Free calculator · no account required
Loft conversion cost calculator for UK builders.
Size it, check it against published averages, then price it from your own rates.
Quick answer
Pick the loft type, enter the floor area and see what published UK averages suggest homeowners pay. Checkatrade's 2026 guide puts a 30 m² dormer at around £50,000, or £1,670 per m². Then enter your own build cost per m², preliminaries, contingency, markup and VAT to see your price and how it compares. The average is a sense-check. Your price comes from your own rates.
Source note: Averages come from Checkatrade's "Loft conversion cost: 2026 UK price guide & breakdown" (type table last updated March 2026, breakdown October 2026). VAT points come from HMRC VAT Notice 708 (last updated 26 August 2026) and GOV.UK's VAT rates page. Planning limits come from the Planning Portal. Figures checked 22 October 2026. Checkatrade calls them ballpark averages and doesn't say whether they include VAT. The per-m² figures are Checkatrade's, rounded by Checkatrade. The calculator scales Checkatrade's 30 m² averages to your floor area and rounds to the nearest £100. That scaling is our arithmetic.
Loft conversion cost calculator
Use square metres and the loft type to rough out the job before doing a full quote.
Published average for this size
£50,000
Published range
£40,000 to £60,000
Checkatrade national averages (March 2026), scaled to your m². What homeowners typically pay, not your cost. VAT treatment not stated.
Want the working file?
Enter your email, then download the CSV. The result stays visible above.
Published averages by loft type
Checkatrade's figures for a typical 30 m² loft (table last updated March 2026):
| Loft type | Average (30 m²) | Range | Per m² (Checkatrade, rounded) | Weeks on site |
|---|---|---|---|---|
| Rooflight (Velux) | £27,500 | £22,500 to £30,000 | £920 | 4 to 6 |
| Dormer | £50,000 | £40,000 to £60,000 | £1,670 | 10 to 12 |
| Hip-to-gable | £60,000 | £50,000 to £70,000 | £2,000 | 10 to 12 |
| Mansard | £65,000 | £50,000 to £80,000 | £2,170 | 8 to 10 |
| Modular | £45,000 | £30,000 to £60,000 | £1,500 | 2 to 3 |
| Shell only | £30,000 | £22,500 to £37,500 | £1,000 | 8 to 10 |
Checkatrade also gives £75,000 for a 50 m² bungalow loft (range £62,500 to £87,500) and notes that London and the South East are often at the upper end. These are national averages. They don't know your roof, your access or your rates.
Where the money goes on a dormer
Checkatrade's breakdown for a 30 m² dormer (last updated October 2026) is a useful checklist for the sections of your quote:
| Section | Checkatrade average |
|---|---|
| Project management | £4,000 |
| Scaffolding (excludes 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 (excludes staircase) | £4,500 |
| Staircase | £2,750 |
| Plastering | £2,750 |
| Decoration | £1,750 |
| Total | £50,000 |
Price each of these lines from your own labour, materials and subcontractor rates. If one of your lines is far from the published figure, check the scope rather than copying the number. To lay the sections out as a client quote, use the loft conversion quote template.
Check planning and VAT before you price
Planning: in England, the Planning Portal says a loft conversion can be permitted development if it stays within limits. These include an added volume of no more than 40 cubic metres for a terraced house, or 50 cubic metres otherwise, and no extension beyond the existing roof slope on the principal elevation facing a highway. If the design is bigger, or the house is on designated land, planning permission is needed. That changes the programme and fees.
VAT: HMRC VAT Notice 708 says work to an existing building is normally charged at the standard rate, which is 20%. A reduced rate of 5% can apply in specific cases, such as converting premises into a different number of dwellings (section 7). A loft room in the same house usually isn't one of them, but check the notice for your job. For quote wording, see VAT quote wording.
Keep contingency and margin separate
Checkatrade tells homeowners to plan for a contingency of around 10% to 15% on top of the quote, because older roofs can need more strengthening once they're opened up. That's the client's buffer. Your contingency covers the risks you price in. Your margin comes from your own overheads. Keep them on separate lines so a buffer isn't mistaken for profit. The construction margin calculator shows the difference between markup and margin.
Primary sources
- Checkatrade, "Loft conversion cost: 2026 UK price guide & breakdown". Page marked "Updated 03/26"; the type table is marked "Last updated: March 2026" and the dormer breakdown "Last updated: October 2026".
- HMRC, "Buildings and construction (VAT Notice 708)". Last updated 26 August 2026. Section 7.1.1.
- GOV.UK, "VAT rates" (standard rate 20%).
- Planning Portal, "Loft conversion: Planning permission" (England, Class B permitted development limits; the page shows no date).
FAQ
How much does a loft conversion cost per m² in the UK?
Checkatrade's 2026 guide gives about £920 per m² for a rooflight conversion, £1,670 for a dormer, £2,000 for hip-to-gable and £2,170 for a mansard. They're national averages based on a 30 m² loft, so use them to sense-check, not to set your price.
How much does a dormer loft conversion cost?
Checkatrade puts a typical 30 m² dormer at around £50,000, with a range of £40,000 to £60,000 (table last updated March 2026). The guide doesn't say whether that includes VAT.
Do I charge VAT on a loft conversion?
Usually at the standard rate. HMRC VAT Notice 708 says work to an existing building is normally standard-rated. The 5% rate covers specific conversions, such as changing the number of dwellings. Check the notice for your job.
Can this calculator replace a site visit?
No. It sizes the job and compares it with published averages. Roof structure, access, steel and stairs all need checking on site before you send a price.
Does a loft conversion need planning permission?
Not always. In England, the Planning Portal says many loft conversions are permitted development within limits, including 40 cubic metres of added volume for a terraced house and 50 cubic metres for others. Checkatrade notes that loft conversions must still meet Building Regulations even when planning permission isn't needed.
Turn the rough number into a real quote.
Put in your own labour, materials and subcontractor rates for each section, add overhead, contingency, markup and VAT, and see a quote outline. It's free and you don't need to sign up to see the result.
Build a quote with your own rates