Do I need planning permission for a side extension?

A single-storey side extension can be permitted development, but the rules are tighter than for rear extensions: it can be no more than 4m tall, no wider than half the original house, and it isn't allowed at all in some protected areas or on a corner plot where the side faces a road.

The short answer

• Single storey only, no more than 4m tall.
• No wider than half the width of the original house.
• Not beside a side wall that faces a road, which rules out most corner plots.
• Not permitted development at all in conservation areas, National Landscapes, national parks or World Heritage Sites.

The limits

To be permitted development, a side extension must meet every one of these:

• A side extension can only be single storey. Schedule 2, Part 1, Class A, Class A.1(j)(ii)
• A side extension can be no more than 4m tall. Schedule 2, Part 1, Class A, Class A.1(j)(i)
• A side extension can be no wider than half the width of the original house. Schedule 2, Part 1, Class A, Class A.1(j)(iii)
• An extension can't go beyond a side wall that faces a road. Schedule 2, Part 1, Class A, Class A.1(e)(ii)
• A wraparound extension is treated as one enlargement, so the whole thing must meet the side extension limits and the rear extension limits at the same time (single storey, no more than 4m tall, no wider than half the house, and within the rear depth limit). Most wraparound designs go beyond this, so check with your council. Schedule 2, Part 1, Class A, Class A.1(ja)
• Within 2m of a boundary, the eaves can be no higher than 3m. Schedule 2, Part 1, Class A, Class A.1(i)
• No further forward than the front of the house.
• No higher than the highest part of the existing roof, and eaves no higher than the existing eaves.
• Together with other extensions and outbuildings, it can cover no more than half the garden around the original house.
• No verandahs, balconies or raised platforms.
• Materials must look similar to the existing house.

Half the width of the original house

The width limit compares the extension with the original house: as first built, or as it stood on 1 July 1948. Measure the width of the house's front, then halve it. A garage or side extension added later doesn't increase the allowance, and counts towards it if the new extension joins onto it.

Corner plots

An extension can't go beyond a side wall that faces a road. On a corner plot, that usually means the side next to the road can't be extended under permitted development, though the other side might be. "Faces a road" includes walls set back behind a front garden or verge, as long as they face the road.

Wraparound extensions

An L-shaped extension that goes along the side and across the back is treated as one enlargement. The whole thing has to meet the side extension limits and the rear extension limits at the same time: single storey, no more than 4m tall, no wider than half the house, and within the rear depth limit. In practice most wraparound designs go beyond this and need planning permission.

Close to the boundary

Side extensions are often built close to a neighbour's boundary. Within 2m of it, the eaves can be no higher than 3m. You'll also usually need to give your neighbour notice under the Party Wall etc. Act 1996 before digging foundations near a shared wall or boundary.

Where the rules are stricter

• Side extensions in a conservation area
• Side extensions in a National Landscape or national park
• Listed buildings
• Article 4 directions
• Flats and maisonettes

Planning permission and building regulations

Planning permission and building regulations are separate. Planning is about whether you can build it and how it looks; building regulations are about whether it is built safely (structure, fire safety, insulation, drainage). Permitted development only covers planning, so you may still need building regulations approval.

Side extensions almost always need building regulations approval. If you are building on or near a shared boundary or wall, you'll probably also need to serve notice on your neighbour under the Party Wall etc. Act 1996.

Getting certainty

If you're confident your project is permitted development, you can apply to your council for a Lawful Development Certificate. It's a formal confirmation that the work is lawful without planning permission, it costs less than a planning application, and it's useful when you come to sell the house. If you're not sure, many councils offer pre-application advice.