2 Storey Extension Cost
Set the width and depth of one floor, pick a finish and see the range for both floors together. Then ask for a quote with those figures already filled in.
£1,700 to £2,900 per m²both floors counted, national 2026 range including VAT
Two storey extension cost calculator
35 m² over two floors
£73,500 to £87,500
Drawn to scale, storeys at about 2.7 m. The dashed line is 3 m from the house: the deepest a two storey rear extension can go under permitted development in England.
- Structural engineer£800 to £1,800
- Building control£900 to £1,300
- Planning application, England£548
- Fees in all, often charged separately£2,248 to £3,648
Typical national ranges for 2026, including VAT. A guide, not a quote.
Where these figures come from
- Build rates: national ranges from published 2026 UK extension cost guides, two storey extensions with both floors counted, £1,700 to £2,900 per m², split here into three finish levels. Expect 20 to 40% more in London and the South East.
- Planning fee: Planning Portal, planning application fees (MHCLG fee table, 1 April 2026).
- Building control: council charge schedules for 2026/27, two storey extensions up to about 40 m². Councils each set their own charges.
- Structural engineer: published 2026 UK fee guides for two storey extensions.
Double storey extension cost by size
The table counts both floors. A 5 x 4 m footprint is 20 m² downstairs and 20 m² up, so 40 m² in all.
| Total floor area | Example footprint | Typical cost |
|---|---|---|
| 30 m² | 3 x 5 m on each floor | £51,000 to £87,000 |
| 40 m² | 4 x 5 m on each floor | £68,000 to £116,000 |
| 50 m² | 5 x 5 m on each floor | £85,000 to £145,000 |
| 60 m² | 5 x 6 m on each floor | £102,000 to £174,000 |
| 80 m² | 5 x 8 m on each floor | £136,000 to £232,000 |
National 2026 figures, VAT included. Jobs in London and the South East usually run 20 to 40% above them. A guide, not a quote.
What the rate per m² covers
The range is for a finished extension: foundations, frame or walls, floor and roof, windows and doors, insulation, wiring, plumbing, plastering and decoration. The low end is a plain box with standard windows. The top end has wide glazing below, an en suite above and better joinery throughout.
A kitchen is extra. So is knocking through the old back wall, which needs a steel beam sized by the engineer at about £1,200 to £2,500 per standard beam, supplied and fitted.
Costs people forget
- Structural engineer's drawings and calculations for two floors: £800 to £1,800.
- Building control: about £900 to £1,300, set by your council.
- Planning application in England: £548.
- A party wall surveyor if you dig or build close to next door: £700 to £1,500 for each neighbour when both sides share one surveyor, and more if they want their own.
- For a prefab frame, a crane: budget about £1,400 for a day's contract lift, meaning a 25 t crane and the crew who plan and sling each load. Many prefab firms build it into their price.
- Piles, if the engineer finds clay near trees or made ground. Nobody can give a fair national figure; on the sites that need them they run to several thousand pounds.
Prefab against brick and block
Finished modular and prefab extensions cost about £2,100 to £3,300 per m², which overlaps the traditional range. Do not expect a large saving. Expect fewer weeks with scaffolding and an open back wall, a frame that is square and well sealed, and less of the build at the mercy of rain. For a family living in the house through the work, that is the real gain.
Cost questions
Why does a two storey extension cost less per m² than a single storey one?
Because the expensive parts are shared. The foundations and the roof do double duty, so the rate for both floors counted together, about £1,700 to £2,900 per m², sits a little under a single storey extension. The total is still higher, because you are building twice the floor.
Is a prefab two storey extension cheaper?
Rarely by much. Finished modular and prefab extensions run about £2,100 to £3,300 per m², close to a traditional build. The gain is fewer weeks on site and less exposure to the weather, which matters most when the back of the house is open.
How much does a first floor over a garage cost?
About £45,000 to £75,000 for a room of roughly 25 m², national figures including VAT. The garage walls and foundations need checking by an engineer first; they often need strengthening to carry a bedroom.
Do I pay VAT on a two storey extension?
Yes, at the standard rate, for work on an existing home. The figures on this page include it. Builders quoting homeowners normally include VAT too, but ask if a price looks low.
Read next
- Planning for two storeys
When an upper floor needs an application, and the permitted development limits.
- Foundations and structure
What carries two floors, and where piles and steel come in.
- Two storey SIPs extensions
Insulated panels for both floors, and what a panel price leaves out.
Get a quote for your two storey extension
Start with the postcode. Four short questions about the extension follow, then your contact details.
We are an introduction service, free to you, and we do not build. Once we have found a firm near you that puts up two storey extensions, we tell you who it is, and your details stay put until you give the go-ahead.
Got it. Your enquiry has arrived.
- We search for a firm that puts up two storey prefab extensions in your part of the country, then get in touch to tell you who it is.
- Nothing goes to that firm unless you agree.
- The firm visits, looks at the ground and the access for a crane, and prices the job. We never send marketing.