Prefab extensions
A prefab extension is made in pieces in a factory, then put together on your foundations. Most are timber-frame wall, floor and roof panels that arrive on one lorry and stand up in a few days, already insulated and with the window openings cut.
Installed, they usually cost £2,200 to £2,800 per m² before VAT. Supply-only kits for a 25 m² room run £25,000 to £35,000, with the groundwork, building and finishing on top.
Installed prefab extension
£2,200 to £2,800 per m²
Factory timber panels, single storey, before VAT
When prefab panels make sense
- Your house has no side access, so whole modules would need a big crane over the roof.
- You want a shell up and watertight in days, then a builder or yourself to finish inside.
- The design is not a simple box: an L-shape, a wraparound or a pitched roof joined to an existing one.
- You want factory-made walls with predictable insulation and airtightness, without the cost of a finished module.
- You are happy to manage the build, and a supply-only kit could save on the main contractor’s margin.
- A lean-to, porch, boot room or garden-side utility room where a crane would be out of proportion.
Prefab extension prices
| Item | Typical price, ex VAT |
|---|---|
| Kit, supply only, 25 m² Timber, steel or oak frame panels delivered; no groundworks, labour or finishes | £25,000 to £35,000 |
| Installed, factory timber frame Per m² of floor, panels erected, roofed, glazed and finished inside | £2,200 to £2,800 per m² |
| Installed, 20 m² single storey A 5 x 4 m kitchen or family room extension | £44,000 to £56,000 |
| Strip foundations Firm ground, single storey; raft or piles on clay or near trees cost more | £1,000 to £3,000 |
| Design, engineer and building control Drawings, structural calculations and inspections | £6,600 to £10,200 |
Where these figures come from
Typical ranges from published UK contractor and builder price guides, read November 2026, before VAT at 20%. Kit prices are supply only, for a 25 m² single-storey kit. 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.
Tell us the size and the access at the back, and we will find a prefab specialist to price it.
Ask for a free quoteKit or fully installed?
A supply-only kit looks cheap at £25,000 to £35,000 for 25 m², but it is only the frame and panels. Foundations, drainage, the roof finish, windows, electrics, plastering and the opening into the house are all extra, and you or your builder carry the risk of getting them right. By the time a kit is finished, most owners spend a similar amount per m² to a fully installed panel build.
A fully installed prefab job is priced as a whole: survey, drawings, foundations, erection, weathering, first and second fix, finishes and sign-off. You have one firm responsible for the joints between factory work and site work, which is where most problems with extensions start.
A middle route works well for many people: the specialist designs, makes and erects a watertight, insulated shell, and a local builder you trust does the groundwork and the inside.
Panel systems compared
Open-panel timber frame is the most common. It arrives as studwork with sheathing on one side; insulation and services go in on site. Closed-panel timber frame arrives insulated, membraned and sometimes with windows fitted, which saves days on site and costs a little more. SIPs are solid sandwich panels of rigid foam between two boards, quick to erect and very well insulated, typically £2,400 to £3,000 per m² installed.
Steel-frame kits are less common for homes but useful where long clear spans or a flat roof with a slim edge are wanted. Oak-frame kits sit at the top of the price range and are usually chosen for the look.
Planning and building regulations
A prefab extension is judged like any other. In England, a single-storey rear extension up to 3 m deep (4 m on a detached house) can be permitted development, and up to 6 m (8 m detached) with prior approval from the council. Materials must be similar in appearance to the house, which usually means brick slips, render or cladding that the council accepts as similar. The government technical guidance sets out every limit.
Building regulations apply in full. The kit maker should supply structural calculations and U-values for their panels, but the building control body inspects the foundations, drainage, the junction with the house and fire separation from the boundary on site.
How long it takes
Panels are usually made in four to six weeks after the drawings are signed off. On site, foundations take a week or two, the shell goes up in two to five days, and the whole job is often finished in four to eight weeks, against ten to sixteen for brick and block. The shorter time on site is the main saving: less disruption, a kitchen out of action for less time, and fewer weeks of site costs.
How a prefab extension goes up
Survey and drawings
Measured survey, design, structural calculations and the planning route. Panel drawings are made from these.
Foundations while panels are made
Strip, raft or screw-pile foundations and drainage go in during the factory weeks.
Erect the shell
Floor cassettes, wall panels and roof go up in two to five days and are made weathertight.
Finish and sign off
Opening into the house, electrics, plumbing, plaster, flooring, then building control completion.
Questions people ask
How much does a prefab extension cost in the UK?
Fully installed, a factory timber-frame extension typically costs £2,200 to £2,800 per m² before VAT, so a 20 m² room comes to about £44k to £56k. Supply-only kits for 25 m² run £25,000 to £35,000 before labour, foundations and finishes.
Are prefab extensions cheaper than brick?
The build cost is similar once the foundations, roof, glazing and finishes are included. The real saving is time on site, which reduces disruption and some site costs. A supply-only kit can be cheaper if you can manage the build well.
Do prefab extensions need foundations?
Yes. Most sit on strip foundations (£1,000 to £3,000 on firm ground), a concrete raft or screw piles. Light panel systems sometimes allow smaller foundations, but the engineer and building control decide.
Do prefab extensions need planning permission?
Not always. They follow the same limits as any extension: in England, up to 3 m deep on a semi or terrace and 4 m on a detached house as permitted development, more with prior approval. See the table on our modular extension page.
How long do prefab extensions last?
A well-detailed timber-frame or SIPs extension built to current building regulations is designed to last as long as a masonry one. What shortens its life is poor detailing: water getting into the frame at the base, the roof edge or the joint with the house. Ask for the warranty terms in writing.
Can I build a prefab extension kit myself?
Many kits are sold to self-builders. You still need building regulations approval, a structural engineer’s sign-off and qualified trades for electrics and gas. Erecting the panels usually needs two or three people and sometimes a small crane.
Get prices for a prefab 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.