Two-storey prefab extensions
A two-storey prefab extension adds a room downstairs and one upstairs using factory-made panels, SIPs or stacked modules. The shell goes up far faster than blockwork, which matters most when scaffolding is up and the back wall of the house is open.
Expect around £2,000 to £3,200 per m² of total floor before VAT. The second floor costs less per m² than the first because foundations and roof are shared.
Two-storey extension
£2,000 to £3,200 per m²
Per m² of total floor, before VAT
Questions to settle first
- Is the extension at least 7 m from the rear boundary? Under that, it needs planning permission in England.
- Will it stay within 3 m of the original rear wall, or is it larger?
- Can the roof pitch match the house, as permitted development requires?
- Will upper side windows be obscure glazed and fixed below 1.7 m?
- What steelwork does the house need where the new rooms open into it?
- Is there access for a crane or panel delivery, and room for scaffolding?
Two-storey extension prices
| Item | Typical price, ex VAT |
|---|---|
| Two-storey extension, per m² Total floor area across both storeys, standard to excellent | £2,000 to £3,200 per m² |
| 3 x 5 m footprint, two storeys 30 m² in total, a typical permitted development size | £60,000 to £96,000 |
| 4 x 6 m footprint, two storeys 48 m² in total, usually needing planning permission | £96,000 to £153,600 |
| Structural engineer Calculations for steels, foundations and the panel frame | £1,500 to £2,500 |
| Party wall surveyor Per neighbour, where notice is needed and they dissent | £1,500 to £3,000 |
Where these figures come from
Typical ranges from published UK contractor and builder price guides, read November 2026, before VAT at 20%. Two-storey rates are from a general extension price guide; no separate published figure exists for modular two-storey work, so treat it as a starting point. A guide, not a quote: the price for your house depends on a survey, the ground and the specification. Work out your own figure with the cost calculator.
Planning a room upstairs as well? Tell us the footprint and we will find a specialist to price it.
Ask for a free quoteWhat permitted development allows upstairs
In England, an extension of more than one storey can be permitted development only if it goes no more than 3 m beyond the original rear wall and stays at least 7 m from the boundary opposite. The roof pitch should match the house as far as practical, and upper-floor side windows must be obscure glazed and non-opening below 1.7 m. There is no prior approval route for two-storey extensions. These limits come from the government technical guidance.
Most two-storey extensions on semis and terraces therefore need a householder planning application, which councils usually decide in about eight weeks.
Panels, SIPs or stacked modules
Timber-frame and SIPs panels are the usual choice: the ground floor walls, first floor cassette, upper walls and roof go up in a few days. Stacked volumetric modules work for detached houses with crane access, but joining two boxes to an existing house at two levels takes careful structural design.
Whatever the system, the existing house needs work where it meets the new rooms: steel beams over the openings, and sometimes new foundations under the old wall if loads change. Your structural engineer designs this.
Costs and time
Per m², a two-storey extension is cheaper than a single storey because the foundations and the roof serve twice the floor. Allow £2,000 to £3,200 per m² of total floor before VAT, plus design and engineering. On site, the shell is often up in under a week; the full job usually takes eight to twelve weeks, compared with four to six months for a traditional two-storey build.
How a two-storey prefab build runs
Survey and design
Measured survey, planning drawings and the structural scheme for both floors.
Planning and party walls
Householder application if over the limits; notices to neighbours in good time.
Foundations and shell
Foundations, then both storeys and the roof erected in days and made weathertight.
Break through and finish
Steels in, openings formed, services, plaster and finishes, then sign-off.
Questions people ask
How much does a two-storey prefab extension cost?
Around £2,000 to £3,200 per m² of total floor area before VAT, so a 3 x 5 m footprint on two floors costs about £60k to £96k, plus design and engineering fees.
Can a two-storey extension be permitted development?
Yes in England if it extends no more than 3 m beyond the original rear wall, is at least 7 m from the rear boundary, and meets the other conditions. Many semis and terraces fall outside this and need planning permission.
Are 2 storey prefab extension kits available in the UK?
Yes, timber-frame and SIPs suppliers sell two-storey kits. They still need foundations, structural engineering for the connection to the house, and qualified trades to finish. Most owners use a firm that erects and finishes too.
How long does a two-storey prefab extension take?
The shell often goes up in a week. The whole build is usually eight to twelve weeks on site, depending on the steelwork and finishes.
Do I need a party wall agreement?
If you dig foundations within 3 m of a neighbour’s building and deeper than theirs, or build on a shared wall, you must serve notice: at least one month for excavation and two months for party wall works (GOV.UK booklet).
Get prices for a two-storey extension
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.