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Loft Conversion Cost Calculator — UK Price Estimates 2026
Estimate loft conversion costs per m² for Velux, dormer, hip-to-gable and mansard conversions. The per-m² rate is all-inclusive (staircase included); add the bathroom fit-out and a contingency separately.
Usable floor area of conversion
En-suite or shower room
Override default cost range
Recommended 10% for unknowns
Per-bathroom fit-out. Default £5,000 = basic loft shower room; a fuller en-suite is £5,000–£12,000 (MyJobQuote 2026)
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How We Calculate This
This calculator estimates loft conversion costs based on floor area and conversion type using current UK market rates. The per-m² rate is all-inclusive — it already covers the new staircase — so the staircase is not added as a separate line (that would double-count it).
The formula
Build Cost = Floor Area × Cost per m² (staircase included in the rate)
Total = (Build Cost + Bathroom) × (1 + Contingency%)
Indicative all-in build rates (2026)
- Velux: £1,000-£1,500/m²
- Dormer: £1,200-£1,900/m²
- Hip-to-gable: £1,400-£2,100/m²
- Mansard: £1,700-£2,500/m²
Indicative market rates from 2026 UK trade cost guides (MyJobQuote, houseUP, Checkatrade, June 2026). These are rule-of-thumb figures, not a published standard — always confirm with builder quotes.
Frequently Asked Questions
As an indicative 2026 all-in build rate (staircase included, bathroom fit-out excluded): Velux/rooflight conversion £1,000-£1,500/m², rear dormer £1,200-£1,900/m², hip-to-gable with dormer £1,400-£2,100/m², and mansard £1,700-£2,500/m² (MyJobQuote/houseUP, June 2026). A typical 20m² dormer works out at roughly £24,000-£38,000 excluding the bathroom. These are market guide figures, not a fixed price — always confirm with quotes.
The per m² rate typically includes structural works (steel beams, floor strengthening), insulation, plasterboarding, electrics, the new staircase, fire doors, building control fees and basic decoration. Because the staircase is already in the rate, this calculator does not add it again. The rate usually excludes the bathroom fit-out, which adds about £5,000–£12,000 depending on specification (MyJobQuote 2026). Use our Loft Bathroom Calculator, Loft Electrical Calculator, and Loft Staircase Space Calculator to budget individual elements.
A Velux (rooflight only) conversion is cheapest at £1,000-£1,500/m² because it doesn't alter the roof structure. However, usable floor area may be limited by head height. A rear dormer adds more space but costs £1,200-£1,900/m². The best value depends on your roof type and head height.
Most loft conversions can be done under Permitted Development rights without planning permission, provided they stay within the roof-volume limits (40m³ for terraced houses, 50m³ for semi-detached and detached houses, per GOV.UK householder PD technical guidance, Class B), don't extend above the ridge, use matching materials, and side-facing windows are obscure glazed. Mansard conversions usually need planning permission.
A typical rear dormer conversion takes 8-12 weeks from start to completion. Velux conversions are quicker at 4-6 weeks. Hip-to-gable and mansard conversions take 10-14 weeks. Allow additional time for party wall agreements (6 weeks) and building control sign-off.
Related Calculators
Mansard Loft Calculator
Calculate mansard loft conversion rear wall, cladding, insulation and structural steel.
Loft Conversion Head Height Calculator
Check if your loft has enough head height for a loft conversion.
Loft Staircase Space Calculator
Calculate floor opening size and area lost for loft staircases by type.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.