The 45-degree rule is a common planning guideline used to ensure that new extensions do not significantly reduce the amount of natural light received by neighbouring properties. It typically involves drawing a 45-degree line from the centre or quarter point of the closest window of a neighbouring property. If the proposed extension crosses this line, it may be deemed to have an unacceptable impact on light and amenity, risking refusal of planning permission. This rule is generally more pertinent to second-storey structures than single-storey extensions, though it can still apply to the latter depending on local regulations. Even with planning permission, if neighbours can demonstrate a direct loss of light, their rights can lead to legal action. Thorough planning usually mitigates such issues, which are rare and typically associated with large property developments.