Loft Conversion Cost Calculator

Count only the area with adequate head height under the finished ceiling — the space under the low eaves is storage, not floor area, and including it flatters the cost per unit of area.

This rate covers what turns a roof void into a room: insulation between and under the rafters, linings and skim, floor finish, decoration, lighting and power, and heating. It does not cover the four items with their own fields below — the structural floor, the stair, the dormer or rooflights, and the fire works — and pricing those twice here is the commonest way a loft budget is overstated. PER SQUARE METRE, AND IT STAYS THERE WHEN THE PAGE SWITCHES SYSTEM. This field carries no dimension, so the unit toggle never touches it, while the floor area above it does convert. Set the page to imperial and you are typing a US-market area into one field and a rate with a metric denominator into this one. So convert the rate before you type it: multiply a square-foot rate by 10.7639 to get the per-m² figure this field wants, and divide by 10.7639 to go back. A 176.52 square-foot rate is the same money as 1900 per m². Typing 176.52 straight in is accepted in silence — it sits well inside this field's 0 to 100,000, so nothing is clamped and nothing is flagged — and it understates the shell by the full factor. On this page's own example figures (38 m² of usable floor, 20% contingency) the shell line falls from 72,200 to 6,708 and the all-in total from 120,480 to 41,889, a 65% under-report of the whole job. The breakdown's all-in cost per unit of usable floor is the one line that does follow the toggle: metric shows it per m², imperial restates it into square-foot terms.

Existing ceiling joists carry a ceiling, not people and furniture. A conversion needs new floor joists, usually spanning wall to wall clear of the old ones, and often steel beams to carry them. This is structural work with engineer's calculations, and it is not optional.

The stair has to land somewhere, and that somewhere is a room on the floor below. Reconfiguring that room, moving a door, and losing part of it to the new flight is a real cost — and on some houses it is the constraint that kills the whole scheme.

Rooflights are dramatically cheaper than a dormer and add no floor area. A dormer adds usable head height across its width and costs several times as much — it is the largest single variable in a loft budget.

Adding a storey usually triggers a requirement for a protected escape route: fire doors to every room off the stairway, upgraded ceilings, interlinked alarms and an escape window. It is a code requirement, not a specification choice, and it is routinely omitted from early budgets.

Design, the structural engineer, building control and any planning application — everything that happens before anyone is on site. A loft conversion needs more of this than its size suggests: the new floor, the steels and the escape route are engineered rather than standard, and a shared wall adds a party wall surveyor on top.

A percentage held back for what opening the roof reveals. Ten per cent is light for a loft — the state of the existing timbers, whether the floor below takes the new loading, and whether the roof is cut or trussed are all unknown until work starts, and a trussed roof changes the structural scheme entirely. This is the one line worth being pessimistic about, because the alternative is stopping mid-job.