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.
Single dormer vs full-width dormer – total cost vs cost per m²
A dormer's total price and its per-m² price move in opposite directions as the box gets bigger. A single rear dormer (5–7m²) – the classic one- or two-window box over a stair or master bedroom – typically lands at £18,000–£28,000 as a stand-alone dormer, or around £2,600–£3,100 per m². A full-width rear dormer (10–15m²) that runs the whole span of the roof usually costs £30,000–£45,000+ as a dormer, but drops to roughly £2,100–£2,600 per m² because fixed costs – scaffold, roof stripping, steels and building-control fees – are spread over far more floor. In other words a full-width dormer costs more in total yet less per square metre. For the wider project, compare our loft conversion cost per m² guide and the full dormer loft conversion cost breakdown, which puts a whole-project dormer conversion at roughly £30,000–£65,000 once the staircase, en-suite and finishes are included.
What drives the price – and permitted development vs planning
Beyond size and roof finish, the biggest cost drivers are access and structure. Terraced houses with no rear access add £800–£1,800 for craned or hand-balled materials; a steeper existing roof pitch means more rafters to cut and reinforce; and any change to the floor structure to carry the new room usually needs steel beams and structural calculations. Region matters too: London and the South East typically run 10–20% above the national figures quoted here, while parts of the North and Scotland sit below.
Planning route is the other swing factor. Many rear dormers are permitted development where the roof-volume addition stays within 40m³ for terraced houses or 50m³ for semi-detached and detached homes, the dormer sits below the existing ridge and is set at least 20cm back from the eaves – the current thresholds published on the Planning Portal loft conversion guidance. Front-facing dormers, conservation areas, Article 4 zones and all flats need a full planning application, which adds £400–£700 in fees plus 8–10 weeks. Confirm your route early with our guide on whether you need planning permission for a loft dormer before you commit to a design.
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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