You've got a narrow strip of land down the side of your house - between the wall and the boundary - and you're wondering whether you can build on it. A side return extension on a terraced house can reclaim dead space and transform a cramped galley kitchen into something that actually works. A wider side extension on a detached or semi-detached house can add a utility room, a downstairs WC, or a whole new room.
Side extensions are permitted development, but the rules are tighter than for rear extensions. They must be single storey only, no wider than half the original house, and no taller than 4 metres. And on designated land, they're not PD at all.
The extension must not extend forward of the principal elevation of the original house. Materials must be of similar appearance to the existing house. And the 50% curtilage coverage rule applies - the side extension, combined with every other structure on the plot, must not exceed 50% of the total curtilage. See our curtilage guide.
Single storey only. No wider than half the house. And not on designated land.
On article 2(3) land - conservation areas, National Parks, AONBs, World Heritage Sites, and the Broads - side extensions are not permitted development. You will need a householder planning application (£548). This is one of the most significant restrictions on designated land, alongside the removal of roof extension rights. For full details on conservation area rules, see our conservation area guide.
A two storey side extension is never permitted development. The PD rules only allow single storey side extensions. If you want to go to two storeys on the side, you need a householder planning application. Councils are often cautious about two storey side extensions because of the "terracing effect" - where a row of semi-detached houses gradually becomes a terrace as each one extends to the side. Your application will need to demonstrate good design to address this concern.
A side return extension reclaims the narrow passage between the side wall of the house and the boundary. On a typical Victorian or Edwardian terrace, this passage is often only 1-2 metres wide - but running the full depth of the ground floor, it can add 5-10 square metres of usable space. Combined with the rear of the house, it creates a wider, more open kitchen-dining room.
A side return technically extends beyond a side wall, so the half-width rule applies. On a 5-metre-wide terrace, a 1-metre side return is well within the 2.5-metre maximum. The eaves will almost certainly be within 2 metres of the boundary, capping them at 3 metres. For full extension rules including rear depth limits, see our extensions guide.
If you know what you want to build, the free eligibility check takes about two minutes.
Common mistakes that cost money
Trying to build a two storey side extension under PD. It's not possible. Two storey side extensions always need planning permission, regardless of width or height.
Not realising side extensions are banned on designated land. If you're in a conservation area, a side extension needs planning permission even if it's tiny. Check your designation status first.
Measuring the half-width from the extended house. The 50% width limit is based on the width of the original house, not the house as it currently stands with any existing extensions.
PD Assessment Tool
A professional permitted development assessment from a planning consultant typically costs £400-600. This tool checks your specific project against every Class A condition and produces a formal document you can share with your builder or council.
Free eligibility check. Full assessment £47.
Content verified against the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended), Class A of Part 1, Schedule 2, paragraphs A.1(e), A.1(j), and A.2(a) (legislation.gov.uk, revised version) and the government's technical guidance (September 2019). This page is for general information only and does not constitute legal advice.
April 2026