Rear Extensions: Permitted Development and the Prior Approval Route

The standard permitted development depths

For a house — permitted development does not apply to flats — a single-storey rear extension can be built without planning permission within set limits. The depth is measured from the rear wall of the "original house":

• Up to 3 metres deep for a terraced or semi-detached house, or 4 metres for a detached house
• No more than 4 metres high overall
• No more than 3 metres at the eaves where the extension is within 2 metres of a boundary
• No more than half the land around the original house covered by extensions and outbuildings combined
• Materials of similar appearance to the existing house

The larger extension "prior approval" route

A deeper single-storey rear extension — up to 6 metres for a terraced or semi-detached house, or 8 metres for a detached house — can be possible under the larger home extension scheme. This is not automatic: it requires a prior approval application under the neighbour consultation scheme. You notify the council with a description and drawings; it consults the adjoining neighbours; and if any object, the council decides whether the extension would have an unacceptable impact on their amenity before it can proceed.

Crucially, this larger allowance does not apply on Article 2(3) designated land — which includes conservation areas — or on a Site of Special Scientific Interest. So in much of prime London the deeper right is unavailable, and a full planning application is the route for anything beyond the standard depths.

Where conservation areas and other limits bite

Even the standard permitted development rights are narrower in conservation areas and can be removed entirely by an Article 4 direction. Two-storey rear extensions, side extensions and anything affecting the roof are more restricted, and cladding materials are controlled on designated land. Because the position turns on the property’s designations and its planning history, it is worth confirming before design rather than assuming the right applies.

Getting it right

Where an extension is permitted development, a lawful development certificate confirms in writing that it is lawful — valuable for you and for a future buyer’s solicitor. Where it is not, a householder planning application is the route. Either way, the extension will need building regulations approval, and the Party Wall Act usually applies where it sits on or near a boundary. We assess which route your extension takes and prepare the right drawings for it.