How big an extension can be before it needs permission
The measurements that decide whether a rear extension is permitted development, and the prior approval route that doubles them.
Applies to England. Facts last checked 5 Aug 2026. Guides describe England; other UK nations run separate planning systems.
Single storey rear extensions
A single storey rear extension is permitted development up to four metres beyond the rear wall of the original house if it is detached, and three metres for any other house — terraced, semi-detached, end of terrace.
"Original house" means as it stood on 1 July 1948, or as first built if later. A house already extended once may have no allowance left, which is a conversation worth having before the drawings.
The prior approval route, which doubles it
There is a larger home extension route: eight metres for a detached house, six metres for any other, subject to prior approval. The council notifies the neighbours, and if none objects within the period the extension can proceed without full permission.
It is not available on designated land — conservation areas, national parks, areas of outstanding natural beauty — or on a site of special scientific interest. That is the trap: the same extension is a form in one street and a full application in the next.
Heights, and the boundary rule that catches people
A single storey rear extension cannot exceed four metres in height. Where any part comes within two metres of a boundary, the height at the eaves cannot exceed three metres — which is the one most often missed, because it is measured to the boundary rather than to the neighbour’s wall.
A side extension can only be single storey, no more than four metres high, and no more than half the width of the original house.
None of this is the whole answer
Permitted development decides whether an application is needed. It says nothing about Building Regulations, which apply either way, and nothing about the Party Wall Act if you are building near or on a boundary.
It also means the job may never appear in a planning search at all — including this one. A four-metre extension on a detached house leaves no public record unless the owner applies for a certificate of lawfulness.
Sources
General explanation, not legal or planning advice. Anything that turns on the detail of a particular site is worth a free phone call to the council’s planning department. Something here wrong or out of date? Tell us.