Loft Conversion Cost Calculator

Loft conversion costs turn mainly on the type of conversion and on the roof structure you already have. A Velux or roof-light job is the cheapest at £20,000 to £35,000, since it adds roof windows without altering the shape of the roof. A dormer, which extends the roof vertically to win headroom, runs £35,000 to £60,000. Hip-to-gable work, which converts a hipped roof end into a vertical gable wall, costs £40,000 to £65,000, and a mansard, replacing the roof slope with a near-vertical wall and a flat top, comes in at £50,000 to £75,000.

Building Regulations approval is required for every loft conversion whatever its size, covering structure, fire safety with a protected staircase, fire doors and smoke alarms, insulation, means of escape and the design of the stairs. You apply either by Building Notice or by Full Plans. Planning permission is a separate question and arises only where you exceed Permitted Development, which allows 40 m³ on terraced houses and 50 m³ on detached and semi-detached ones.

Enter your roof type, the conversion you want and its size, and the calculator estimates the build cost, the additional items such as staircase, en-suite, electrics and plastering, and the professional fees, then checks whether the scheme is likely to sit inside Permitted Development. Rear-facing dormers usually do, provided nothing projects beyond the plane of the existing roof slope at the front and the materials are similar to what is already there. Side dormers and roof extensions facing the street need permission, conservation areas apply tighter rules, and listed buildings always need listed building consent.

Head height decides whether the loft is worth converting at all. The regulations require 2.0 m above the staircase, and a habitable room in England needs at least 2.3 m of headroom over 50% of its floor area, with Scotland set slightly differently. Trussed roofs, standard in British houses built after the 1960s, have to be structurally altered by replacing the trusses with a rafter and joist system, which adds £8,000 to £15,000. Older cut roofs are usually easier. Either way, the original ceiling joists are rarely strong enough to carry a habitable floor and need strengthening.

Measured per square metre, build costs land between £1,200 and £2,500 depending on complexity. That figure covers structural calculations, insulation, plasterboard, an en-suite and dormer windows, but not kitchen-grade fittings or statement features, which sit on top of it. A loft conversion typically adds 15 to 25% to the value of a house, which puts most British homes at break-even or in profit. What you do with the room changes that, since a bedroom with an en-suite tends to add £20k to £40k against roughly £10k to £20k for an office.

The standard layout is one bedroom with an en-suite shower. Adding a kitchenette or a wet room costs £3,000 to £8,000 more. Built-in storage in the eaves, where the ceiling drops below 1.5 m, is what keeps the usable part of the room from feeling cramped, and knee walls are normally built to 1.5 m for the same reason. Skylights and Velux roof windows cost £600 to £1,500 each installed, while a dormer buys full-height windows and extra floor area. An en-suite for a guest room, a walk-in wardrobe and plain storage all compete for the same square metres, so decide early.

Expect the work to take 8 to 12 weeks. Scaffolding and deliveries fill the first 2 weeks, weeks 3 to 6 go on opening the roof, structural work and the dormer, weeks 7 to 10 on insulation, plasterboard, plumbing and electrics, and weeks 11 to 12 on finishing, fixtures and snagging. Living in the house throughout is possible but noisy, dusty and awkward around scaffolding access, and clearing the loft first often means self-storage at £80 to £200 a month. VAT is charged at 20% on labour and materials for nearly all of this, the reduced 5% rate applying only to properties empty for over 2 years.