How Much Does a Semi-Detached Loft Conversion Cost in 2026?
A semi-detached loft conversion typically costs £40,000–£75,000 in 2026, or roughly £1,250–£1,650 per m². A Velux conversion starts around £28,000; an L-shaped dormer can reach £95,000. Get 3 free quotes from vetted local specialists.
Semi-detached loft conversion cost at a glance
The average semi-detached loft conversion in the UK costs £40,000–£75,000 in 2026, depending on the conversion type and finish.
- Velux / rooflight: £28,000 – £42,000
- Rear dormer: £45,000 – £65,000
- Hip-to-gable: £52,000 – £78,000
- L-shaped (hip-to-gable + rear dormer): £65,000 – £95,000
- Per m² average: £1,250 – £1,650
Prices include structural work, staircase, insulation, Building Control and VAT, but exclude furnishing. London and the South East sit 20–30% above the UK average.
2026 Semi-Detached Loft Conversion Cost Breakdown
Use the table below as a realistic 2026 starting point for a semi-detached home, then get 3 free quotes for prices specific to your roof. On a semi the most popular routes are a hip-to-gable, which builds the single sloping end up into a vertical gable wall to reclaim width, and a rear dormer that adds standing headroom at the back; the two are often combined into an L-shape. Head height is the deciding factor — you generally want about 2.2m or more under the existing ridge before conversion. Building Regulations then govern the new staircase, a protected fire-escape route, mains-interlinked alarms, floor-joist strengthening and insulation, and the added roof volume must stay within the permitted-development allowance to avoid a full planning application. Because you share one wall with the neighbour, the Party Wall etc. Act 1996 applies to that boundary. See our step-by-step guide to planning a semi-detached loft conversion and the per-square-metre loft conversion cost guide.
| Conversion Type | Best For | 2026 Cost Range |
|---|---|---|
| Velux / rooflight | Tight budget, good existing headroom | £28,000 – £42,000 |
| Rear dormer | Extra bedroom, most popular for semis | £45,000 – £65,000 |
| Hip-to-gable | Hipped side roof, maximising width | £52,000 – £78,000 |
| L-shaped dormer | Bedroom + ensuite, maximum space | £65,000 – £95,000 |
| Mansard | Conservation areas, near-vertical rear | £65,000 – £80,000 |
Prices include structural steel, staircase, insulation, electrics, plastering, Building Control and VAT. A new ensuite adds roughly £6,000–£12,000.
5 Factors That Change Your Semi-Detached Loft Cost in 2026
Hipped vs gable
A hipped side roof needs a hip-to-gable rebuild to gain usable width, adding £8,000–£18,000 over a simple dormer. Existing gable ends are cheaper to convert.
Shared wall with neighbour
Semis always share a party wall, so the Party Wall Act applies. If your neighbour will not consent, a surveyor costs £900–£1,800. Serve notice two months ahead.
Access & layout
Fitting a Building Regs-compliant staircase into a semi's landing can require reconfiguring a bedroom below, adding £2,000–£6,000 to the project.
Plumbing runs
Adding a bathroom needs new soil and water runs and often a pump. Budget £6,000–£12,000 depending on spec and distance to existing drainage.
Finish & glazing
Velux vs aluminium dormer windows, built-in storage, and bespoke joinery can swing a project by £8,000–£15,000. Standard finishes keep costs predictable.
More 2026 Loft Conversion Guides
All cost guides →Dormer Loft Cost 2026
Full dormer loft conversion price breakdown for 2026.
Hip-to-Gable Cost 2026
Ideal for semis with a hipped side roof.
UK Loft Conversion Cost
Every loft conversion type compared with ROI data.
Loft Planning 2026
When a loft conversion needs planning permission.