What is the short answer?
In England a single-storey rear extension can usually go 3 metres from the original back wall of an attached house, or 4 metres on a detached house, without a planning application. Wales allows 4 metres for many houses, Northern Ireland 3 or 4 metres, and Scotland sets depth limits only where the extension is within 1 metre of a boundary. Every one of those limits comes with conditions.
England
In England a single-storey rear extension can often be built under permitted development: up to 3 metres beyond the original rear wall of an attached house, or 4 metres on a detached house, no higher than 4 metres, with eaves no higher than 3 metres if any part is within 2 metres of a boundary.
England also has the larger home extension route: a single-storey rear extension of up to 6 metres on an attached house or 8 metres on a detached house, after prior approval from the council. The council notifies your neighbours and has 42 days to decide; the fee is £249 (£310 for applications made from 8 December 2026). The route is not available on designated land such as conservation areas.
Measure from the original house, meaning the house as it was first built or as it stood on 1 July 1948 if older. An extension added since then counts against your allowance, even if a previous owner built it. Extensions and outbuildings together must not cover more than half the land around the original house.
Wales
In Wales a single-storey rear extension can often be permitted development if it projects no more than 4 metres beyond the rear wall and is no more than 4 metres high. Wales has no prior approval route for deeper extensions, so anything bigger needs a householder planning application.
In Wales a rear extension of more than one storey can be permitted development if the ground floor projects no more than 4 metres and the upper floor no more than 3 metres, with at least 10.5 metres to the rear boundary. It is not permitted development in conservation areas, National Parks or National Landscapes, so many double-storey schemes need a householder application.
Scotland
In Scotland a single-storey rear extension can be permitted development under Class 1A: eaves no higher than 3 metres and an overall height of 4 metres, and where any part is within 1 metre of a boundary it can project no more than 3 metres on a terraced house or 4 metres on other houses. The rights do not apply in conservation areas, and a building warrant is still needed before work starts.
In Scotland an extension of more than one storey is only permitted development (Class 1B) if no part is within 10 metres of any boundary and it is no higher than the existing house, so almost every two-storey rear extension needs planning permission and a building warrant.
Northern Ireland
In Northern Ireland the Planning (General Permitted Development) Order (Northern Ireland) 2015 allows a single-storey rear extension of up to 3 metres on an attached house or 4 metres on a detached house, no higher than 4 metres, with eaves no higher than 3 metres within 2 metres of a boundary. Building control approval from the council is still required.
In Northern Ireland a rear extension of more than one storey can be permitted development if it projects no more than 3 metres and stays at least 7 metres from the rear boundary. Two-storey extensions in conservation areas, and most larger schemes, need planning permission from the council.
When do the rules not apply?
Flats and maisonettes have no householder permitted development rights. Councils can remove rights with a planning condition (common on estates built since the 1980s) or an Article 4 direction. Conservation areas, National Parks, National Landscapes and the Broads restrict the rules further, and listed buildings always need listed building consent.
Permitted development is never a pass on Building Regulations: the extension still needs approval for foundations, structure, drainage, fire safety and insulation.
How do you prove an extension was permitted development?
Apply to the council for a lawful development certificate before you build. It is optional but cheap insurance when you sell, and in England it costs half the householder planning fee. Councils should decide within eight weeks.
About this guide
Written by the Rear Extensions design team and checked against GOV.UK, Welsh and Scottish Government and Northern Ireland guidance in November 2026. It is general guidance, not legal advice; we confirm the rules for your house at the survey.