Short answer: many single-storey extensions in England can be built under your permitted development rights, which means no planning application is needed — as long as you stay within set size and siting limits. Larger extensions, anything at the front of the house, flats, listed buildings and homes in designated areas usually do need permission.
Permitted development vs planning permission
There are two ways an extension can be lawful. Either it falls within "permitted development" (PD) — a national set of allowances that let you build without applying — or you apply to your council for planning permission. Knowing which route applies is the single most important thing to get right before you spend money on drawings.
When you usually do NOT need planning permission
For a typical house (not a flat or listed building) on non-designated land, you can generally extend under permitted development if you stay within limits such as:
- Single-storey rear extension up to 3m deep for a terraced or semi-detached house, or 4m for a detached house (up to 6m / 8m under the "Larger Home Extension" prior-approval route).
- Maximum height of 4m for a single-storey rear extension; no higher than 3m at the eaves if within 2m of a boundary.
- Side extensions that are single storey, no more than 4m high and no wider than half the width of the original house.
- Two-storey rear extensions up to 3m deep, kept at least 7m from the rear boundary, with matching roof pitch and similar materials.
- Materials similar in appearance to the existing house, and no balconies, verandas or raised platforms.
When you DO need planning permission
- Anything that extends forward of the principal (front) elevation facing a highway.
- Extensions that would cover more than half the land around the "original house" (the 50% rule).
- Flats and maisonettes, listed buildings, or homes on designated land — conservation areas, AONBs and National Parks — where PD rights are restricted or removed.
- Any area covered by an Article 4 direction that withdraws permitted development.
- Anything that exceeds the permitted development limits above.
The 50% rule and the "original house"
Permitted development is measured against the "original house" — as it was built, or as it stood on 1 July 1948 if older. Previous extensions count towards your allowances even if you didn’t build them. You also can’t cover more than 50% of the land around the original house with extensions and outbuildings combined.
Permitted development doesn’t mean "no paperwork"
Even if your project is permitted development, it’s wise to get a Lawful Development Certificate from your council as formal proof it was lawful — invaluable when you sell. And every extension still needs building regulations approval, which is separate from planning.
Before you commit to drawings: Unformed’s Planning Review checks your scheme against permitted-development and NPPF rules and shows what your council is likely to flag — so you can fix it before you apply.
This guide is general information, not legal or planning advice. Planning rules change over time and are applied differently by each local planning authority (LPA). Always confirm the position with your council before starting work. Unformed’s Planning Review gives an AI-assisted risk assessment of a scheme but does not guarantee any planning outcome.