Do You Need Planning Permission?

Whether your extension needs planning permission comes down to two routes: permitted development and full planning permission. Many single-storey rear extensions fall under permitted development and need no application at all, provided they stay inside a fixed set of limits. This checker asks about your property type and how far the extension will project, then tells you which route you are on.

How the depth limit decides your route

The depth of your extension, measured from the original house rear wall, is the figure that usually settles it. Under permitted development rights, a semi-detached or terraced house can extend to 3m, and a detached house to 4m. Go beyond that and you move into the Larger Home Extension scheme, which allows up to 6m for a semi or terrace and 8m for a detached house through a prior approval process where the council consults your neighbours. Deeper still, and you need a full householder application.

Depth is not the only test. All your extensions and outbuildings together must cover no more than 50% of the curtilage, the land around the original house. Height, eaves, materials, and boundary distances each have their own limits, and every one of the seven permitted development conditions has to be met at the same time.

When permitted development never applies

Some properties have no permitted development rights to begin with. Flats and maisonettes always need planning permission for an extension. A listed building needs both planning permission and listed building consent for any external change. A conservation area restricts side extensions and cladding changes, and an Article 4 direction can strip specific rights street by street. Previous work counts too: the limits are measured against the original house, so additions made by earlier owners eat into your allowance.

A worked example

Take a semi-detached house with a planned 3.5m rear extension. That is past the 3m standard permitted development depth, so it is not automatic PD. It does sit within the 6m Larger Home Extension limit, so the owner can apply for prior approval: the council notifies the neighbours, waits 42 days for objections, and signs off if none are upheld. Now change the house to detached with the same 3.5m depth. It drops back under the 4m standard limit and needs no application at all, subject to the other conditions. Same extension, different answer, decided by property type.

Even when your extension is clearly permitted development, a Lawful Development Certificate from the council gives you legal proof of that for a future sale. And whichever route applies, building control is a separate approval you still have to arrange.