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.
Planning the work? Check what it usually costs, or work out your own estimate. The free quote form is at the top of this page.
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.