line is drawn at 45 degrees from the neighbour's nearest main window; if your extension crosses that line, the council may think it harms their light and outlook. This checker works out where the line falls on plan and on elevation, and tells you whether your design clears it under the most common council methods.

How to use this calculator

1. Tell us which room your neighbour's nearest window lights, and whether it is a standard window or a full-height glazed door.
2. Measure how far the window is from the side wall of your planned extension, and how wide it is.
3. Enter how far your extension projects past the neighbour's window wall and how high it is at the far end, then press See my result.
4. Your headline result, with the maximum depth that clears the line, is free with no sign-up. Add your details to unlock the full working, how each council method treats your case and a free planning report.

The rules explained

The 45-degree rule is not set by national law. It is local guidance that each council writes for itself, so the details vary. The common ground and the main differences are:

• Which window: the nearest window that is the main source of light to a habitable room. Warwick District Council counts dining rooms, kitchens, lounges, studies and bedrooms, and excludes WCs, bathrooms, utility rooms, landings and hallways. Birmingham City Council also counts conservatories. Hart District Council treats patio doors and glazed French doors as windows.
• Plan test: on a plan drawing, a line is drawn at 45 degrees from the midpoint of the window. The extension should not cross it. Wokingham Borough Council draws it from the midpoint of the principal ground floor window to the corner of the development.
• Quarter point for two storeys: Birmingham measures single storey extensions from the midpoint of the nearest ground floor window, but two storey extensions from the quarter point. Warwick uses the quarter point for first floor extensions, defines it as a quarter of the window's width in from the edge nearest the extension, and treats anything over 4 m high as first floor.
• Elevation test: some councils also draw a 45-degree line upwards on an elevation drawing. North Norfolk District Council judges it against the centre of the window, or a point 2 m above ground for full-height windows, and says light is likely to be significantly blocked if the window centre falls within the 45-degree lines on both plan and elevation. Where it falls within the lines on plan and/or elevation, a more detailed daylight and sunlight assessment is needed.
• Either test: Gloucester City Council says an extension that breaks the 45-degree line in either test would normally be unacceptable, and draws its elevation line from 1 m above ground floor level. It applies the rule to single and two storey extensions.
• Single storey: councils differ. Wokingham's note applies the test to development of two or more storeys, while Birmingham and Gloucester apply it to single storey extensions too.
• Not an automatic refusal: Wokingham says failing the test does not mean automatic refusal, and it will also consider the distance to the neighbouring windows, the overall scale and massing, and any detailed daylight and sunlight assessment.

Councils in Wales, Scotland and Northern Ireland publish their own guidance and may use different angles or methods, so always check the design guide for your area. The 45-degree test is separate from permitted development, which has its own national limits; try our permitted development rear extension calculator for those.

For more on extensions next to a neighbour, read our guides to the side return extension, the householder planning application, prior approval for a larger home extension and planning drawings. Building in the garden instead? Use our outbuilding permitted development checker.

How the checker works it out

• Plan test: a 45-degree line moves out one metre for every metre across. So the extension clears the line if its depth past the window wall is no more than the distance from its side wall to the window's reference point (the midpoint, or the quarter point for the two storey check).
• Elevation test: the line rises one metre for every metre out from the wall. So the extension clears it if its height at the far end is no more than the window's reference height plus the depth.
• An extension exactly on the line is treated as not crossing it.

Assumptions and limits

• It assumes level ground and that the rear walls of both houses are in line. Changes in level, staggered walls, angled walls and existing boundary walls all change the result, and councils can take them into account.
• It checks one window at a time. If you have neighbours on both sides, check each one.
• It uses the common methods described above, with the elevation line taken from the window centre. Some councils, such as Gloucester, start the elevation line at 1 m above floor level, which is stricter. Your council's own guide may differ, so check it before you design.
• It is a quick daylight test, not a full daylight and sunlight assessment, and it does not cover privacy, overlooking or the overbearing effect of a design.

Frequently asked questions

Last checked 12th November 2026. This tool gives a guide only, based on the rules published at the date above. It is not professional advice, and your council has the final say.