The short answer
In England a single-storey rear extension can go 3 m beyond the original rear wall of a semi or terrace, or 4 m on a detached house, without a planning application. Through the prior approval process that rises to 6 m and 8 m. It must be no more than 4 m high, and two-storey extensions are limited to 3 m deep and 7 m from the rear boundary.
On this page
Permitted development is the set of building work a house can have without a planning application, because Parliament has already granted the permission in general terms. For rear extensions in England the rights sit in Class A of the General Permitted Development Order, and the government’s technical guidance explains how councils read them. The rules are the same whether the walls are brick, timber frame or a factory-built module.
The depth limits
The key measurement is how far the extension projects beyond the original rear wall of the house. “Original” means the house as first built, or as it stood on 1 July 1948 if older. Any earlier extension counts against the allowance.
| House type | Without notifying anyone | With prior approval |
|---|---|---|
| Semi-detached or terraced | Up to 3 m | Up to 6 m |
| Detached | Up to 4 m | Up to 8 m |
| Any house, two storeys or more | Up to 3 m, and 7 m from the rear boundary | Not available |
So a 5 m deep kitchen extension on a semi is not permitted development as of right, but it can go ahead under prior approval if the council agrees.
Height, eaves and boundaries
A single-storey rear extension must be no more than 4 m high at any point. If any part sits within 2 m of a boundary, its eaves must be no higher than 3 m. The eaves and ridge must also be no higher than those of the existing house.
Buildings added since the house was built, including outbuildings and earlier extensions, cannot cover more than half the land around the original house. On a small terrace garden that limit can bite before the depth limit does.
Materials and windows
The outside should look similar to the existing house. That does not mean identical, but a timber-clad box on a red-brick semi can fail this test, while brick slips or a render close to the existing finish usually pass. Conservatories are exempt from the materials rule.
For two-storey extensions, upper-floor windows in a side wall must be obscure glazed and fixed shut unless the opening part is more than 1.7 m above the floor. The roof pitch should match the house as far as practical.
Where permitted development does not apply
The rights are restricted or removed in several places:
- Flats and maisonettes have no householder permitted development rights for extensions.
- Conservation areas, national parks, the Broads, National Landscapes and World Heritage Sites (known as article 2(3) land): no cladding in stone, render, timber, plastic or tiles, no side extensions, and rear extensions limited to a single storey.
- Listed buildings need listed building consent for almost any change.
- Article 4 directions let a council remove some rights in a defined area.
- Planning conditions on newer estates sometimes remove permitted development rights. Check the original planning permission for the house.
How prior approval works
For the larger single-storey limits, you send the council a description, a plan and the extension’s depth, height and eaves height, with the fee, currently £249 in many English councils. The council writes to the neighbours who share a boundary, and they can object. The council then considers only the effect on their amenity. You must not start until the council says prior approval is not needed, grants it, or 42 days pass without a decision.
Proving it is lawful
You do not have to apply for anything for an extension within the basic limits, but a lawful development certificate gives you a formal record that it is allowed. Buyers’ solicitors often ask for one.
What this means for a modular extension
Factory-made extensions are often sized to suit these limits: a module 3 m deep fits a semi without any application, and one 4 m deep fits a detached house. The configurator on our modular extension page draws your chosen size against the 3 m, 4 m, 6 m and 8 m lines so you can see the route straight away. Building regulations approval is still needed whichever planning route applies.
If you want a figure for your size, the cost calculator adds the right planning fee for you. When you are ready, use the form below to get prices.