Most UK lofts with around 2.2 metres of head height between the ceiling joists and the ridge can be converted. A rooflight conversion typically costs £30,000 to £45,000, a dormer £45,000 to £65,000, and a hip-to-gable or mansard £55,000 to £80,000. Many conversions fall under permitted development, but every loft conversion needs Building Regulations approval. Building work usually takes 6 to 10 weeks.
Can my loft be converted?
Three things decide it: head height, roof structure and space for a staircase. Measure from the top of a ceiling joist to the underside of the ridge. Around 2.2 m or more is a good sign. Older houses with rafters and purlins are easier to convert than modern houses with W-shaped trussed rafters, which need replacing with steel and new timbers. The new staircase needs headroom of 2 m over its centre line, reducing to 1.9 m at the centre and 1.8 m at the side where it rises under a sloping roof.
The four main types of loft conversion
| Type | What it is | Typical cost | Usually suits |
|---|---|---|---|
| Rooflight | Windows added to the existing roof slope, no change to the roof shape | £30,000 to £45,000 | Lofts that already have good head height |
| Rear dormer | A box structure with vertical walls built out from the rear slope | £45,000 to £65,000 | Terraces and semis |
| Hip-to-gable | The sloping side roof is rebuilt as a vertical gable wall, often with a rear dormer | £55,000 to £80,000 | Semi-detached and detached houses with hipped roofs |
| Mansard | The rear slope is rebuilt almost vertical with a flat top | £60,000 to £80,000+ | Terraces where planning allows |
Costs are indicative for Northamptonshire and exclude VAT where applicable, fees and high-end finishes.
Do I need planning permission?
Many loft conversions are permitted development in England. The main limits are:
- Up to 40 m³ of additional roof space for terraced houses, and 50 m³ for semi-detached and detached houses.
- No part of the extension can be higher than the existing ridge.
- Nothing can extend beyond the plane of the roof slope that faces the highway on the principal elevation.
- Side-facing windows must be obscure glazed and non-opening below 1.7 m above the floor.
- Materials should be similar in appearance to the existing house.
- Permitted development does not apply in conservation areas and other designated land, or to flats.
If you want certainty, apply for a Lawful Development Certificate from your council before building.
What Building Regulations cover
Every loft conversion needs Building Regulations approval. Building control checks the structure (new floor joists and steels designed by an engineer), fire safety (usually fire doors on the escape route and mains-powered interlinked smoke alarms), the staircase, insulation and ventilation. At the end you receive a completion certificate, which you will need when you sell.
Party walls
If you live in a semi or terrace, the steels for your floor usually bear into the party wall, so you must serve notice on your neighbour under the Party Wall etc. Act 1996 at least one month before that work starts. If they consent in writing, no surveyor is needed.
How long does a loft conversion take?
A typical programme runs: scaffold and roof opening in week one; structure and dormer or gable in weeks two to four; roofing, windows and first fix services in weeks four to six; staircase, plastering and second fix in weeks six to nine. Rooflight conversions can be done in around six weeks; mansards may take ten or more.
Further reading
This guide is general information for England and is not a substitute for advice on your specific property. Planning rules differ in Wales, Scotland and Northern Ireland. Always confirm with your local planning authority.