About this page: Home Extensions London is a free enquiry service run by Web Designer Limited (trading as Zoib). We pass enquiries to independent extension builders and design-and-build firms; we don’t carry out the work, and we can’t guarantee that every enquiry is taken up. Figures below are quoted from the named sources with their dates; they are not our prices. More about this site.
Published costs by type
We don’t set prices: we pass enquiries to independent builders. These figures are taken from Checkatrade’s loft conversion cost guide (table last updated March 2026, compiled with estimators who are RICS and CIOB members) for a typical 30 m² loft, as it read on 20 October 2026.
| Type (30 m² unless stated) | Average | Range | Typical build time |
|---|---|---|---|
| Rooflight (Velux) | £27,500 | £22,500 – £30,000 | 4–6 weeks |
| Dormer | £50,000 | £40,000 – £60,000 | 10–12 weeks |
| Hip-to-gable | £60,000 | £50,000 – £70,000 | 10–12 weeks |
| Mansard | £65,000 | £50,000 – £80,000 | 8–10 weeks |
| Modular | £45,000 | £30,000 – £60,000 | 2–3 weeks |
| Shell only (structure, no finishes) | £30,000 | £22,500 – £37,500 | 8–10 weeks |
| Bungalow (50 m²) | £75,000 | £62,500 – £87,500 | 10–12 weeks |
The HomeOwners Alliance loft conversion guide quotes the same Checkatrade averages and gives property-type averages of about £50,000 for a semi-detached house, £62,500 for a detached house and around £50,000 for a dormer on a terraced house.
Cost per square metre
Checkatrade’s per-m² averages are about £920 for a rooflight conversion, £1,670 for a dormer, £2,000 for hip-to-gable, £2,170 for a mansard, £1,500 for modular and £1,000 for a shell. HomeOwners Alliance gives a wider band of about £1,250 to £2,500 per m², and explains that the area counted is usually the usable floor area with headroom of 1.5 m or more. That is why a small loft with little full-height space can be expensive per m².
Where the money goes
Checkatrade’s example breakdown for a £50,000, 30 m² dormer conversion (last updated August 2026):
| Item | Average |
|---|---|
| Project management | £4,000 |
| Scaffolding (excluding temporary roof) | £4,750 |
| Internal preparation | £7,000 |
| Alterations to roof structure | £5,000 |
| Steelwork | £6,000 |
| Roof coverings | £8,250 |
| Electrics and plumbing | £3,250 |
| Joinery (excluding staircase) | £4,500 |
| Staircase | £2,750 |
| Plastering | £2,750 |
| Decoration | £1,750 |
Loft conversion cost in London
Neither guide publishes a London-only table. Checkatrade says London homeowners should expect to pay towards the upper end of the UK range for each type, and HomeOwners Alliance says location has a major impact in London because contractors charge more and moving materials is harder. On a typical London terrace, add the practical factors: scaffolding over a pavement usually needs a licence from the borough under section 169 of the Highways Act 1980, there may be two adjoining owners for Party Wall purposes, and conservation-area rules can push you from a dormer to a design that needs planning permission. See loft conversions in London for the rules.
Extras and contingency
- Architect or technologist drawings: Checkatrade suggests about £1,000 to £2,000.
- Structural engineer’s calculations, Building Regulations charges, any planning or lawful development certificate fee, and Party Wall surveyors.
- A bat survey if one is needed (Checkatrade quotes about £400 to £750).
- An en-suite, built-in storage, flooring and decoration if they are outside the builder’s price.
- Contingency: Checkatrade suggests planning for about 10–15% on top of the initial quote.
Turning ballparks into a real quote
Ask each loft company to price the same drawings, to list provisional sums separately, to say whether VAT is included, and to state who handles Building Regulations and Party Wall notices. Link staged payments to completed work. Our page on choosing an extension builder has a fuller checklist. When you are ready, the enquiry form lets us pass your loft conversion enquiry to an independent builder who covers your area.
Questions people ask
How much does a loft conversion cost in 2026?
Checkatrade’s 2026 guide puts most UK loft conversions between about £27,500 and £75,000, with averages of £27,500 for a rooflight conversion, £50,000 for a dormer, £60,000 for hip-to-gable and £65,000 for a mansard (30 m² examples).
How much does a loft conversion cost in London?
There is no published London-only table. Checkatrade says London prices tend towards the top of each UK range, and HomeOwners Alliance says location has a major impact in London. Get several itemised quotes for your house.
What is the cheapest type of loft conversion?
A rooflight (Velux) conversion, which keeps the existing roof shape. Checkatrade gives an average of about £27,500 for 30 m², but it also creates the least full-height space.
How long does a loft conversion take?
Checkatrade suggests most take about 6 to 10 weeks on site once approvals are in place; its table gives 4–6 weeks for a rooflight conversion and 10–12 weeks for a dormer or hip-to-gable.
Sources
- Checkatrade – Loft conversion cost: 2026 UK price guide & breakdown
- HomeOwners Alliance – Loft conversion costs and how to reduce them
- Highways Act 1980, section 169 (scaffolding on the highway)
- GOV.UK – Permitted development rights for householders: technical guidance
Sources checked on 20 October 2026. General information only, not legal, planning, structural or financial advice.
Related pages
- Extension builders in London
How to shortlist, check and compare extension builders in London before you sign anything. - Kitchen extension cost
Published 2026 figures for kitchen extension costs, what quotes usually include and leave out, and the London premium. - Loft conversions in London
Velux, dormer, hip-to-gable and mansard lofts: permitted development limits, Building Regulations, Party Wall and London issues.