Prefab extension cost
A prefab extension built and finished by one firm costs £2,200 to £2,800 per m² including VAT. A kit on its own is £1,200 to £1,700 per m², and a factory-finished module is £2,100 to £3,300 per m². Set your size below to see a range for your room.
Prefab extension cost calculator
Panel-built, 20 m², one storey
£47,500 to £64,000
including VAT
Builders on site for 4 to 8 weeks.
A guide, not a quote. Your type and size go into the form with you.
Prefab extensions cost by size
Single storey, structure only: add foundations and fees from the next table.
| Floor area | Kit, panels only | Panel-built | Finished module |
|---|---|---|---|
| 12 m²About 4 m by 3 m | £14,500 to £20,500 | £26,500 to £33,500 | £25,000 to £39,500 |
| 20 m²About 5 m by 4 m | £24,000 to £34,000 | £44,000 to £56,000 | £42,000 to £66,000 |
| 30 m²About 6 m by 5 m | £36,000 to £51,000 | £66,000 to £84,000 | £63,000 to £99,000 |
| 40 m²About 8 m by 5 m | £48,000 to £68,000 | £88,000 to £112,000 | £84,000 to £132,000 |
Typical national ranges for 2026, including VAT, rounded to the nearest £500. London and the South East usually cost 20 to 40% more. A guide, not a quote.
Reading a prefabricated extension cost
The three routes are not priced on the same footing, and that trips people up. A kit price is for panels on a lorry. Somebody still has to dig footings, stand the panels, roof them, glaze them, wire them and plaster them. By the end, a kit usually costs about what a panel-built job does. The calculator assumes exactly that.
A module price normally includes standard groundworks and one day of crane hire. Awkward access changes it quickly: a larger crane, a road closure or a lift over the roof all add to the £400 to £1,500 that a simple crane day costs.
What moves a price within its range
Glazing comes first. A wall of sliding doors can cost more than the wall panels around it. Roof shape is next: a flat roof with one rooflight is cheapest, and a pitched roof tied into the house costs more. Then the finish inside, from a plastered shell ready for your own kitchen fitter up to a complete room.
Where you live matters too. London and the South East usually cost 20 to 40% more than the national figures. Tick the box in the calculator to apply it.
Costs the calculator leaves out
A new kitchen or bathroom, moving a boiler or a drain, landscaping the garden afterwards and somewhere to cook while the back wall is open. Keep a contingency of 10 to 15% of the build cost for what the ground or the old house reveals. If the ground needs piles, your engineer designs and prices them after a ground investigation, and they can add several thousand pounds. The party wall figure assumes one surveyor agreed by both sides; if your neighbour appoints their own, you pay for both.
The sources behind these prices
Rates per m², foundations, crane hire and professional fees are typical national ranges for 2026, including VAT. Planning fees are set by MHCLG from 1 April 2026. VAT treatment is from HMRC.
Extras, item by item
| Item | Typical price including VAT |
|---|---|
| Strip footings on firm ground | £1,000 to £3,000 |
| Concrete raft on softer ground | £3,000 to £6,000 |
| Piles, on clay or close to trees | Priced by your engineer after a ground investigation |
| Planning drawings by an architect or designer | £1,000 to £2,500 |
| Building control fee, single storey | £700 to £1,100 |
| Building control fee, two storeys | £900 to £1,300 |
| Structural engineer's calculations, single storey | £600 to £1,200 |
| Structural engineer's calculations, two storeys | £800 to £1,800 |
| Party wall surveyor, each neighbour, one agreed surveyor | £700 to £1,500 |
| Crane for a finished module, one day | £400 to £1,500 |
| Householder planning application, England | £548 |
| Prior approval for a larger rear extension, England | £249 |
Cost questions
What is a typical prefab extension cost in the UK?
For a single storey, £2,200 to £2,800 per m² panel-built or £2,100 to £3,300 per m² as a finished module, including VAT. A 20 m² panel-built room is £44,000 to £56,000, plus foundations and fees.
What does a prefabricated extension cost per square metre?
Kit only: £1,200 to £1,700 per m². Panel-built: £2,200 to £2,800 per m². Finished module: £2,100 to £3,300 per m². Two storeys: £1,700 to £2,900 per m² of total floor.
Why do prefab extensions cost as much as brick ones?
Because the shell is only part of the bill. Foundations, drains, glazing, the opening into the house, wiring and finishes cost the same whatever the walls are made of.
Is VAT included in these prices?
Yes. The tables show prices including VAT, and the calculator can show the total without it. Extensions to an existing home are charged at 20% (VAT Notice 708).
What pushes the price up most?
Glazing, ground conditions and location. Large sliding doors and piled foundations can each add several thousand pounds, and a London or South East postcode usually adds 20 to 40%.
How accurate is this calculator?
It gives a budget range from national averages. It cannot see your drains, your soil or your access. A firm that has visited will give a fixed price.
Keep reading
- Prefab extension kitsWhat a kit price covers
- Flat pack extensionsThe panel-built route
- 2 storey prefab extensionsCost per m² over two floors
- Prefab extensionThe three routes side by side
Get a quote for your extension
Postcode, four quick questions, then your contact details. It takes about a minute.
We are an introduction service, not a prefab extension builder. We search for a company that takes work near you and give you its name first. Your details stay with us unless you agree to the introduction, and asking is free.
That is everything. Your request is with us.
- We look for a firm that builds the kind of extension you chose and covers your postcode.
- We email or phone you with that firm's name. You decide whether they get your details.
- If you say yes, they arrange to see the house and price the work. We never send marketing.