The 45-Degree Rule for Extensions in London
The 45-degree rule is one of the most common reasons extensions get refused planning permission in London. It is not law, but almost every council uses it to assess whether your extension will block a neighbour's light. Here is exactly how it works, how it is measured, and what you can do if your design breaches it.
Quick Answer
The 45-degree rule is a planning guideline used by London councils to protect neighbours' daylight. A 45-degree line is drawn from the centre of your neighbour's nearest habitable-room window. If your extension crosses that line, the council may refuse permission on grounds of loss of light. It does not apply to permitted development — only to schemes needing full planning permission.
Full planning
Applies to
Neighbour's window
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Guideline, not law
Legal status
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Free Property CheckWhat Is the 45-Degree Rule?
The 45-degree rule is a planning guideline used by local authorities to assess whether a proposed extension will cause unacceptable loss of daylight to a neighbouring property. It originates from the BRE (Building Research Establishment) guide “Site Layout Planning for Daylight and Sunlight,” which is the standard reference document for daylight assessment in England and Wales.
The rule is not enshrined in statute. It is a rule of thumb. But in practice, most London boroughs treat it as a starting point when assessing any extension that requires planning permission. If your extension breaches the 45-degree line, you will likely receive an objection from the planning officer. If it complies, the daylight question is usually considered resolved.
The purpose is straightforward: to prevent extensions from creating a wall of building that cuts off daylight to a neighbour's habitable rooms. Kitchens, living rooms, bedrooms, and studies all count. Bathrooms, utility rooms, hallways, and garages do not.
How the 45-Degree Rule Is Measured
The rule is assessed in two planes: plan view (looking down from above) and elevation view (looking from the side). Both must pass for the extension to comply.
Plan test (horizontal plane)
From a bird's-eye view, draw a 45-degree line from the centre of the nearest ground-floor habitable-room window of the neighbouring property. The line extends at 45 degrees towards your proposed extension. If the extension wall sits entirely behind this line (i.e. does not cross it), the plan test passes.
Elevation test (vertical plane)
From a side-on view, draw a 45-degree line upward from the midpoint of the neighbour's window sill. If the top of your extension sits below this line, the elevation test passes. This is primarily relevant for two-storey extensions or where the extension is close to and directly beside the neighbour's window.
Which window is used?
The nearest habitable-room window on the neighbouring property that faces or is adjacent to your extension. Most councils measure from the centre of the window. Some measure from the nearest edge of the windowpane to your extension. Check your council's specific guidance, as this varies.
In simple terms: imagine your neighbour standing at their kitchen window and looking towards your extension at 45 degrees. If they can see sky above the top of your extension at that angle, you pass. If your extension blocks that view, you fail.
How London Boroughs Apply the Rule
Every London borough references the 45-degree rule in some form, but application varies. The BRE guidance itself says the rule should be interpreted flexibly. In practice, some boroughs are stricter than others.
Stricter boroughs
Boroughs like Camden, Islington, and Kensington & Chelsea tend to apply the rule more rigidly, particularly in conservation areas and on densely built streets. Even a minor breach may trigger a refusal unless you submit a full daylight and sunlight assessment demonstrating acceptable light levels.
More flexible boroughs
Outer London boroughs such as Croydon, Bromley, and Barnet may be more flexible, especially in suburban areas where properties are spaced further apart. A minor breach in context (e.g. the neighbour already has limited light from that direction due to an existing structure) may be considered acceptable.
Measurement variations
Some councils draw the 45-degree line from the centre of the nearest window. Others draw it from the nearest edge of the window closest to your extension, which is more restrictive. Always check your borough's supplementary planning document (SPD) for the precise methodology before finalising your design.
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How the Rule Affects Different Extension Types
Rear extensions
Rear extensions on semi-detached and terraced houses are where the 45-degree rule matters most. The line is drawn from the neighbour's rear-facing habitable window (usually the kitchen or living room). A single-storey rear extension of 3–4m depth on a semi-detached house will usually pass the plan test. Beyond 4m, you start to risk breaching the 45-degree line depending on how close the windows are to the boundary.
For terraced houses, the same principle applies on both sides. A deep rear extension may breach the line on one or both neighbours, which makes it significantly harder to get approval.
Side extensions
Side extensions on semi-detached houses often trigger the 45-degree rule because they extend the building towards the neighbour's front-facing habitable windows. The line is drawn from the neighbour's nearest front or side window. Two-storey side extensions are particularly sensitive — the added height means the elevation test frequently fails.
Two-storey extensions
The 45-degree rule is far more restrictive for two-storey extensions. The additional height means both the plan and elevation tests become harder to satisfy. Many London councils will not approve a two-storey rear extension that projects more than 3m on a semi-detached house specifically because of the 45-degree rule. On terraced houses, two-storey rears are often limited to 2–2.5m depth.
| Extension type | Typical depth before breach | Risk level |
|---|---|---|
| Single-storey rear (semi) | 3–4m | Low |
| Single-storey rear (terraced) | 3–4m | Medium (both sides) |
| Two-storey rear (semi) | 2.5–3m | High |
| Two-storey rear (terraced) | 2–2.5m | High |
| Two-storey side (semi) | Varies | High |
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Check now — it's free →What to Do If Your Extension Breaches the Rule
A breach does not automatically mean a refusal. The 45-degree rule is a starting point, not a pass-or-fail threshold. Here are your options.
- Commission a daylight and sunlight assessment. A specialist consultant uses 3D modelling to calculate the actual daylight impact on your neighbour's windows using the BRE methodology (Vertical Sky Component, Daylight Distribution, Annual Probable Sunlight Hours). If the results show acceptable daylight levels despite the breach, you have strong grounds for approval. Cost: £1,500–£3,500 inc VAT.
- Redesign to reduce the breach. Your architect can lower the ridge height, step the extension back at first-floor level, or reduce the depth. Even pulling back 300–500mm can be enough to bring the extension within the 45-degree line.
- Argue existing context. If the neighbour's window already receives limited light due to an existing structure, fence, or tree, the actual impact of your extension may be minimal. Planning officers can take this into account.
- Joint planning with your neighbour. If your neighbour is also planning an extension, you can both apply at the same time. The council assesses both proposals together, and what would be a breach for one property alone may be acceptable when both are considered.
- Appeal a refusal. If your application is refused on daylight grounds and your daylight assessment shows acceptable levels, you can appeal to the Planning Inspectorate. Inspectors weigh the BRE assessment results more heavily than the simple 45-degree test.
When the 45-Degree Rule Does Not Apply
The rule is only relevant when you need full planning permission. There are several scenarios where it does not apply at all.
- Permitted development. If your extension falls within permitted development limits (3m depth for terraced/semi, 4m for detached, single storey), you do not need planning permission and the 45-degree rule is irrelevant. However, building something that severely blocks a neighbour's light could still create issues under the Right to Light.
- Prior approval (Neighbour Consultation Scheme). The prior approval process for larger single-storey rear extensions (up to 6m terraced/semi, 8m detached) does not assess daylight using the 45-degree rule. The council can only consider impact on amenity of adjoining neighbours, but this is a narrower test.
- Non-habitable room windows. The rule only applies to windows serving habitable rooms. Bathrooms, utility rooms, hallways, and garages are excluded. If the only nearby window is a bathroom window, the 45-degree rule does not apply to that window.
- No neighbouring windows on the affected side. If the neighbouring property has a blank wall facing your extension (no windows at all), the rule has nothing to assess. This is common with flank walls on semi-detached houses.
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The 45-Degree Rule vs the 25-Degree Rule
The BRE guide contains two separate angular tests. They are often confused but serve different purposes.
45-degree rule
- Used when the extension is beside or perpendicular to the neighbour's window
- Drawn at 45 degrees in both plan and elevation
- Most relevant for rear and side extensions on adjoining properties
- The test most commonly applied in London residential planning
25-degree rule
- Used when the extension is directly opposite the neighbour's window (facing it)
- Drawn at 25 degrees in the vertical plane from the centre of the window
- More relevant for developments opposite a property, e.g. across a road or garden
- Less commonly triggered by typical residential extensions
If an extension passes both the 45-degree and 25-degree tests, it is generally considered acceptable from a daylight perspective. If it fails either test, the council may request a full daylight and sunlight assessment.
How the Rule Affects Extension Size and Cost
The 45-degree rule is one of the biggest practical constraints on extension depth in London, particularly for two-storey designs. Here is how it typically limits what you can build.
Real impact on project size
The decision between complying with the rule and commissioning an assessment to justify a breach comes down to cost. If the extra floor space is worth more than the assessment fee, it is usually worth testing the breach.
Common Mistakes Homeowners Make
Assuming permitted development means no light issues
The 45-degree rule does not apply to permitted development, but the Right to Light does. If your PD extension substantially blocks a neighbour's light that they have enjoyed for 20+ years, they can still take legal action. Just because you do not need planning permission does not mean you can ignore the impact on neighbours.
Measuring from the wrong window
The 45-degree line is drawn from the nearest habitable-room window, not any window. Homeowners sometimes draw the line from a distant window and conclude their extension complies, only for the planning officer to measure from the closer kitchen or living room window and refuse the application.
Not checking both plan and elevation
Some homeowners only check the bird's-eye view and miss the elevation test. A single-storey extension may pass in plan but if the roof height is excessive or the extension is very close to the window, it can still fail the elevation test. Both must be assessed.
Ignoring upper-floor windows
While the ground-floor window is usually the primary concern, two-storey extensions can also affect first-floor windows. If a first-floor bedroom window is the nearest habitable window, the 45-degree line is drawn from that window instead, which can be even more restrictive.
Not requesting pre-application advice
For £200–£600, most London councils offer a pre-application service where a planning officer reviews your proposal before you submit. This is the fastest way to find out if the 45-degree rule is going to be a problem. Skipping this and submitting a full application that breaches the rule wastes the £528 application fee and 8–10 weeks.
Frequently Asked Questions
What is the 45-degree rule for extensions?
The 45-degree rule is a planning guideline used by councils to assess whether an extension will unacceptably reduce daylight to a neighbouring property. A 45-degree line is drawn from the centre of the nearest habitable-room window on the neighbouring property. If your extension crosses that line, the council may refuse planning permission on grounds of loss of light.
Does the 45-degree rule apply to permitted development?
No. The 45-degree rule only applies when you need full planning permission. Extensions built under permitted development rights or the prior approval scheme are not assessed against the 45-degree rule. However, the Right to Light (a separate legal concept) still applies to permitted development extensions.
How is the 45-degree rule measured for a semi-detached house?
The 45-degree line is drawn from the centre of the nearest habitable-room window on your neighbour's property (usually the rear kitchen or living room window). It is drawn at 45 degrees in both plan view (bird's eye) and elevation view (side-on). If your extension sits entirely behind this line in both views, it complies.
Can I still build if my extension breaches the 45-degree rule?
Yes, it is possible. A breach does not guarantee a refusal. You can commission a daylight and sunlight assessment (£1,500–£3,500 inc VAT) to demonstrate that actual light levels remain acceptable. You can also redesign to reduce the breach, argue that existing context limits the impact, or appeal a refusal to the Planning Inspectorate.
What is the difference between the 45-degree rule and the 25-degree rule?
The 45-degree rule applies when an extension is beside or perpendicular to a neighbour's window (typical for rear and side extensions). The 25-degree rule applies when a development is directly opposite a window, drawn at 25 degrees in the vertical plane from the window sill. Both come from the BRE guide on daylight and sunlight.
Is the 45-degree rule a legal requirement?
No. The 45-degree rule is a guideline from the BRE guide, not a legal requirement. However, it is widely used by planning authorities across England and Wales as a standard test for assessing daylight impact. Councils can apply it flexibly and can approve extensions that breach it if other evidence shows acceptable light levels.
How much does a daylight and sunlight assessment cost?
A daylight and sunlight assessment for a residential extension in London typically costs £1,500–£3,500 inc VAT. The assessment uses 3D modelling to calculate Vertical Sky Component (VSC), Daylight Distribution, and Annual Probable Sunlight Hours for affected windows. It is the standard way to justify a breach of the 45-degree rule.
Does the 45-degree rule apply to bathroom and utility room windows?
No. The rule only applies to windows of habitable rooms: kitchens, living rooms, bedrooms, dining rooms, and studies. Bathroom windows, utility room windows, hallway windows, and garage windows are excluded from the assessment.
Summary
The 45-degree rule is not law, but it is the single most used daylight test in London residential planning. If your extension needs full planning permission, you need to know where it stands relative to the 45-degree line before you submit.
For single-storey rear extensions on semi-detached houses, the rule is usually manageable within 3–4m depth. For two-storey extensions or deeper single-storey schemes, check the line early in the design process. If you breach it, a daylight and sunlight assessment is your best route to getting approval.
Start with pre-application advice from your council. It costs a fraction of a full application and tells you exactly where you stand.
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