Standard permitted development size limits
Under Class A of the GPDO (England), single-storey rear extensions are permitted development without any planning application when: the rear projection does not exceed 4m on a detached house, or 3m on any other house (semi-detached, terraced, end-of-terrace); the eaves height does not exceed 3m within 2m of a boundary; the overall height does not exceed the original house roof height; and the extension does not extend beyond the existing side elevation (except as a limited side extension). Side extensions under PD must be single storey, no taller than 4m at the highest point, and must not exceed half the width of the original house. Extensions on properties that are within a conservation area, on listed buildings, or subject to Article 4 Directions may have different or no PD rights.
The Larger Home Extension scheme — up to 8m or 6m
The Larger Home Extension (LHE) scheme (prior approval under Part 1, Class A of the GPDO) allows single-storey rear extensions beyond the standard PD limits — up to 8m deep on detached houses, and up to 6m on any other house. To use the LHE scheme: you must apply to the local authority for prior approval (a lightweight application, not full planning); the authority notifies neighbours, who have 21 days to object; the authority assesses whether the extension would have an unacceptable impact on the amenity of any adjoining premises; if impact is acceptable (or if no objections are received), prior approval is granted. The LHE route takes 6–10 weeks and costs approximately £120 in application fees. Refusal rates are low because the test is narrow (amenity impact only), but conservation area and Article 4 restrictions still apply.
