Answer in brief
Behind an attached English house you can usually build out 3 m on one storey with no application, and 4 m behind a detached one. Prior approval from the council stretches that to 6 m and 8 m. Going up two storeys, the limit is 3 m and the extension must leave 7 m to the back boundary.
Whether you need to apply depends on permitted development rights. For houses in England they are set out in Schedule 2, Part 1 of the General Permitted Development Order (Class A covers extensions), and for a modest room at the back they are generous: most small kit extensions are built without a planning application at all.
Two groups miss out from the start. Flats and maisonettes have no householder rights to extend, and a listed building needs consent for almost any change.
How far out you can go
The measurement runs from the original rear wall. For a house built before 1 July 1948, original means as it stood on that date. That catches people out: if a previous owner added a 2 m kitchen, a semi has only 1 m of allowance left.
| House | One storey, no application | One storey, with prior approval | Two storeys |
|---|---|---|---|
| Semi-detached or terraced | 3 m | Up to 6 m | 3 m |
| Detached | 4 m | Up to 8 m | 3 m |
Prior approval sits between doing nothing and a full application. You send the council your plans; it notifies the adjoining neighbours, then has 42 days to rule on the effect on them. Silence after 42 days means you may go ahead.
How high, and how close to the fence
Projection is only part of the test. A one-storey back extension may not top 4 m. Where a wall comes within 2 m of the boundary, its eaves stop at 3 m. Nothing may rise above the eaves or ridge of the house itself.
The 2 m clause matters most on semis and terraces, where a full-width room is close to both side boundaries. A flat-roofed kit clears it without effort. A sloping lean-to roof needs its side walls drawing carefully.
Garden coverage and other conditions
Add up every extension and outbuilding: together they may take no more than half of the land around the original house. External materials should look broadly like the existing ones. Verandas, balconies and any raised platform more than 30 cm off the ground fall outside the rules.
Two storeys at the back can still count as permitted development, but only to 3 m, with 7 m kept clear to the rear boundary, a roof pitch that follows the house where it can, and obscured, mostly fixed side windows upstairs.
Places where the answer changes
Protected areas. In conservation areas, National Parks, National Landscapes and World Heritage Sites, the back extension limits are unchanged, but side extensions and external cladding need permission.
Rights taken away. On some estates the council removed permitted development rights when the houses were built, through a condition or an Article 4 direction. A short email to the planning department settles it.
Wales. Back extension limits look much like England’s, but there is no larger-home prior approval route, so 3 m and 4 m are the ceiling.
Scotland. Scottish householders work to separate permitted development classes, and need a building warrant before starting, whatever the planning position.
Get it in writing
When a design sits close to any limit, a lawful development certificate is worth having. It is optional, but it puts the council’s view on paper, and a buyer’s conveyancer is likely to ask for it when you sell.
Planning aside, building regulations approval is always required. Most prefab extension kits for a single room are drawn to sit inside the 3 m allowance for exactly this reason, which is part of why they move from order to finished room so quickly.