How much does a dormer loft conversion cost in 2026?

Indicatively £1,900–£2,600 per square metre of finished floor area in London for construction alone. On a 28 square metre room that is £63,000–£91,000 once fees, statutory costs, contingency and VAT are included.

Is a loft conversion cheaper than an extension?

Usually yes, per square metre, because there are no foundations and no drainage. A loft is also more likely to be permitted development. The saving disappears if your head height or roof structure is wrong.

How long does a loft conversion take?

On site, indicatively 8–12 weeks for a dormer. End to end from first drawing to finished room, closer to 28 weeks once design, the lawful development certificate, party wall and tender are included.

Cost8 minute read

How much does a loft conversion cost in 2026?

Cost by type of conversion, a worked example with every line, and the two checks to do before you budget anything.

The short answer

Expect £63,000–£91,000 all in for a 28 square metre rear dormer loft conversion in London, or £48,000–£70,000 outside the South East. That is construction, professional fees, statutory costs, contingency and VAT. A rooflight conversion in a loft that already has the head height starts nearer £40,000. A mansard on a London terrace can pass £130,000.

01Cost by type of conversion

Rooflight only, head height already there	£1,400–£1,900 per sqm
Rear dormer	£1,900–£2,600 per sqm
Hip to gable, with a rear dormer	£2,100–£2,900 per sqm
Mansard	£2,600–£3,600 per sqm

Indicative construction rates for London in 2026, measured over the finished floor area, and excluding professional fees and VAT. Outside the South East, take roughly 20% to 30% off. A loft costs less per square metre than an extension because there are no foundations.

The type is not a style choice. It is set by your roof shape, your head height and whether the council will accept the change to the roof line.

02A worked example: 28 square metres in south London

Construction, 28 sqm at £1,900–£2,600	£53,200–£72,800
Structural engineer and drawings	£2,500–£4,500
Lawful development certificate	£300–£900
Building control	£800–£1,400
Party wall award, one neighbour	£900–£3,200
Contingency at 10%	£5,770–£8,280
All in, rounded, VAT included	£63,000–£91,000

Indicative. The construction line is the dormer rate above applied to 28 sqm. Contingency is 10% of everything above it. The lines add to £63,470–£91,080, rounded here. Domestic work is standard rated, so VAT at 20% is included in every line. Furniture and window dressing are excluded, and the party wall range assumes one neighbour, an agreed surveyor at the low end and two surveyors at the high end.

A quote from a loft company will usually read close to the construction line alone. The gap here is £9,800–£18,300, and none of it is optional.

03The two checks worth doing before you budget anything

Both take an afternoon, both are free, and either one can end the project. Do them before you pay an architect or believe a per-square-metre figure.

—Head height. Measure from the top of the existing ceiling joist to the underside of the ridge. Below about 2.4 metres, you lose the room to insulation and floor build-up, and the fix is lowering the ceilings below or raising the roof. Either one changes the budget by a third.
—Roof structure. Look for a web of thin diagonal timbers rather than a clear void. That is a trussed roof, common after 1965, and converting it means a new structural floor and steels before you gain a single square metre.

04What actually moves the number

—Escape route. A loft conversion needs a protected route to the front door, which usually means fire doors and upgraded plasterboard on the landings below. Indicatively £2,500–£6,000, and it is often missing from an early quote.
—En-suite. A shower room in the loft needs a soil connection, a pump and a route for it. Indicatively £6,000–£14,000 fitted.
—Dormer size and cladding. Lead or zinc cheeks cost several times a rendered or tile-hung finish, on the same opening.
—Steels. Beams have to be craned or carried up and bear on the party walls. Awkward access is charged for.
—Rooflights. A conservation rooflight is indicatively double a standard one, and a conservation area usually requires it.
—Insulation route. A warm roof keeps the rafter depth shallow and protects head height. It costs more and it is often the reason the room works.

Every figure in this list is indicative, at 2026 London supply and fitting levels. They show which decisions carry the weight, not what your job costs.

Common questions

How much does a dormer loft conversion cost in 2026?: Indicatively £1,900–£2,600 per square metre of finished floor area in London for construction alone. On a 28 square metre room that is £63,000–£91,000 once fees, statutory costs, contingency and VAT are included.
Is a loft conversion cheaper than an extension?: Usually yes, per square metre, because there are no foundations and no drainage. A loft is also more likely to be permitted development. The saving disappears if your head height or roof structure is wrong.
How long does a loft conversion take?: On site, indicatively 8–12 weeks for a dormer. End to end from first drawing to finished room, closer to 28 weeks once design, the lawful development certificate, party wall and tender are included.

Rules described here apply in England and Wales, and were last checked on 29 August 2026. Permitted development limits and fees change. Check your council’s current guidance before you rely on a figure.