How Much Does a Loft Dormer Cost Per m² in 2026? (UK)
A UK loft dormer in 2026 typically costs £2,100–£3,000 per m² fitted. A small 4m² gable-end dormer runs £8,500–£12,500; a popular 8m² flat-roof box dormer is £17,000–£24,000; a large 15m² full-width rear dormer reaches £32,000–£45,000+. Pitched dormers add 10–18% over flat-roof equivalents but tend to integrate better with traditional roofscapes.
Loft dormer cost per m² by size (2026 UK)
Dormer cost scales with footprint and roof type. As a 2026 UK guide: 4m² (small gable dormer) roughly £8,500–£12,500; 6m² (single-room box) roughly £12,500–£18,000; 8m² (popular 2-window box dormer) £17,000–£24,000; 12m² (master-bedroom rear dormer) £26,000–£36,000; 15m²+ (full-width) £32,000–£45,000+. Figures assume timber framing, insulation to current Part L, plasterboard finish, double-glazed casements and tiled or EPDM roof finish.
How the roof finish changes the price
EPDM single-ply membrane on a flat box dormer is the cheapest (£60–£90 per m² covered). GRP fibreglass is comparable (£80–£110 per m²). Tile-hung cheek with a small pitched roof is mid-range (£110–£170 per m²). Slate-clad dormers in heritage areas are premium (£170–£260 per m²). On an 8m² dormer the choice typically adds or subtracts £1,500–£4,000.
Glazing, structural openings and extras
Most dormers add a structural opening or steel beam: RSJ for a new opening adds £1,800–£3,500 including structural calcs. Juliet balcony with French doors adds £2,200–£3,800. Aluminium casement windows add £400–£800 over standard UPVC per window. En-suite shower in the new room adds £4,500–£8,500. Velux on the slope alongside the dormer is £1,500–£2,500 fitted each.
Flat vs pitched dormer — cost difference
A pitched dormer typically costs 10–18% more than a flat-roof equivalent due to extra rafters, tiling and ridge. The trade-off: pitched dormers blend more naturally with traditional UK housing (especially semis and terraces in conservation areas) and tend to add more resale value. Flat-roof box dormers maximise internal headroom for the same footprint — typically gaining 200–400mm of usable head height in the room.
How to budget step by step
1. Confirm the dormer footprint and type
Cost scales at roughly £2,100–£3,000 per m² fitted. Decide if you need full-width (max space), single-room box, or smaller gable-style.
2. Pick the roof finish
EPDM is cheapest; tile-hung mid-range; slate premium.
3. Decide on structural openings and en-suites
RSJs, Juliet balconies and en-suites add the most to the headline figure.
4. Compare three matched quotes
FMB or TrustMark members, on the same written scope including building control.
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