Modular extension cost
Pick a size and the details below for a budget built line by line, with fees and VAT. The rates come from published UK contractor and builder price guides, read November 2026, and every figure is listed further down. A guide, not a quote.
£42k to £66k
20 m² single storey, before VAT
Prior approvalThe council asks your neighbours first.
Price this size| Line | Low | High |
|---|---|---|
| Total |
Modular extension cost by size
Single storey, installed, standard to excellent finish, before VAT and fees.
| Floor area | Before VAT | With VAT |
|---|---|---|
| 10 m² | £21,000 to £33,000 | £25k to £40k |
| 15 m² | £31,500 to £49,500 | £38k to £59k |
| 20 m² | £42,000 to £66,000 | £50k to £79k |
| 25 m² | £52,500 to £82,500 | £63k to £99k |
| 30 m² | £63,000 to £99,000 | £76k to £119k |
| 40 m² | £84,000 to £132,000 | £101k to £158k |
| 50 m² | £105,000 to £165,000 | £126k to £198k |
Rates by finish
| Finish | Per m² |
|---|---|
| Standard finish uPVC or basic aluminium glazing, standard flooring and lighting | £2,100 to £2,500 |
| Good finish Aluminium sliders or bifolds, roof light, better floor and joinery | £2,500 to £2,900 |
| Excellent finish Large-format glazing, triple glazing, bespoke cladding and joinery | £2,900 to £3,300 |
| Two storeys Per m² of total floor, general extension guide | £2,000 to £3,200 |
Extras and fees
| Item | Typical cost |
|---|---|
| Strip foundations, firm ground | £1,000 to £3,000 |
| Raft, weaker ground | £3,000 to £6,000 |
| Piles, poor ground or near trees | from £6,000 |
| Crane for a one-day lift | £800 to £3,000 |
| Extra for difficult crane access | £1,500 to £3,000 |
| Architect or designer | £4,500 to £6,500 |
| Structural engineer | £1,500 to £2,500 |
| Building control | £600 to £1,200 |
| Party wall surveyor, per neighbour | £1,500 to £3,000 |
| Householder planning application, England | £548 |
| Prior approval for a larger home extension | £249 |
| Lawful development certificate, proposed | £274 |
How the calculator works
- Floor area times the per m² rate for your finish. Single-storey rates include standard strip foundations and a one-day crane lift.
- London and the South East add 20 to 30%, as published guides suggest.
- Raft or piled foundations replace the strip foundation with the difference added.
- Tight access adds the published extra for a bigger crane or longer lift.
- Fees cover a designer, a structural engineer and building control, at the low and high ends of published ranges.
- The planning fee follows the route the drawing shows for England. VAT is added at 20% to everything except the council fee.
Where these prices come from
Build rates per m², foundations, crane, professional fees and the London uplift: published UK contractor and builder price guides, read November 2026, which include a modular single-storey rate of £2,100 to £3,300 per m² for 2026. Planning fees: council fee schedules for England, 2026. Planning limits: MHCLG permitted development technical guidance. VAT: standard rate 20%.
Questions about modular extension costs
How much does a modular extension cost per m²?
Typical installed prices run £2,100 to £3,300 per m² before VAT for a single storey, including standard groundworks and the crane. Two-storey extensions run about £2,000 to £3,200 per m² of total floor because the foundations and roof are shared.
What does a 25 m² modular extension cost?
About £52,500 to £82,500 before VAT, from a standard to an excellent finish. Add VAT at 20% and about £7k to £10k for design, engineering and building control.
Is VAT charged on a modular extension?
Yes, at the standard 20% for an extension to an existing home. Reduced rates apply only in narrow cases, such as converting a building that has been empty for years. Most quotes are given before VAT, so check.
What is not included in a typical modular price?
Usually: design and planning fees, building control, a party wall surveyor, unusual foundations, moving drains, a new boiler or consumer unit if the house needs one, flooring and kitchen units if not specified, and landscaping afterwards.
Why do London prices run higher?
Labour, parking, access and crane permits all cost more. Published UK guides add 20 to 30% for London and the South East, which the calculator applies when you choose it.
How accurate is this calculator?
It gives a budget range from published figures, not a price for your house. A specialist needs to see the access, the ground and your drawings before quoting. Ask for quotes on the same specification so you can compare them.
Get prices for the extension you sized
Three short steps: where the house is, what you want to add, and how to reach you. About two minutes.
This is a free matching service, not a modular extension specialist. Once we have found a firm that builds in your part of the country, we tell you its name, and your details stay with us until you agree.
Thanks, your enquiry is in.
- We search for a firm that makes and fits the type of extension you picked, in your part of the country, then email or ring you with its name.
- Your details reach that firm only if you say yes.
- They look at the house and the access, then price the job. No marketing from us, ever.