Do You Need Planning Permission for a Loft Conversion in London?

The short answer

A loft conversion to a house can often be carried out under permitted development — but two things frequently change that in prime London. First, permitted development for loft conversions and roof extensions is removed on designated land, which includes conservation areas, so in much of Hampstead, Highgate and the prime central boroughs a loft conversion needs planning permission. Second, permitted development does not apply to flats and maisonettes at all. Where the rights do apply, they come with specific volume and design limits.

The permitted development limits

For a house outside a conservation area, a loft conversion can be permitted development within these limits:

• Up to 40 cubic metres of additional roof space for a terraced house, or 50 cubic metres for a detached or semi-detached house (any previous roof enlargement counts towards this)
• No part higher than the highest part of the existing roof
• No extension beyond the plane of the existing roof slope on the principal elevation that fronts a highway — so no street-facing dormer under permitted development
• Materials similar in appearance to the existing house
• Side-facing windows obscure-glazed, and non-opening below 1.7 metres from the floor
• The roof enlargement set back at least 20cm from the original eaves where practicable, and not overhanging the wall
• No verandas, balconies or raised platforms — so a roof terrace is not permitted development

Why conservation areas change the answer

On Article 2(3) designated land — which includes conservation areas — permitted development for loft conversions and roof extensions is removed. A dormer or mansard that would be permitted development on an unprotected street therefore needs a full planning application in a conservation area, and the design is assessed for its effect on the roofscape and the character of the area. Given how much of prime London is designated, this is the norm here rather than the exception.

Flats, party walls and building regulations

For a flat, a loft or roof-space conversion is not permitted development and, where the roof space belongs to the freeholder, needs the freeholder’s consent through a Licence to Alter as well as any planning permission. Every loft conversion — house or flat — must meet building regulations for the new structure, the stair, fire escape and insulation, and usually engages the Party Wall Act with neighbours. Where a house conversion is permitted development, a lawful development certificate confirms in writing that it is lawful.