Permitted Development Rear Extension Calculator
In England, a single-storey rear extension can usually go 3 metres beyond the original rear wall of a semi-detached or terraced house, or 4 metres for a detached house, without a planning application. Under the larger home extension scheme that rises to 6 metres or 8 metres, but only after the council confirms prior approval is not needed or grants it. Two-storey rear extensions are limited to 3 metres and must stay at least 7 metres from the rear boundary.
How to use this calculator
- Choose your house type. Semi-detached and terraced houses share the same limits; detached houses get more depth.
- Say whether the extension is single storey or two storeys, and whether the house is in a protected area.
- Enter the depth from the original rear wall to the outside face of the new wall, including any earlier rear extension it joins.
- Enter the height and eaves height, and say whether it comes within 2 metres of a boundary.
- Press "Check my extension". The result shows which route applies and how we worked it out.
The rear extension rules explained
The rules come from Class A of Part 1, Schedule 2 of the Town and Country Planning (General Permitted Development) (England) Order 2015, explained in the government's technical guidance for householders.
Limits measured from the rear wall of the original house.
| Extension | Detached house | Semi-detached or terraced |
|---|
| Single storey, permitted development | Up to 4m deep | Up to 3m deep |
| Single storey, larger home extension (prior approval) | Over 4m, up to 8m | Over 3m, up to 6m |
| Two or more storeys | Up to 3m deep, at least 7m from the rear boundary | Up to 3m deep, at least 7m from the rear boundary |
- Height: a single-storey rear extension can be no more than 4 metres high. No extension can be higher than the highest part of the existing roof.
- Eaves: the eaves cannot be higher than the eaves of the existing house. If any part is within 2 metres of a boundary, the eaves cannot be more than 3 metres high.
- Protected areas: the larger home extension scheme does not apply on article 2(3) land (conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites) or on a Site of Special Scientific Interest. Extensions of more than one storey at the rear are not permitted development on article 2(3) land, and there you also cannot clad the outside in stone, artificial stone, pebble dash, render, timber, plastic or tiles.
- Coverage: extensions and outbuildings together cannot cover more than half of the land around the original house.
- Original house: the house as it stood on 1 July 1948, or as first built if later. Earlier extensions count towards the limits.
- Materials and windows: materials must look similar to the existing house (conservatories aside). Upper-floor side windows must be obscure glazed and non-opening unless the opening part is more than 1.7 metres above the floor.
- Larger home extension process: you notify the council, which tells your adjoining neighbours and gives them at least 21 days to comment. You cannot start until the council confirms prior approval is not needed, grants it, or 42 days pass without a decision. Read more in our guide to prior approval for a larger home extension.
Flats and maisonettes do not have these householder rights (see what is a maisonette), and councils can remove rights with an Article 4 direction or a planning condition. If your project falls outside the limits you will need a householder planning application.
Wales, Scotland and Northern Ireland
This calculator covers England only. Wales, Scotland and Northern Ireland have their own permitted development rules, with different limits and no English larger home extension scheme, so check with your local council there.
Assumptions and limits
- It checks the depth, height, eaves and rear boundary rules for rear extensions only. It does not check side extensions, wrap-around extensions, the 50% coverage rule, materials or windows.
- It assumes your house was not created through a change of use under Part 3 of the Order and was not built under Part 20, as those houses do not have these rights.
- Your measurements must be taken from the original house, not from an earlier extension.
- A result of "permitted development" depends on every other condition being met. A lawful development certificate gives you formal proof.
Frequently asked questions
How far can I extend at the back without planning permission?
For a single-storey rear extension in England, up to 3 metres on a semi-detached or terraced house and 4 metres on a detached house, measured from the original rear wall. With prior approval under the larger home extension scheme, up to 6 metres and 8 metres.
Does an existing conservatory or extension count?
Yes. The limits are measured from the original house, so any earlier rear extension it joins counts towards the depth.
Can I build an 8 metre extension in a conservation area?
Not under permitted development. The larger home extension scheme does not apply in conservation areas and other article 2(3) land, so anything over 4 metres (detached) or 3 metres (other houses) needs a planning application there.
How long does prior approval for a larger home extension take?
The council has 42 days from receiving your notification. If it does not respond in that time you can go ahead, building in line with the details you sent.
Last checked 9th November 2026.
This tool gives a guide only, based on the rules published at the date above. It is not professional advice, and your council has the final say.
