The volume limit
Under permitted development, the new roof space (including any previous roof enlargement) must not add more than 40 cubic metres to the original roof space of a terraced house, or 50 cubic metres for detached and semi-detached houses.
Design limits
- No higher than the highest part of the existing roof.
- No extension beyond the plane of the existing roof slope on the principal elevation facing a road. In practice, front dormers usually need planning permission.
- Materials similar in appearance to the existing house.
- Side-facing windows must be obscure-glazed, and any opening part must be at least 1.7 metres above the floor.
- Roof enlargements (other than hip-to-gable) should be set back, as far as practicable, at least 20cm from the original eaves.
- No verandas, balconies or raised platforms. A Juliet balcony with no platform would normally be acceptable.
When you always need permission
- Flats and maisonettes (permitted development rights for houses do not apply).
- Houses on designated land, including conservation areas, National Parks, Areas of Outstanding Natural Beauty, the Broads and World Heritage Sites.
- Listed buildings, which also need listed building consent.
- Homes where permitted development rights have been removed by a planning condition or Article 4 direction.
Prove it with a certificate
If your loft conversion is permitted development, you can apply for an optional lawful development certificate. It confirms the work is lawful and can save questions when you sell.
Building Regulations still apply
Permitted development is about planning only. Converting a loft into a habitable room always needs Building Regulations approval, and terraced and semi-detached homes may need a party wall notice.
Wales, Scotland and Northern Ireland have their own rules. This guide covers England. If you live elsewhere in the UK, check with your local council.
Last updated: 14 October 2026
