How much does a loft conversion cost in London?

There is no single average that helps, because the type sets the bracket. The most common London job, a rear dormer on a terrace, sits at £55,000–£80,000 across the 2026 guides we compiled, or £60,000–£92,000 with an en-suite. A Velux conversion starts around £30,000, a hip-to-gable with a dormer runs £63,000–£93,000 and a mansard £75,000–£120,000. Fees, sanitaryware and any redecoration below come on top.

Not as a habitable room at 2026 London prices, according to every published guide we found. The typical Velux range starts at £30,000, and the starting figures some companies advertise below that describe a bare room in a roof that already has the height, without an en-suite and before the fire-door and alarm work on the floors below. Boarding and a ladder for storage is a different job and a different budget.

A Velux, or rooflight, conversion in a roof that already has the head height, with the stair continuing the existing flight, no en-suite, and the work kept within permitted development so there is no planning application. Every one of those choices removes cost, and the last three apply to a dormer too. What does not save money is skipping building control: a conversion without a completion certificate becomes a problem when you sell.

Per square metre of new room, usually the loft, because the walls, roof and floor structure already exist. Published London guides put a single-storey extension at £2,650–£4,500 per square metre to build before the kitchen and glazing, and a loft at £1,800–£2,700 per square metre on the thin evidence available. The better question is which space the house needs: a loft adds bedrooms, an extension adds living space. Our house extension cost guide covers the other side.

In England, since 25 April 2024, unauthorised building work becomes immune from planning enforcement only after ten years; work substantially completed before 25 April 2024 still falls under the old four-year limit (gov.uk planning enforcement guidance, checked September 2026). Check the position with your council before relying on it. It is not a strategy. A buyer's solicitor will ask for the planning paperwork and the building control completion certificate, and Building Regulations approval is a separate matter with no such cut-off for a conversion that was never inspected.

Not straight away. In England the Valuation Office Agency does not normally re-band a house while you own it; a property that has increased in size may move up a band when it is next sold, and a self-contained annexe can trigger a change sooner (gov.uk, council tax band changes, checked September 2026). So a conversion that adds a bedroom and a bathroom is usually the buyer's band to inherit rather than yours to pay, but check the position with the VOA or your council if the loft becomes a separate unit.

London guides rarely price the stair separately, because it cannot be separated from the fire-protection work it triggers: the new flight, the opening in the floor below, the protected enclosure, fire doors to the rooms off the stairwell and interlinked alarms. What moves the figure is position. A stair that continues the existing flight and lands under the ridge is the cheapest; one that turns, needs a bedroom reconfigured or lands under the slope is not.

Head height discovered to be marginal after the floor goes in, party wall notices served late and the two-month period pushing the start date, fire strategy on the floors below that nobody budgeted for, an en-suite planned where no soil pipe can reach, condensation from insulation fitted without ventilation, and planning refused in a conservation area for a dormer that would have been permitted development a street away. Almost all of them are avoidable at survey stage.

Usually not for a rear dormer on a house. Permitted development allows 40 cubic metres of roof extension on a terrace and 50 on other houses, provided nothing extends beyond the roof slope on the principal elevation facing the road, the dormer sits back from the eaves and the materials look similar. Mansards and front dormers almost always need permission, and there is no permitted development for flats, in conservation areas or under an Article 4 direction (Planning Portal, loft conversions, checked September 2026). Confirm with your borough, or apply for a lawful development certificate.

From London cost guides published in 2026 by loft companies, architects and planning consultants, principally Alstruct, houseUP, Mayfair Studio, Buildaway, LoftCompare, Draw and Plan and PlanWatch, with Advance Architecture, Structural Engineer Calcs, Proficiency and MyJobQuote for the derived rows. For each type we took the median of the published lows and of the published highs and rounded to the nearest thousand pounds. Fees are the boroughs' and the Planning Portal's own published schedules. They are market ranges, not MO Team quotes.