The short answer
Many single-storey extensions fall under “permitted development” and don’t need full planning permission — but larger extensions, anything at the front of the house, and most work on flats or listed buildings usually do. Even permitted-development work must meet size limits and still needs building regulations approval.
Permitted development limits
Broadly: a single-storey rear extension can extend up to 3m (attached house) or 4m (detached) under standard rules, or further under the Larger Home Extension scheme with a neighbour consultation. Height, materials and how much garden you cover are all capped.
When you almost always need permission
Double-storey extensions close to a boundary, extensions to the front (the “principal elevation”), anything in a conservation area or on a listed building, and most extensions to flats and maisonettes typically need a full planning application.
Planning vs building regulations
They’re different things. Planning permission governs what you can build and how it looks; building regulations govern that it’s structurally sound, insulated, safe and properly drained. Almost every extension needs building-regs sign-off even when planning isn’t required.
Not sure? Get local advice
Rules vary by council and property. A good local builder or architect will confirm what applies before you start — tell us about your project and we’ll match you with experienced local specialists.