Loft conversions are one of the most profitable jobs in residential building — and one of the most variable to price. The difference between a simple Velux conversion and a mansard can be three times the cost for the same floor area. This guide walks through pricing each type correctly.
These are all-in prices including labour, materials, and fixtures at a mid-spec finish. They exclude architect fees, structural engineer fees, and planning fees where applicable.
| Type | Typical cost range | Notes |
|---|---|---|
| Velux / rooflight conversion | £20,000 – £35,000 | No structural changes to roofline. Cheapest and fastest. |
| Dormer conversion (rear) | £35,000 – £55,000 | Most common. Creates a box-shaped extension on the roof slope. |
| Hip-to-gable conversion | £40,000 – £65,000 | Extends the hipped end of a roof. Usually combined with a rear dormer. |
| Mansard conversion | £45,000 – £75,000 | Flat roof with steep sides. Maximum space but always needs planning. |
| Over-structure / above garage | £25,000 – £45,000 | Converting above a garage or flat-roofed extension. |
Most loft conversions fall under permitted development (PD) — meaning no planning application needed — provided they meet specific rules: no more than 40m³ added volume for terraced houses (50m³ for detached/semi), no extension beyond the existing roof plane at the front, and materials that match the existing house. Mansard conversions almost always need full planning permission because they change the roof profile significantly. Always check with the local authority before quoting — a job that needs planning adds 8–12 weeks and £500–£2,000 in fees before you start.
Unlike extensions, loft conversions always require building regulations approval regardless of whether they need planning permission. Key areas covered: structural calculations (steels, floor joists), fire safety (30-minute fire doors on all rooms opening onto the escape route), insulation to Part L standards, and a fixed staircase meeting Part K requirements. Budget for a structural engineer (£400–£900) and building control fees (£700–£1,200) on every job.
| Trade | Days | Notes |
|---|---|---|
| Carpenter / roof specialist | 5 – 10 days | Structural work, dormer frame, floor joists, staircase |
| Roofer | 2 – 4 days | Felt, battens, tiles/slates, Velux or dormer weatherproofing |
| Electrician | 2 – 3 days | New circuit, lighting, sockets, smoke detectors on all floors |
| Plumber | 1 – 3 days | Only if en-suite included. Add £3,000–£6,000 for a basic en-suite. |
| Plasterer | 3 – 5 days | Boarding and skimming throughout |
| General labourer | 2 – 4 days | Clearout, waste removal, general assist |
Never give a firm price on a loft conversion without measuring the ridge height yourself. Many customers report measurements that turn out to be wrong, and a low roof changes the entire scope of the job.
Describe the job by voice and get a professional quote in seconds. Free to try.
Start free →