Loft conversion cost London 2026 — dormer vs mansard vs rooflight, real ranges

Loft quotes scatter more than any other project's, because 'loft conversion' covers four structurally different jobs. Price the roof form, not the word: a rooflight conversion and a mansard differ by a factor of three on the same house.

Cost by roof form

Rooflight / Velux conversion (no volume change)	£45,000–£75,000
Rear dormer (the standard terrace job)	£75,000–£120,000
Hip-to-gable + rear dormer (semis)	£90,000–£140,000
Mansard (full roof rebuild)	£110,000–£180,000+
Per square metre, across forms	£3,000–£4,500 /m²

Inner London, 2026 — build cost, typical total project

Where the money goes

Structure: new floor joists sized for habitable load, steels into the party walls, and — on mansards — rebuilding the roof itself.
The stair: the single hardest design problem. A well-resolved stair costs space and money; a badly resolved one costs the first floor its best room.
Bathroom: an en-suite adds £8,000–£15,000 with the soil-stack run often deciding the layout.
Fire strategy: a three-storey house needs a protected stair enclosure, fire doors and interlinked alarms — built into the budget, not bolted on.
Conservation spec: natural slate, lead cheeks and timber sashes on a conservation-area mansard add 15–25% over standard materials.

The non-build budget

Design + structural engineering	£4,000–£10,000
Planning application or Lawful Development Certificate	£100–£300
Party wall (both neighbours on a terrace)	£1,800–£4,500
Building control + warranty	£1,000–£2,000
VAT	20%

On top of the build cost

Value: the bedroom-bracket effect

Lofts earn their keep by moving a house up a bedroom bracket — three-bed to four-bed is the classic London jump, and sold-price data shows it clearly on family streets. The effect is strongest where the new room is a genuine principal bedroom with en-suite, weakest where headroom compromises make it a 'study with a bed in it'. Compare converted and unconverted sold prices on your street before deciding the spec.

The Siteline area report shows real Land Registry sales for your postcode — see what converted houses actually fetch before you commit.

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How much does a loft conversion cost in 2026?

In inner London: £45,000–£75,000 for a rooflight conversion, £75,000–£120,000 for a standard rear dormer with en-suite, and £110,000–£180,000+ for a mansard — before VAT and fees. Outer London runs 10–20% less.

Which loft type adds the most value?

The mansard adds the most floor area and, on high-value terraces, the most value. The rear dormer is the best cost-to-value ratio on most streets. The rooflight conversion is the cheapest entry but often fails the 'real bedroom' test that drives the uplift.

How long does a loft conversion take to build?

Ten to fourteen weeks on site for a dormer, longer for a mansard — plus design and consent time up front (a Lawful Development Certificate in 4–8 weeks, or a full application in 8–12). Scaffold and a temporary roof mean it's far less disruptive than a rear extension; most families stay in residence.

Do I need my neighbour's permission?

Not their permission — but on a terrace you need party wall awards, because the new steels bear on shared walls. Serve notices two months before works; budget for surveyors if a neighbour dissents.

Loft conversion costs in London: dormer vs mansard