Flat pack extensions
A flat pack extension reaches your house as a stack of numbered panels on the back of a lorry. A small team bolts them together on foundations that are already in place. Bought as a kit, expect £1,200 to £1,700 per m² for the panels. Built and finished by one firm, expect £2,200 to £2,800 per m².
Flat pack is also called a pre built or ready made extension, though the name oversells it. The parts are ready made. The room is not.
Built and finished
£2,200 to £2,800 per m²
Single storey, including VAT. On site for 4 to 8 weeks.
Flat pack works well when
- The only way to the garden is a side path or the front door.
- Your street is too narrow, or too busy, to park a crane for a day.
- You want a pitched roof, an L-shape or a room that wraps a corner.
- You live a long way from the factory and haulage for a whole module would be costly.
- You want the walls up fast and are relaxed about a few weeks of finishing.
Flat pack extension prices
| What | Typical price including VAT |
|---|---|
| Panels only, per m²Delivered to the kerb, nothing fitted | £1,200 to £1,700 per m² |
| Built and finished, per m²Foundations to decoration by one firm | £2,200 to £2,800 per m² |
| 15 m² room, built and finishedAbout 5 m by 3 m | £33,000 to £42,000 |
| 30 m² room, built and finishedAbout 6 m by 5 m | £66,000 to £84,000 |
| Concrete raft on softer groundWhere strip footings will not do | £3,000 to £6,000 |
Typical national ranges for 2026, including VAT. London and the South East usually cost 20 to 40% more. A guide, not a quote: only a firm that has seen your house can price it. Try your own size in the calculator
What comes off the lorry
The delivery is smaller than people expect: the wall panels for a single-storey room make a modest stack. With them come floor cassettes, roof cassettes or trusses, a pallet of fixings and tapes, and a drawing that shows where each numbered piece goes.
Panels are light enough for two people to carry, or four for the long ones. That is the whole point of flat pack: it goes where a crane cannot.
Three kinds of panel
Open timber panels are a stud frame with board on the outer face. Insulation, a vapour layer and plasterboard are fitted on site. They are the cheapest to buy and the slowest to finish.
Closed timber panels leave the factory insulated and lined on both faces, often with service channels and windows in place. The shell is weathertight within days.
Structural insulated panels sandwich rigid foam between two boards. They are thinner than a timber wall for the same warmth, which gains you a few centimetres of floor on a tight plot.
How long assembly takes
Standing the panels is quick: two to five days for a single-storey room. The weeks either side are what set the programme. Footings and drains take one to two weeks and must be accurate, because factory panels will not forgive a slab that is out of level. After the shell come the roof covering, the opening into the house, wiring, plaster and floors. Altogether, builders are on site for 4 to 8 weeks, against 10 to 16 for brick and block.
Flat pack or a finished module?
A prefab extension can also arrive as a completed room, craned over the house in a morning. That route has the shortest spell on site, 2 to 6 weeks, but it needs clear space for a crane and a road wide enough for the load. A crane day costs £400 to £1,500.
Choose flat pack if access is tight or the shape is unusual. Choose a module if the room is a simple box, the access is open and you want the mess over quickly.
Planning and building control
The council treats a flat pack room as it treats any extension. In England many single-storey rear extensions fall within permitted development, with a 4 m height limit and a 3 m limit on eaves within 2 m of a boundary (Planning Portal). Outside walls should look similar to the house, so plan for brick slips, render or cladding the council will accept.
Building control inspects on site as usual. Give the inspector the maker's structural calculations and panel specifications at the start.
Tight access at the back? Tell us how you reach the garden and we will look for a panel firm.
Get a quoteHow a flat pack extension is built
Measure and design
A survey of the back of the house fixes the sizes the factory will cut to.
Make the base
Footings, drains and slab go in while the panels are made.
Stand the shell
Panels are carried through, fixed down and roofed in under a week.
Finish inside and out
Cladding, the opening to the house, services, plaster and floors.
Flat pack questions
How much does a flat pack extension cost?
Panels alone cost about £1,200 to £1,700 per m² including VAT. A flat pack extension built and finished by one firm costs about £2,200 to £2,800 per m², so £44,000 to £56,000 for 20 m². Try your own size in the cost calculator.
Is a flat pack house extension cheaper than brick?
Usually not on the build price: brick and block is typically £1,800 to £3,000 per m², so a finished flat pack room costs about the same or a little more. Flat pack saves weeks on site, which cuts scaffold and skip hire and the time you live with an open wall.
Can a flat pack extension kit be built by a DIYer in the UK?
The shell, yes, with help and care. Foundations, electrics, gas and the structural opening into the house need qualified people, and building control must approve the work.
How long does a flat pack extension last?
Timber frame houses in the UK are designed for a life of 60 years or more, and a panel extension built to the same standard should match the house. Keep the cladding and roof edges in good repair so the frame stays dry.
Does a flat pack extension need foundations?
Yes. Most sit on strip footings, at £1,000 to £3,000 on firm ground, or on a raft where the soil is poor. Piles on clay or near trees are priced by your engineer after a ground investigation.
Keep reading
- Prefab extension kitsBuying the panels on their own
- 2 storey prefab extensionsPanels over two floors
- Cost calculatorPut your own size in
Get a quote for a flat pack 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.
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- 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.
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