Planning permission for a loft conversion
Most loft conversions are permitted development, but dormers, roof extensions and any home on designated land can tip a project into needing a full application.
Loft conversions are usually permitted development, which is why they are such a popular way to add a bedroom or office. The key control is volume: how much you add to the original roof space.
- Added roof volume within 40m³ (terraced) or 50m³ (semi-detached and detached) of the original roof
- No part higher than the existing ridge; dormers set back at least 20cm from the original eaves where practical
- Rooflights that project no more than 150mm above the roof slope
- Side-facing windows obscure-glazed and, if opening, at least 1.7m above the floor
- Any dormer or roof extension on the principal elevation fronting a highway
- Conversions that exceed the 40m³ / 50m³ volume allowance
- Homes in conservation areas and other designated land, and any flat or maisonette
- Balconies, verandas or raised platforms formed as part of the conversion
Don't forget building regs
Every habitable loft conversion needs building regulations approval: covering the new floor structure, stairs, fire escape and protected routes, and insulation. We handle those drawings and calculations in-house.
We check the volume and the roof form against permitted development, then design a dormer or rooflight scheme that works. Our structural team sizes the new floor and steels under the same roof, so the planning and building regulation drawings stay coordinated. See our loft conversion projects.
Frequently asked
How much can I add to my loft without planning permission?
Permitted development allows up to 40 cubic metres of added roof volume on a terraced house and 50 cubic metres on a semi-detached or detached house, measured against the original roof. Dormers on the front roof slope always need permission.
Do rooflight (Velux) loft conversions need permission?
Usually not. Rooflights that sit within 150mm of the roof slope and do not raise the ridge are typically permitted development, which is why rooflight conversions are the simplest route. We still confirm it for your property.
Not sure where your project stands?
Checking whether you need planning permission is one of the first things we do: on the house, before any drawings. Tell us about your project and we'll confirm the route.