Free UK cost estimates
Know the cost before you call a builder.
Extensions, loft conversions and garden rooms, priced the way a quantity surveyor prices them: line by line, with fees and contingency included.
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What are you planning?
Extensions
Typically £44,400 to £87,500 all-in
Single and two-storey, kitchen, side return and wrap-around.
Costs, planning and quotes →Loft conversions
Typically £35,000 to £95,900 all-in
Velux, dormer, hip-to-gable and mansard.
Costs, planning and quotes →Garden rooms
Typically £16,500 to £41,100 all-in
Insulated offices, studios and gyms, fitted and wired.
Costs, planning and quotes →Two checks before you build
Do you need planning permission?
Answer a few questions and see whether your project is likely to be permitted development in England, with the reasons.
Check my project →QuotesIs your builder's quote fair?
Compare a quote with a QS estimate for the same job, and see what's missing before you sign.
Check a quote →Typical UK costs in 2026
Building work alone, then all-in with design fees and a 10% contingency. National baseline for a standard finish, including VAT.
Price your own project| Project | Building work | All-in |
|---|---|---|
| Single-storey extension, 20 m² | £48,000 | £59,200 |
| Two-storey extension, 36 m² over both floors | £71,000 | £87,500 |
| Velux (roof light) loft conversion | £29,400 | £35,000 |
| Dormer loft conversion | £65,100 | £77,300 |
| Hip-to-gable loft conversion | £75,100 | £89,200 |
| Mansard loft conversion | £80,000 | £95,900 |
| Insulated garden room, 15 m² | £21,700 | £24,600 |
Cross-checked against published averages from Checkatrade loft conversion costs, MyJobQuote house extension costs, The Garden Room Guide.
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Read exactly how it worksCost guides
All guidesThe first guides are being written. Until then, the calculator covers every project type.
Who works these out
Written by a quantity surveyor
The guides are written by a quantity surveyor who prices building work for a living. They break costs down the way a QS does, so you can see what drives the price before you speak to a builder.
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