What Size Extension Can I Build Without Permission?

Adding space to your home can be a great way to improve your living environment without the stress of moving. But before you start, it’s crucial to understand what you can legally build without formal planning permission. In the UK, certain extensions fall under what's called Permitted Development Rights—meaning you don’t need to go through the full planning application process.

What Are the Rules?

Permitted Development Rights apply only to houses, not flats or maisonettes. Your property must also not be in a designated area like a conservation zone or national park.

Here are the general rules:

• No more than 50% of the land around the original house can be covered by extensions or other buildings.

• The extension must not be higher than the existing roof.

• Materials must be similar in appearance to the existing house.

• Extensions must not include verandas, balconies or raised platforms without permission.

If your home has had previous extensions or alterations, those may reduce how much more you can add under Permitted Development.

Single Storey Extensions       

For a single-storey rear extension, these are the usual limits under Permitted Development:

• Maximum depth of 3 metres for an attached house.

• Maximum depth of 4 metres for a detached house.

• Maximum height of 4 metres.

• If the extension is within 2 metres of a boundary, the eaves must not exceed 3 metres.

Under the larger home extension scheme (available until 30 May 2025), you may be able to extend:

• Up to 6 metres for a semi or terraced house.

• Up to 8 metres for a detached house.

But for these larger extensions, you must go through a prior approval process with your local authority, even if it’s not full planning permission.

Two-Storey Extensions

Two-storey extensions are more tightly restricted:

• Must not extend beyond the rear wall of the original house by more than 3 metres.

• Must be at least 7 metres away from the rear boundary.

• The roof pitch must match the existing house as far as possible.

• Side-facing windows must be obscure-glazed and non-opening below 1.7m from the floor.

Also, the extension cannot be forward of the principal elevation or front of the house under Permitted Development—so you’d need full planning for any front-facing two-storey build.

How High Can I Build an Extension Without Planning Permission?

The height restrictions depend on how close the extension is to your boundary:

• For single-storey, max overall height is 4 metres.

• If within 2 metres of a boundary, the eaves must not be over 3 metres.

• For two-storey extensions, the ridge height must not exceed the existing roof of the house.

In all cases, height is measured from the natural ground level.