How Much Does a Dormer Window Cost in 2026? (UK)
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.
Dormer Window Cost by Size (2026)
Supplied and fitted, including structure, roofing, glazing and making good — on a standard pitched roof.
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.
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.
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