Cost Guide · Updated June 2026

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.

Updated June 2026 Based on 1,200+ recent quotes FMB & insured builders
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By the BestBuilders Editorial Team
Researched, written & fact-checked by our editorial team. Reviewed . How we research & fact-check
✅Reviewed by the BestBuilders editorial team on 17 June 2026. All cost ranges, planning and Party Wall references verified against current Q2 2026 UK market data. Editorial standards: /editorial-standards.
Quick Answer

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.

Transparent Pricing

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.

Conversion TypeBest For2026 Cost Range
Velux / rooflightTight budget, good existing headroom£28,000 – £42,000
Rear dormerExtra bedroom, most popular for semis£45,000 – £65,000
Hip-to-gableHipped side roof, maximising width£52,000 – £78,000
L-shaped dormerBedroom + ensuite, maximum space£65,000 – £95,000
MansardConservation areas, near-vertical rear£70,000 – £100,000

Prices include structural steel, staircase, insulation, electrics, plastering, Building Control and VAT. A new ensuite adds roughly £6,000–£12,000.

What Affects the Price

5 Factors That Change Your Semi-Detached Loft Cost in 2026

Roof Type

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.

Party Wall

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.

Staircase

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.

Ensuite

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.

Specification

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.

£1,250+
Average cost per m² (2026)
8wk
Typical dormer build time
18%
Typical value uplift
3
Free quotes to compare
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Dormer

Dormer Loft Cost 2026

£45k – £65k

Full dormer loft conversion price breakdown for 2026.

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Hip-to-Gable

Hip-to-Gable Cost 2026

£52k – £78k

Ideal for semis with a hipped side roof.

Full guide ›
All Types

UK Loft Conversion Cost

£28k – £100k

Every loft conversion type compared with ROI data.

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Planning

Loft Planning 2026

PD vs Full

When a loft conversion needs planning permission.

Planning rules ›
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Frequently Asked Questions

Semi-Detached Loft Conversion FAQs

A semi-detached loft conversion costs £40,000–£75,000 in 2026 depending on type. Velux is cheapest at £28,000–£42,000, a rear dormer is £45,000–£65,000, a hip-to-gable is £52,000–£78,000, and an L-shaped dormer is £65,000–£95,000. London and the South East sit 20–30% higher.
Many semis have a hipped (sloping) side roof that limits headroom. A hip-to-gable rebuilds that slope into a vertical gable wall, unlocking far more usable floor area. Combined with a rear dormer (an L-shaped conversion) it can create a full double bedroom with an ensuite. Hip-to-gable typically costs £52,000–£78,000.
Most fall under Permitted Development if the added roof volume stays within 50m³, nothing extends forward of the principal elevation, and materials match. Conservation areas, Article 4 zones and listed buildings need full planning. A Lawful Development Certificate (£103) is strongly recommended to prove PD compliance.
Yes — because you share a party wall with your neighbour, the Party Wall etc. Act 1996 almost always applies. You must serve notice at least two months before work starts. If your neighbour consents in writing it costs nothing; if a surveyor is needed, budget £900–£1,800.
A Velux conversion takes 4–6 weeks, a rear dormer 6–8 weeks, and a hip-to-gable or L-shaped 8–10 weeks on site. Add 6–10 weeks upfront for design, structural calculations, Party Wall notices and Building Control — budget around 4–5 months end to end.
A conversion that adds a double bedroom and ensuite typically adds 15–20% to a semi's value. On a £350,000 semi that is roughly £52,000–£70,000, often exceeding the build cost. The return is highest when it creates a genuine extra bedroom rather than a study.
A Velux (rooflight) conversion at £28,000–£42,000, because it keeps the existing roof shape and simply adds windows in the slope. It needs no structural change to the roofline, so it is usually Permitted Development. The trade-off is less headroom and floor area than a dormer.
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Sources & Further Reading

Our sources for this guide

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