Do You Need Planning Permission for a House Extension?

Most single-storey rear extensions in England can be built under permitted development, meaning no planning application is needed, but the limits are specific rather than a rule of thumb. Getting the measurements wrong before you plan a layout around them is one of the more common (and avoidable) causes of a retrospective planning application after work has already started.

Here's how to work through it in order, from the questions that decide which rules apply to the checks that catch the exceptions people miss.

1

Check whether it's single-storey or two-storey

Permitted development limits are different for single-storey and two-storey extensions, so this decides which set of rules applies before anything else.

2

Measure the projection beyond the original rear wall

For a single-storey rear extension under permitted development, a detached house can extend up to 4 metres beyond the original rear wall; a semi-detached or terraced house up to 3 metres. “Original” means the house as first built, or as it stood on 1 July 1948, not as it's been extended since.

3

Check the height limits

A single-storey rear extension can't exceed 4 metres in overall height. If it's within 2 metres of a boundary, the eaves height is capped at 3 metres.

4

Check side extensions separately

Side extensions must be single-storey, no more than 4 metres high, and no wider than half the width of the original house.

5

Add up total garden coverage

Extensions and outbuildings together can't cover more than 50% of the land around the original house, once you count anything already built.

6

Rule out anything that removes permitted development rights

Listed buildings, homes in a conservation area, an Article 4 direction from the council, or a new-build with planning conditions restricting further work can all remove permitted development rights entirely, meaning a full application is needed regardless of size.

7

Consider the larger-extension route if you're over the limit

A single-storey rear extension up to 8 metres (detached) or 6 metres (semi-detached/terraced) can sometimes still avoid a full planning application through the “larger home extension” prior approval route, which notifies neighbours through the council rather than requiring a standard application.

None of this replaces an actual planning check against your specific property, but it's enough to know roughly where you stand before you commission drawings or a quote. We check every extension and renovation project against these rules before quoting, and handle planning and building control liaison directly where the project needs it, rather than leaving that to you to sort out separately.