How the calculator works
Building work starts from our range for a typical conversion of about 30 m², the middle half of every price five UK cost guides publish for that type. No government series prices loft conversions, so we show the guides' figures side by side in our loft conversion cost guide rather than pick one.
| Type | Building work, about 30 m² |
|---|---|
| Rooflight (Velux) | £22,500 to £37,500 |
| Dormer | £37,500 to £52,500 |
| Hip to gable | £47,500 to £60,000 |
| Mansard | £60,000 to £75,000 |
The data behind this table
The lower and upper quartile of every figure MyJobQuote, HomeOwners Alliance (Checkatrade's averages), the Federation of Master Builders, Rated People and Bark publish for each type, rounded to the nearest £2,500, read on 11 November 2026.
Size scales that range by floor area to the power 0.62, a curve fitted to Rated People's prices for the same dormer and hip to gable at several sizes: twice the floor area costs about 1.5 times as much. Extras and fees are the middle half of the published figures for each item, and the contingency is 10%. Heating uses our heat model, and the EPC line our EPC model. The working is all in js/loft-conversion.js.
We leave out what we cannot source: regional prices, raising the roof, lowering ceilings, decorating and furniture. For the planning rules and building regulations, read the planning section of the guide.
Frequently asked questions
How accurate is this loft conversion calculator?
It gives the range a typical job of your type and size falls in, not a quote. The building work is the middle half of the prices five UK cost guides publish, and those guides disagree widely, so a local builder can quote outside it. Get at least three itemised quotes for the same drawings.
What is not included in the estimate?
Decorating, furniture, raising the roof or lowering the ceilings below, and a planning application for anything but a mansard. Some builders include drawings and building control in their price and some do not, which is why the calculator shows them as separate lines.
Why is there no adjustment for my region?
We only adjust prices by region using official pay data, and we have not yet built that for building trades. The guides that mention region all say London and the South East cost more; Rated People puts labour there up to 20% higher.
How is the heating cost worked out?
From our heat model: the median gas use per square metre for a home like yours, scaled to the level our model measures for homes rated EPC band A or B, because a converted roof must meet current insulation standards. It is priced at the October to December 2026 Ofgem price cap for your heating.
Data sources
- MyJobQuote, Loft Conversion Cost (updated 21 October 2026): prices by type, fees, en-suite, stairs and time on site.
- HomeOwners Alliance, Loft conversion costs (undated): Checkatrade's averages by type and house type.
- Federation of Master Builders, Loft conversion cost: a breakdown of costs (3 September 2025): averages from member firm Marken Lofts, building control, en-suite and the 10% contingency.
- Rated People, How much does a new loft conversion cost in 2026?: prices by type and size, fees and time on site.
- Bark, How much does a loft conversion cost (undated): prices by type and en-suite.
- GOV.UK, fees for planning applications: a lawful development certificate costs half the planning fee, and fees rise each 1 April.
- Approved Document L, Volume 1, 2021 edition and 2026 edition: insulation limits.
- Heating from our heat model (DESNZ NEED 2026) at the Ofgem price cap for October to December 2026. All guides read on 11 November 2026.