Prefab extension kits in the UK
A prefab extension kit is the shell of a room, made to your drawings and delivered in panels: walls, roof and usually the floor. In the UK a kit for a 25 m² single-storey room costs £30,000 to £42,500 including VAT, which works out at £1,200 to £1,700 per m².
That figure buys the structure and nothing else. This page sets out what else you pay for, how small a kit can sensibly be, and how to tell a good kit from a cheap one.
Kit only, delivered
£1,200 to £1,700 per m²
Single storey, including VAT. Foundations, labour and finishes are extra.
A kit suits you if
- You have run building work before, or you have a builder you trust who will take the kit on.
- You can be at the house most days for six to twelve weeks.
- The room is a plain rectangle with a flat or single-pitch roof.
- Panels can be carried round the side, because the garden cannot take a crane.
- You would sooner pay trades directly than pay one firm to manage them.
What a kit costs
| What | Typical price including VAT |
|---|---|
| Small kit, 12 m²A utility room, study or porch, about 4 m by 3 m | £14,500 to £20,500 |
| Mid-size kit, 25 m²A kitchen and dining room, about 5 m by 5 m | £30,000 to £42,500 |
| Larger kit, 40 m²Full-width room on a detached house | £48,000 to £68,000 |
| Strip footings on firm groundDug and poured by a groundworker before delivery | £1,000 to £3,000 |
| Drawings, engineer and building controlNeeded whoever makes the panels | £2,300 to £4,800 |
Kit prices are supply only and national, including VAT; London and the South East usually cost 20 to 40% more. Small kits cost more per m² than large ones because design, delivery and the door openings cost much the same at any size. A guide, not a quote. Try your own size in the calculator
What the price leaves out
Read the quotation line by line. A typical kit includes wall panels, a roof structure, fixings, membranes and a set of assembly drawings. Many include the floor cassette. Windows and doors are sometimes in the price and sometimes a separate order.
Almost everything else is yours to arrange: foundations (£1,000 to £3,000 for strip footings on firm ground, more on clay), drains, the roof covering, external cladding or brick slips, the opening into the house and its steel, electrics, heating, plastering, flooring and decoration. Add those up and the finished room usually lands near the price of a panel-built job, £2,200 to £2,800 per m². What you save by buying a kit is the main contractor's margin, and you earn it by doing their work.
How small can a kit be?
Kit makers will build almost any size, but the sums change below about 12 m². A 3 m by 3 m room still needs a designer, an engineer, a delivery lorry and a door knocked through, so its cost per m² climbs well above the £1,700 top of our range. For a small room, ask two or three makers for a price on the same drawing and compare the total, not the rate.
Small kits do have one real advantage. In England a rear room up to 3 m deep behind an attached house, or 4 m deep behind a detached one, often needs no planning application (government technical guidance). Scotland and Wales set their own limits.
How to judge the best kit
Price per m² tells you little. These five things tell you more.
Ask for the U-values of the wall and roof panels in writing. In England a new extension wall should reach 0.18 W/m²K and a roof 0.15 under Approved Document L. A kit that misses those figures will need extra insulation on site, at your cost.
Ask whether the panels are open or closed. Open panels arrive as a timber frame with one face boarded, and your builder insulates them. Closed panels arrive insulated and lined, sometimes with windows fitted, so the shell is weathertight sooner.
Ask who signs off the structure. You need calculations from a chartered engineer that cover the panels, the roof and the fixings to the house. Building control will ask for them.
Ask what happens when something does not fit. A maker who sends a fitter for the first day, or will erect the shell for a fixed price, is worth paying a little more for.
Finally, check the firm. Membership of the Structural Timber Association shows a timber frame maker has been audited, and a product certificate matters for structural insulated panels.
Rules a kit still has to meet
A kit is an extension like any other in law. It needs building regulations approval, which means inspections of the footings, drains, structure, insulation and electrics. If you dig within 3 m of a neighbour's house, the Party Wall Act may require a written notice at least a month ahead.
Running the job yourself also changes who carries health and safety duties. On a home project those duties sit with the contractor, so make sure one named builder is in charge of the site.
Sources for this page
- Householder permitted development: the government's technical guidance (GOV.UK)
- Approved Document L on energy efficiency in homes (GOV.UK)
- How to get building regulations approval (GOV.UK)
- GOV.UK: Party walls and building work
- HSE: Construction work for domestic clients (CDM 2015)
- Structural Timber Association: member audit scheme
Have a kit drawing or just a size in mind? Send it and we will look for a firm to price it.
Get a quoteFrom order to a finished room
Drawings and approval
A designer draws the room, an engineer checks it, and you confirm whether planning is needed.
Order and footings
The maker takes three to six weeks to cut the panels. Use that time to dig, pour and lay drains.
Delivery and shell
One lorry, a small team, and the walls and roof stand in two to five days.
Trades and sign-off
Roofer, electrician, plasterer and joiner follow. Building control makes its last visit.
Kit questions
How much are prefab extension kits in the UK?
About £1,200 to £1,700 per m² for the kit alone, including VAT, so £30,000 to £42,500 for a 25 m² room. Finished, with foundations, trades and fittings, expect something close to £2,200 to £2,800 per m².
Can I put a kit up myself?
Two or three capable people can stand a single-storey panel kit. You may not do your own gas work, and electrical work must be certified. Most people who buy a kit pay a builder to erect it.
What are the best prefab extension kits in the UK?
There is no single best kit. The best one for your house has panel U-values that pass building regulations without extra work, engineer's calculations included, and a maker who will help on erection day. Judge it on those points and on the finished cost.
Do kits include windows and doors?
Some do. Others leave openings for you to fill. Glazing is often the most expensive single item after the panels, so check before you compare two kit prices.
Is VAT charged on an extension kit?
Yes, at 20%. The zero rate applies to building new homes, not to extending one (VAT Notice 708).
How long does a kit take to arrive?
Three to six weeks from signed drawings is typical for timber panels. Windows can take longer than the panels, so order them at the same time.
Keep reading
- Flat pack extensionsWhat arrives and how it goes together
- Cost calculatorKit against panel-built against module
- Prefab lean-to extensionsThe simplest shape to build from a kit
- Do I need an architect for an extension?Who can draw it and what they charge
Get a quote for a kit 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.