Do I Need Planning Permission for an Extension?

A single-storey rear extension falls within permitted development if it projects no more than 3 metres from the original rear wall for a terraced or semi-detached house, or 4 metres for a detached house. Within those limits there is no application, no neighbour consultation and no fee — you build.

Two conditions catch people out. The overall height cannot exceed 4 metres. And where the extension comes within 2 metres of a boundary, the eaves height cannot exceed 3 metres. That second rule affects almost every terraced side return, because a side return by definition runs along a boundary.

"Original" also means the house as it stood on 1 July 1948, or as first built if later — not as you bought it. If a previous owner already extended, that extension counts against your allowance.

Under the Larger Home Extension scheme a single-storey rear extension can go up to 6 metres on a terraced or semi-detached house, or 8 metres on a detached house. This is still permitted development, but it is conditional: you must apply to the council for prior approval before starting.

The council notifies your adjoining neighbours, who have 21 days to object. If nobody objects, the council signs it off. If someone does object, the council decides whether the impact on neighbouring amenity is acceptable. It is not a planning application — the council cannot refuse on design grounds — but it is a real process with a real timescale, and it needs to be started well before you want to be on site.

The rules tighten considerably. A two-storey rear extension may not project more than 3 metres from the original rear wall, and must be at least 7 metres from the rear boundary. In practice, that second condition rules it out on most London terraced plots, where gardens are shorter than the rule requires.

Permitted development rights belong to houses. They do not apply to flats or maisonettes — any alteration there needs planning permission and, separately, the freeholder’s consent under the lease.

Listed buildings need listed building consent for almost any alteration, inside or out. In conservation areas the rights are reduced rather than removed, and side extensions in particular are usually restricted.

An Article 4 direction can remove permitted development rights from a specific area entirely. Local authorities use these to protect the character of particular streets, and they are not always obvious — the only reliable way to know is to check the specific address with the local planning authority.

Even where planning permission is not needed, Building Regulations approval almost always is. They are two separate systems: planning governs what a building looks like and how it affects its surroundings; Building Regulations govern whether it is structurally sound, insulated, ventilated and safe. Removing a load-bearing wall requires a structural calculation regardless of planning.

Where an extension is built on or near a shared boundary, the Party Wall etc. Act 1996 also applies and notice must be served on the affected neighbours. That is a separate legal process with its own notice periods — see the party wall guide.