Permitted development rules for rear extensions
Under Class A of the Town and Country Planning (General Permitted Development) Order 2015, you can build a single-storey rear extension without planning permission subject to these limits: maximum 4m projection beyond the original rear wall on a detached house; maximum 3m on a semi-detached or terraced house; maximum eaves height of 3m (4m if within 2m of a boundary); maximum overall roof height not exceeding the original house. Under the Larger Home Extension scheme (prior approval route), limits increase to 8m (detached) and 6m (semi/terrace) — but you must notify neighbours and get prior approval from the local authority. A double-storey rear extension always needs full planning permission.
When you always need planning permission
Full planning is required when: the extension exceeds PD depth limits; the extension is double-storey; the property is in a conservation area (most of the standard PD rights are restricted); the property is a listed building; the property is a flat (no PD rights); an Article 4 direction applies to the area; the extension affects the principal elevation; or the roof of the extension would be higher than the existing roof. London has extensive conservation coverage — large parts of Hackney, Islington, Camden, Kensington & Chelsea, Wandsworth, Hammersmith and Westminster are conservation areas where standard PD rights are removed.
The prior approval process for larger extensions
The Larger Home Extension (prior approval) scheme allows single-storey rear extensions up to 8m (detached) or 6m (semi/terrace) — double the standard PD limits. You must apply to the local authority for prior approval. The council notifies neighbours, who have 21 days to object. If no objections are raised, or if objections are not material in planning terms, approval is granted. If objections are raised, the council assesses the impact on amenity and responds within 42 days. This is a lighter-touch process than full planning — no design quality assessment, no planning policy compliance, just amenity impact — but it does add 6–10 weeks versus the standard PD route.
