Rear extension planning permission in London — rules, limits & exceptions

The honest answer is: it depends on three things you can check in minutes — what you want to build, whether your house still has permitted development rights, and what designations sit over your street. Get those three right and the planning route for a London rear extension is rarely a surprise.

The short answer

If you own a house (not a flat), it isn't listed, and your street has no conservation-area or Article 4 designation, a single-storey rear extension within the permitted development limits needs no planning application at all. Outside those conditions — which, in inner London, is often — you need a full householder application. Most refusals we see were avoidable: the project was designed before anyone checked which regime applied.

Permitted development: the exact limits

Attached house (terraced / semi)	3m beyond the original rear wall
Detached house	4m beyond the original rear wall
Larger home extension (prior approval)	6m attached / 8m detached
Maximum height	4m (3m eaves within 2m of a boundary)
Materials	Similar appearance to the existing house
Garden coverage (all extensions + outbuildings)	No more than 50% of the original curtilage

GPDO Schedule 2, Part 1, Class A — single-storey rear extensions

'Original' means the house as first built (or as it stood on 1 July 1948). If a previous owner already extended, that allowance may be spent — the planning history matters as much as the tape measure.

When you definitely need full permission

Your home is a flat or maisonette — flats have no permitted development rights at all.
The house is listed (you'll likely need listed building consent as well).
An Article 4 direction covers your street and withdraws Class A rights.
The extension is two storeys in a conservation area, or exceeds any Class A limit.
A previous extension already used up the original allowance.
You want to go wider than the original house or wrap around the side return in a conservation area.

What decides a full application in London

Three things dominate inner-London rear-extension decisions: your neighbour's daylight (the 45-degree test from their nearest habitable window), materials (matching brick and roof finish in conservation areas), and precedent — what's already been approved on your street. Officers lean on consistency; an application that cites three recent consents at the same depth on the same terrace is hard to refuse.

Check your address before you design: the Siteline area report tests your exact coordinates against the official conservation-area and Article 4 boundaries and shows recent sold prices for context — free, cited to source.

Check an address

Do I need planning permission for a 3m rear extension?

On a house with intact permitted development rights, no — 3m single-storey is within Class A even in most conservation areas. You do need building regulations approval, and a Lawful Development Certificate is worth getting for resale. Flats, listed buildings and Article 4 streets are the exceptions.

What is prior approval and is it quicker?

The 'larger home extension' route (up to 6m attached / 8m detached) requires notifying the council, which consults your adjoining neighbours for 21 days. No objections and the council confirms within 42 days. It's faster and cheaper than a full application — but it isn't available in conservation areas.

Can my neighbour stop my rear extension?

Under prior approval, an adjoining neighbour's objection triggers a council assessment of the impact on their amenity — it can result in refusal. Under a full application, objections are weighed but don't decide alone. Separately, the Party Wall Act gives neighbours procedural rights over the construction itself, not the permission.

How long does planning permission take for an extension?

The statutory target for householder applications is eight weeks from validation. London conservation-area cases with design negotiation often run longer. Prior approval, where available, is 42 days.

Do you need planning permission for a rear extension in London?