A dormer window in the UK costs roughly £4,000–£12,000 supplied and fitted in 2026 for the dormer box on its own — or £40,000–£75,000 when it forms part of a full dormer loft conversion with stairs, insulation and a finished room. The big cost drivers are roof type (flat-roof dormers are cheapest), size and number of windows, structural steelwork, and the internal finish. This guide breaks down every line of the price and where you can safely save.
Price by dormer size Box only vs full conversion What affects the price
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Dormer Window Cost by Size (2026)
Supplied and fitted, including structure, roofing, glazing and making good — on a standard pitched roof.
Dormer type
Dormer box only
As full loft conversion
Single flat-roof dormer (small)
£4,000–£7,000
£40,000–£55,000
Standard rear dormer
£6,000–£12,000
£50,000–£65,000
Large or full-width dormer
£12,000–£20,000
£60,000–£75,000
Rule of thumb in 2026: the dormer structure costs roughly £1,500–£2,500/m² of added floor area; the finished room as a whole runs £2,000–£3,000/m².
What You Pay For
Where the Money Goes
Design & structural calcs (£800–£2,500). Drawings plus a structural engineer for the steel beams that carry the new opening.
Scaffolding (£800–£1,800). Needed for safe roof access and usually hired for the full build period.
Strip & structural opening (£1,500–£4,000). Cutting the roof, fitting steels and forming the dormer frame is the core of the box cost.
Roofing & cladding (£1,500–£4,000). Flat-roof felt/EPDM or tiled cheeks, lead flashing and weatherproofing.
Windows & glazing (£600–£2,500). Double-glazed casements or full-width glazing; rooflights are cheaper than dormer glazing.
Internal fit-out (£8,000–£25,000+, conversion only). Stairs, insulation, plasterboard, electrics, heating and a building-regs sign-off turn the box into a usable room.
Frequently Asked
Common Questions
Expect £4,000–£12,000 supplied and fitted for the dormer box itself, depending on size, roof type and glazing. A large or full-width dormer can reach £20,000 before any internal fit-out.
Yes — adding only the dormer box is far cheaper than a complete conversion, because you avoid stairs, insulation, electrics and building-regs fit-out. But a dormer on its own does not create a habitable room; most homeowners add it as part of a £40,000–£75,000 conversion.
A rear dormer is often permitted development within the 40m³ (terraced) or 50m³ (semi/detached) volume limit, but a dormer on the front or principal elevation facing a highway almost always needs full planning permission.
Roof type and size are the biggest factors — a flat-roof rear dormer is cheapest, while pitched or full-width dormers cost more in structure and roofing. Steelwork, access, glazing spec and finish all add to the price.
The dormer box itself is typically 2–4 weeks once scaffolding is up. As part of a full loft conversion, budget 6–10 weeks all-in for structure, fit-out and building-regs sign-off.
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