Permitted development covers a surprising amount of work, but the limits are specific and the penalties for getting them wrong are expensive.
The usual limits
For most houses, a single-storey rear extension can go up to three metres from the original rear wall — four for a detached home — provided it stays under four metres high and does not cover more than half the garden.
The word that catches people out is original. It means the house as first built, not as you bought it. If a previous owner already extended, your allowance may already be spent.
When the rules do not apply
Permitted development rights are restricted, or removed entirely, in conservation areas, on listed buildings, in national parks, and on some newer estates where the original planning consent stripped them out. Flats and maisonettes have no permitted development rights at all.
Why we often apply anyway
Even when a job is clearly permitted, a Lawful Development Certificate is worth the fee. It is a formal confirmation from the council that what you built was allowed — and it is the first thing a buyer’s solicitor asks for when you come to sell.