Loft conversion cost calculator for London
What a loft conversion costs to build in London in 2026, by type — Velux, rear dormer, L-shaped dormer, hip-to-gable or mansard — with or without an en-suite. Pick a specification and get a low–high range, then add the professional fees for the whole-project figure.
Guide prices, London, ex VAT, 2026. Not a quote. Build cost only — no fees, VAT or contingency. Ranges come from published London loft prices listed underhow we worked out these figures.
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The five types of loft conversion, and what they cost
Velux or rooflight — about £30,000–£50,000
Windows in the existing roof slope, a new floor, insulation and a staircase. The cheapest route because nothing structural is rebuilt, but it only works with enough existing head height.
Rear dormer — about £40,000–£75,000
The rear slope is rebuilt as a full-height wall with a flat roof, giving standing height across most of the floor. The standard answer on a London terrace, and often permitted development.
L-shaped dormer — about £65,000–£95,000
A rear dormer that wraps over the back outrigger of a Victorian or Edwardian terrace — more roof to rebuild, and usually room for a double bedroom plus en-suite.
Hip-to-gable with rear dormer — about £55,000–£90,000
For semis and end-terraces with a sloping side roof: the hip is rebuilt as a vertical gable and combined with a rear dormer.
Mansard — about £60,000–£110,000
The rear roof is replaced with a near-vertical new storey. The most space and the most structure, and usually a full planning application.
What sits outside this calculator
Professional and statutory fees (drawings, planning or a Lawful Development Certificate, structural engineer, party wall, building control), VAT, and a contingency — older London roofs often need strengthening that only shows up once they are opened. Therenovation fees calculator adds the fees; for the build context read extension and loft conversion costs in London and party wall agreements.
How we worked out these figures
For each type we combine two published London loft price guides (one West London, one North London, both ex VAT): the low end is the lower of the two published lows and the high end the higher of the two published highs. The specification setting does not add any new figures — it positions you in that published range (basic: lower half; standard: the middle half; high: upper half). The en-suite range combines both sources’ en-suite figures. Totals are rounded to the nearest £1,000. Sources checked on 27 September 2026.
- GSM Builders — Loft conversion cost West London 2026 — Ex VAT: Velux £35,000–£50,000; rear dormer £50,000–£75,000; L-shaped dormer £65,000–£90,000; hip-to-gable + rear dormer £60,000–£90,000; mansard £75,000–£110,000+. Includes a mid-range bathroom/en-suite. En-suite adds £6,000–£12,000.
- The Extension Company — Loft conversion cost London 2026 — North London, ex VAT and fees: Velux £30,000–£45,000; rear dormer £40,000–£75,000; hip-to-gable + rear dormer £55,000–£90,000; L-shaped dormer £65,000–£95,000; mansard £60,000–£100,000. En-suite fit-out £4,000–£10,000 on top.
- Planning Portal — Fees for Planning Applications in England (from 1 April 2026) — Householder application £548; Lawful Development Certificate (proposed) = half the full application fee; larger home extension prior approval £249.
One of the two sources already includes a mid-range bathroom in its loft prices, so the top of a range plus an en-suite may overstate slightly. Guide prices, London, ex VAT, 2026. Not a quote.
Common questions
How much does a loft conversion cost in London in 2026?
Published London prices, ex VAT and before fees, run at roughly £30,000–£50,000 for a Velux or rooflight conversion, £40,000–£75,000 for a rear dormer, £55,000–£90,000 for a hip-to-gable with rear dormer, £65,000–£95,000 for an L-shaped dormer and £60,000–£110,000 for a mansard. Where your job lands in the range depends mostly on size, specification and access.
What is the cheapest type of loft conversion?
A Velux or rooflight conversion, because the roof shape does not change — windows go into the existing slope, and the work is insulation, a new floor, a staircase and finishes. It only works if you already have enough head height; if you do not, a dormer is usually the next step up.
How much does a dormer loft conversion cost in London?
A rear dormer is published at around £40,000–£75,000 ex VAT in London, and an L-shaped dormer that extends over the rear outrigger of a Victorian or Edwardian terrace at around £65,000–£95,000. Both are often permitted development, within the volume limits.
How much does a mansard loft conversion cost?
Around £60,000–£110,000 ex VAT in London on published figures. A mansard rebuilds the rear roof slope almost vertically, so it gives the most space and involves the most structure, and it usually needs full planning permission — it is often the answer in conservation areas where a dormer would be refused.
How much does an en-suite add to a loft conversion?
London builders publish en-suite fit-outs at around £4,000–£12,000 on top of the conversion, depending on sanitaryware, tiling and whether the hot water system needs upgrading. Some published loft prices already include a mid-range bathroom, so check what a quote covers before adding it twice.
Do I need planning permission for a loft conversion?
Many rear dormers, hip-to-gables and rooflight conversions are permitted development, subject to volume and design limits and to conservation area or Article 4 restrictions. Even then, a Lawful Development Certificate — £274 for proposed work in England from April 2026 — gives you proof the work was lawful when you sell. Mansards usually need a full householder application (£548). Building Regulations approval is always needed.
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