Advanced options
Check FIRST: you need ~2.2m from the top of the existing joists to the ridge for a workable conversion (measure before you dream). Rooflight conversions are the budget route where height allows; dormers buy headroom and floor space; mansards are near-rebuilds of the roof. Most lofts convert under permitted development, but building regs always apply: fire doors, escape windows, insulation and a proper staircase (the stair steals space from the floor below: plan where it lands).
What The Loft Conversion Cost Calculator Works Out
- Build cost: loft floor area multiplied by the rate for the conversion type, plus GBP 6,000 for an en-suite. Rooflight conversions run around GBP 1,500/m2 where the ridge is generous, the ubiquitous rear dormer GBP 2,000 to GBP 2,400, hip-to-gable about GBP 2,500 for semis, and the mansard, effectively a new near-vertical roof, close to GBP 3,000.
- Realistic total: build cost plus 20% VAT plus about 15% for structural calculations, building control and contingency. A typical dormer with bathroom lands in the GBP 50,000 to GBP 75,000 band.
- The head-height gate: about 2.2 m from joist top to ridge is the workable minimum. It takes five minutes with a tape and decides whether the job is standard, dormer-dependent, or the expensive exotica of roof raising and ceiling lowering.
- VAT and fees (advanced): override the 20% and 15% assumptions, for example if a quote already includes design fees, or to test a 10% contingency against the 15% default.
- Roof structure (advanced): post-1960s trussed-rafter roofs (the W-shaped webs) are convertible but need steel beams and structural rework, typically adding around 15% to the structural cost. Older cut-timber roofs convert obligingly.
- Location (advanced): London and South East labour rates add roughly 20% to the same specification.
- Eaves storage and party wall (advanced): fitted eaves cupboards and wardrobes are the compensating craft for the storage a conversion removes, and terraces and semis sharing structure usually need party wall notices at about GBP 1,500 per neighbour where a formal award is required.
- Cost per m2 all-in (advanced): the grand total divided by the loft floor area, the honest figure to compare against other ways of adding space.
- Versus a ground-floor extension (advanced): the same floor area built outward at a mid-spec GBP 2,700/m2 with the same VAT and fees. The loft usually wins on cost per added square metre and always wins on keeping the garden.
- Rules that always apply: most lofts convert under permitted development, but Building Regulations require a fire-protected escape route and doors, structural calculations for the new floor, current insulation standards and a compliant staircase whose landing footprint downstairs is the design problem owners discover last.