CPD Fact Sheet 1
Will a development or extension affect daylight to neighbouring properties?
45 degree rule and 25 degree rule explained
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A practical guide to how the 25 degree rule and 45 degree rule are used to assess the impact of development on light.
- Understand how the 25° and 45° tests are applied in planning
- Assess whether daylight and sunlight impacts are likely
- See how the tests are used as an initial screening tool by local authorities
- Identify when more detailed daylight and sunlight analysis may be required
Based on BRE guidance referenced in UK planning decisions
Who this
guide is
useful for
Planning a development or extension
Assessing the impact on neighbouring properties
Architects, designers and planning consultants
How the 25 degree rule and 45 degree rule are used
The 25 degree rule and 45 degree rule are commonly used as initial screening tools in the planning process to assess whether a proposed development is likely to affect neighbouring daylight.
They are based on guidance set out in the BRE document Site Layout Planning for Daylight and Sunlight, which is widely referenced by local authorities.
45° rule
(rear extensions affecting neighbouring rear windows)
The 45 degree rule is typically applied where a rear extension may affect the light to a neighbouring property’s rear-facing window.
25° rule
(buildings positioned in front of windows)
The 25 degree rule is generally used where a proposed building is positioned in front of existing windows.
Important limitations
The 25 degree rule and 45 degree rule are only rules of thumb.
They do not provide a full assessment of daylight and sunlight and:
• A scheme can pass these tests and still cause a noticeable impact
• A scheme can fail them and still be acceptable
Where there is uncertainty, a full daylight and sunlight assessment is usually required.
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