Loft Conversion Cost Calculator

Loft conversion costs depend on the type of conversion and the existing roof structure. The main types are: Velux/roof-light (cheapest, £20,000–£35,000, adds roof windows without changing the roof shape), dormer (£35,000–£60,000, extends the roof vertically to create more headroom), hip-to-gable (£40,000–£65,000, converts a hipped roof end to a vertical gable wall) and mansard (£50,000–£75,000, replaces the roof slope with a near-vertical wall and flat top).

Building Regulations approval is required for all loft conversions, covering structure, fire safety (protected staircase, fire doors, smoke alarms), insulation, means of escape and staircase design. Planning permission is needed if you exceed Permitted Development limits (40 m³ for terraced houses, 50 m³ for detached/semi-detached).

Enter your roof type, desired conversion type and size. The calculator estimates the build cost, additional costs (staircase, en-suite, electrics, plastering) and professional fees. It also checks whether your plans are likely to fall within Permitted Development limits.

UK loft conversion costs 2026. Dormer conversion (most common): £35,000-£60,000 for 2-bed semi. Velux (rooflight only, simplest): £25,000-£40,000. Hip-to-gable: £45,000-£75,000. Mansard (most extensive, planning needed): £55,000-£100,000+. Per m² rate: £1,200-£2,500/m² depending on complexity. Includes: structural calcs, insulation, plasterboard, en-suite, dormer windows. Excludes: kitchen-quality fittings, statement features. ROI: typically adds 15-25% to property value — break-even/profit on most UK homes.

Planning permission rules. Many loft conversions are permitted development (no planning needed): rear-facing dormers up to 40m³ (terraced) or 50m³ (semi/detached); no extension beyond plane of existing roof slope on front; materials similar to existing. Side dormers + roof extensions to front: planning permission required. Conservation areas: tighter rules. Listed buildings: always need listed building consent. Always: Building Regulations approval mandatory regardless of planning — Building Notice or Full Plans.

Headroom and head height requirements. Building Regs minimum: 2.0m above staircase. Minimum 2.3m headroom over 50% of floor area for habitable room (England). Scotland slightly different. Trussed roofs (most post-1960s UK homes): require structural alteration — replace truss with rafter/joist system (£8,000-£15,000 extra). Traditional cut-roof (pre-1960s): often easier to convert. Floor joists usually need strengthening — original ceiling joists rarely strong enough for habitable floor.

Bedrooms, en-suites and storage. Standard loft conversion: 1 bedroom + en-suite shower. Adding kitchenette/wet room: £3,000-£8,000 extra. Built-in storage in eaves (where ceiling under 1.5m): essential to maximise space. Knee walls 1.5m tall standard. Skylights + Velux roof windows: £600-£1,500 each installed. Dormer adds full-height windows + extra m² floor. Choose carefully: en-suite for guest bedroom vs walk-in wardrobe vs extra storage. Resale value: en-suite + bedroom typically adds £20-£40k vs office £10-£20k.

Timeline and disruption. Project duration: 8-12 weeks typical. First 2 weeks: scaffolding, materials delivery. Weeks 3-6: roof opening, structural work, dormer construction. Weeks 7-10: insulation, plasterboard, plumbing, electrics. Weeks 11-12: finishing, fixtures, snagging. Lived-in during build: possible but disruptive — noise, dust, scaffolding access. Storage during build: clear loft first (cost £80-£200/month self-storage). VAT: 20% on labour and materials (most). Reduced 5% rate: only for empty properties (over 2 years). Most loft conversions are full 20% VAT.