Loft Conversion Cost Calculator

A loft conversion in the UK is priced per m² of new floor area, and the type of conversion sets the rate. A rooflight (Velux) job sits around £900 – £1,300 per m², a dormer around £1,400 – £2,100 per m², a hip-to-gable around £1,700 – £2,400 per m², and a mansard, the most involved, around £1,800 – £2,700 per m². An L-shaped dormer on a terrace sits around £1,600 – £2,300 per m² and usually adds the most floor area of the lot.

This calculator turns that rate into a ballpark for your own loft. Pick the type, enter the floor area, choose your region and finish level, and it splits the build cost across the trades so you can check a loft company's quote line by line instead of staring at a single number.

How the cost per m² changes with the type

The cheapest conversion keeps the roof as it is. A rooflight job at £900 – £1,300 per m² just adds a new floor, insulation, a staircase, rooflights and finishes inside the existing roof shape, so there is no structural rebuild. A dormer at £1,400 – £2,100 per m² builds a box out of the roof to win headroom and floor space. A hip-to-gable at £1,700 – £2,400 per m² rebuilds a sloping hip end as a vertical gable wall, usually alongside a rear dormer. A mansard at £1,800 – £2,700 per m² rebuilds most of a roof slope and gives the most space, which is why it costs the most and normally needs planning permission rather than falling under permitted development.

Region and finish move the figure as well. London and the South East run roughly a fifth above the Midlands baseline, while the North, Scotland and Wales run below it. Once the shell is up, the finish is one of the biggest drivers: better joinery, a bespoke staircase and a high-end bathroom push the whole range upward.

What loft quotes leave out

Most lofts are bought design-and-build from a specialist loft company, and the headline price on that quote rarely covers everything. Common exclusions are VAT, the structural engineer's calculations and drawings, building control fees, a party wall surveyor where steels bear onto a shared wall, a non-standard or space-saver staircase, decoration, flooring and blinds, and the ensuite. An ensuite shower room typically adds £3,000 – £7,000, often with a pumped waste or macerator because the loft sits above the soil stack. Tick the ensuite box in the calculator to fold that into the total.

A worked example

Take a 25m² dormer in the Midlands at a standard finish. At the dormer rate of £1,400 – £2,100 per m², the build cost lands in the range the calculator shows above the breakdown. Add an ensuite at £3,000 – £7,000, then add 10 to 15% contingency on top for the things that only appear once the roof is open: undersized existing joists, old wiring, a chimney in the way, or building control asking for extra fire protection. Enter your own type, size, region and finish in the calculator for a figure tuned to your project.