Fact Sheet 1: 25 and 45 Degree Rules of Thumb

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This fact sheet uses simple diagrams to help explain the correct application of the 25 degree rule and 45 degree rule of thumb planning tests.

25 degree test – applied where the window is opposite the development or extension.

45 degree test – applied where the development or extension is perpendicular to the window. This test is often used to assess rear extensions.

The tests described in this fact sheet originate from the Building Research Establishment (BRE) document ‘Site Layout Planning for Daylight and Sunlight: A guide to good practice’. The rule of thumb tests are used to determine whether or not more detailed daylight and sunlight calculations are required.

This fact sheets discusses the impact on neighbouring properties; as well as light levels within proposed new developments themselves.

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    Fact Sheet 1: 25 and 45 Degree Rules of Thumb
    CPD

    Other Fact Sheets you may be interested in:

    Fact Sheet 2: BRE ‘Site Layout Planning for Daylight and Sunlight’
    Fact Sheet 3: A Guide to Legal Rights of Light
    Fact Sheet 4: Light Obstruction Notices
    Fact Sheet 5: Important Rights of Light Cases
    Fact Sheet 6: Rights of Light Risk Mitigation Tips
    Fact Sheet 7: Advice for Neighbours

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