Do you need planning permission for a side or wraparound extension?

Development is not permitted by Class A if ... the enlarged part of the dwellinghouse would extend beyond a wall forming a side elevation of the original dwellinghouse, and would (i) exceed 4 metres in height, (ii) have more than a single storey, or (iii) have a width greater than half the width of the original dwellinghouse

Which walls count as side walls

The technical guidance treats any wall that is not the front or the rear as a side wall, so a house can have several. The side wall of an original rear outrigger is one, so a side return infill is held to these limits. The front is the principal elevation, usually the wall facing the road that gives the house its address. Class A permits no enlargement beyond the principal elevation, or beyond a side elevation that fronts a highway (A.1(e)). On a corner plot, if the side wall faces the road, a side extension on that side is not permitted development. The guidance says a wall at more than 45 degrees to the road does not normally front it, and a wall a long way from the road may not. A highway includes an unadopted street or private way (paragraph I).

Half the width of the original house

The guidance measures the original house at its widest point. The half-width limit covers everything beyond the side wall, including an earlier extension that the new one joins (A.1(ja)). If a side extension already takes up half the width, a later rear extension that joins it and runs further out to the side fails.

Wraparound extensions

A wraparound fills the side and the rear, so it must:

• be a single storey, no more than 4 metres high;
• be no wider, beyond the side wall, than half the width of the original house;
• extend no more than 3 metres beyond each original rear wall, or 4 metres on a detached house;
• keep its eaves to 3 metres wherever it comes within 2 metres of a boundary (A.1(i)).

The rear part can go to 6 metres, or 8 metres on a detached house, through prior approval. Where the original rear wall is stepped, each step is a rear wall and the depth limit applies from each. A side extension that runs past the rear wall is held to the rear depth limit as well (technical guidance).

Conservation areas and other Article 2(3) land

On Article 2(3) land, which includes conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites, Class A permits no enlargement beyond a side wall of the original house at any size (A.2(b)). A side extension or wraparound there needs planning permission. A single-storey rear extension can still qualify, within 3 metres, or 4 metres on a detached house; the prior approval route is not available.

What this means for you

Work out the width of the original house first, from drawings or a measured survey, then check whether any wall you plan to build from faces a road. Put your figures into the permitted development checker, and check the original house rules if the house has been extended before.

Getting certainty

A lawful development certificate under section 192 of the Town and Country Planning Act 1990 confirms that the extension as drawn would be lawful. From 1 April 2026 the fee is £274 and the council has 8 weeks to decide. A design that goes beyond half the width or above one storey needs a householder planning application instead, £548 from the same date.