Permitted Development (PD) is a set of national rules that let homeowners carry out certain kinds of building work — including many single-storey rear and side extensions — without submitting a full planning application. It exists precisely so that straightforward, modest extensions don't need to go through the same process as a major development. But "permitted" doesn't mean "unlimited," and it doesn't mean "everyone."
What Permitted Development actually allows
For most houses, a single-storey rear extension can be built under PD provided it stays within set limits on how far it extends, how tall it is, and what it's built from. Go past any of these limits and the project needs a full planning application instead — not because it's forbidden, but because it now needs individual assessment.
| Extension type | Typical PD limit | Beyond this |
|---|---|---|
| Single-storey rear extension (detached/semi) | Up to 4m depth | Full planning application |
| Single-storey rear extension (terraced) | Up to 3m depth | Full planning application |
| Maximum height | Up to 4m eaves and ridge | Full planning application |
| Materials | Must be similar in appearance to the existing house | Full planning application |
Simplified for clarity — exact limits depend on your specific property and local authority. Confirm your own figures before relying on them.
The exceptions that remove PD rights entirely
The rules above assume a standard house with unrestricted PD rights. Several situations remove that right altogether, meaning even a modest extension needs a full application: conservation areas, Grade II (or higher) listed buildings, Article 4 Directions, and houses where a previous planning condition specifically removed PD rights. If any of these apply to you, the size limits above don't matter — the starting point is different.
Long description: start from the existing house, add the proposed rear extension, then ask whether it falls within Permitted Development limits. If yes, a Lawful Development Certificate confirms this formally. If no — because of size, height, materials, or a removed PD right — a full planning application is required instead.
Why this is worth checking before you start
Building outside your PD rights without realising it can create real problems later — from a mortgage lender or buyer's solicitor questioning the work at sale, to the local authority requiring changes after the fact. A Lawful Development Certificate isn't compulsory, but it's the only way to have this confirmed formally and in writing.
"We assumed our extension was within Permitted Development because it looked similar to next door's. It turned out our house had a removed PD right from a previous planning condition nobody had mentioned to us."
Real client experience, resolved before work began — name withheldThat case is exactly why this question is worth asking properly, not assuming — a quick check at the start avoids exactly this kind of surprise.