Do I need planning permission for a house extension?

In most cases a single-storey rear extension up to six metres from a detached house, or four metres from a semi or terrace, falls under permitted development and does not need planning permission. You still need building regulations approval and you should apply for a Lawful Development Certificate from your council to have proof in writing. If you are in a conservation area, listed building, or your property has had its permitted development rights removed, you will need a full planning application regardless of size.

Two-storey extensions almost always require planning permission. The rules around proximity to boundaries, ridge heights, and rear wall projections are strict and your council will want drawings from an architect or architectural technician. I always recommend clients speak to their local planning department before committing to a design. A pre-application enquiry costs around £200 to £400 and gives you informal feedback before you spend thousands on full drawings. It saves a lot of heartache if there is an issue.

Side extensions are another area where people get caught out. If your side extension is more than half the width of the original house, you need planning. Even smaller side extensions require planning if you are in a conservation area. I also tell clients to check whether they need a party wall agreement if the extension is near a neighbour's boundary. Getting that sorted early avoids delays once the build starts.