Timber frame extension kits
A timber frame extension kit is the structure of the room, made to your drawings and delivered flat on a lorry: wall panels, roof timbers, sheathing, membrane, fixings and the engineer's calculations. Everything else is yours to organise.
Kits are priced per project, and what each supplier includes varies, so there is no fair national figure per m². As a rule the kit is well under half the cost of the finished room, which typically runs at £1,400 to £2,400 per m² including VAT.
Who a kit suits
- You have a builder or carpenter you trust and want to buy the frame direct.
- You are a capable DIYer with a helper, a free fortnight and a single-storey design.
- You are managing the project yourself and hiring trades one by one to save a main contractor’s margin.
- Your builder normally works in block and wants the frame made off site so the build is not held up.
- The site is awkward, and small hand-carried panels are the only thing that will fit down the side passage.
Kit prices
| What | Typical price including VAT |
|---|---|
| Frame kit, deliveredPanels and roof structure; contents vary by supplier | Priced per project |
| Trench or strip footingsFirm ground, single storey | £1,000 to £3,000 |
| Finished room for comparison25 m², built and decorated by a contractor | £35,000 to £60,000 |
| Building control, single storey of 10 to 40 m²You apply, not the kit supplier | £700 to £1,100 |
About these figures
These are typical national ranges for 2026, including VAT. London and the South East usually cost 20 to 40% more. Kit prices vary with what the supplier includes; compare lists line by line. Read them as a guide, not a quote: only drawings and a look at the ground give a firm enough to price your job.
Want a kit priced from your drawings? Start with your postcode.
Get a quoteWhat is in the kit, and what is not
Nearly every kit includes the sole plates, external wall panels with their sheathing board, lintels and beams over openings, the roof structure (flat roof joists, cut rafters or trusses), roof deck, breather membrane, nails, straps and holding-down fixings, plus panel drawings and structural calculations for building control.
What varies: insulation, vapour control layer, windows and doors, internal stud partitions and the floor structure if it is suspended. Ask each supplier for a written list and line them up, because a kit that looks dearer may include more than the difference in insulation.
What is never in a frame kit: foundations and slab, drains, roof covering, cladding or brickwork, plasterboard, electrics, plumbing, plastering, flooring and decoration. Nor is anyone to put it up, unless you pay for the supplier's erection crew, which many offer as a priced extra.
Open panels or closed panels
Open panels are studs with sheathing on one face. They are light enough for two people to carry, cheap to make and forgiving to fit, and you insulate and line them on site. Most extension kits are open panel.
Closed panels leave the workshop with insulation, membranes and sometimes windows and service battens already fitted. They give better airtightness and a faster build, but they are heavy, need a crane or telehandler, and have to be kept bone dry until the roof is on. On a back-garden extension reached by a side gate they are often impractical.
Stick-building, where a carpenter cuts and nails the frame on site from loose timber, is the third route. It needs no factory lead time and suits odd shapes. You lose the workshop accuracy and you must commission your own structural calculations.
Can you build it yourself?
Putting up a single-storey open-panel kit is within reach of a careful DIYer who has built a shed or a garden room. The supplier's drawings number every panel; you set out and level the sole plate, stand the panels in order, plumb, brace and nail them, then fix the roof. With two people it takes three to five days.
The frame is the easy part. Foundations and drainage, the roof membrane, the junction with the house and the steel over the knock-through are where extensions leak, crack or fail inspection, and most self-builders pay trades for those. Electrical work in the new room has to be signed off by a registered electrician.
Two storeys is a different matter. Lifting first-floor panels and trusses needs plant, a scaffold and people who do it every week. Buy the erection service.
Approvals are your job
A kit supplier sells you a product. The planning position, the building regulations application, party wall notices and the completion certificate are the owner's responsibility. Submit a full plans application to the council's building control team or to a registered building control approver before you order, and send the supplier's calculations with it.
The inspector will want to see the excavations before concrete, the damp-proof course and sole plate, the frame and its fixings before it is covered, insulation, and the finished job. Book each visit; work covered up without inspection may have to be opened.
In Scotland the council has to grant a building warrant first, and for most timber frame work a structural engineer's certificate goes in with it.
Delivery, storage and the weather
Kits arrive on a rigid lorry or an articulated one, usually with a lorry-mounted crane that can set packs down a few metres from the road. Measure the route to the back garden. Panels are made to suit: a supplier told there is only a 900 mm side passage will keep every panel under 2.4 m long.
Do not take delivery until the slab and kerb are finished and dry-fit measured. Panels stored for weeks on a lawn swell and twist. If they must wait, stack them flat on bearers off the ground, covered but ventilated.
Get the roof membrane or a temporary sheet on within a day or two of the walls going up. Sheathing board and softwood tolerate a shower; they do not tolerate a fortnight of standing water.
VAT on a kit
Extensions to an existing house are standard-rated, so the kit and all labour carry VAT at 20%. The DIY housebuilders' refund scheme only covers new dwellings and certain conversions. Reduced or zero rates exist for narrow cases such as some work for disabled occupants; HMRC's Notice 708 sets them out.
From order to weathertight shell
Drawings to the supplier
Approved plans go to the frame maker, who returns panel drawings and calculations for you to sign off. Allow four to eight weeks for manufacture.
Slab and kerb
Foundations, slab and a level brick or block kerb, measured against the panel drawings before delivery is booked.
Delivery and erection
Sole plates fixed on a damp-proof course, panels stood in numbered order, braced, nailed and strapped down.
Roof and wrap
Roof structure, deck and membrane, then breather membrane on the walls. The kit is finished; the trades take over.
Questions we get asked
How much is a timber frame extension kit?
Each supplier prices from your drawings, and contents vary, so there is no fair national figure. Expect the kit to be well under half the finished cost of the room. Insulation and windows may or may not be included, so compare what each supplier lists.
How much does a kit save over a contractor build?
Less than the gap suggests. The kit is under half the finished cost; foundations, roofing, cladding, services and finishes make up the rest. Managing the trades yourself saves a main contractor's margin, as long as nothing goes wrong. See the cost calculator.
Is building regulations approval needed for a kit?
Yes, exactly as for any extension. The supplier provides structural calculations, but you make the application and book the inspections.
How long does a kit take to arrive?
Four to eight weeks from signing off the panel drawings is normal. Order once planning and building regulations approvals are in hand, and time the groundworks to finish just before delivery.
Can I get a kit with the cladding and windows?
Some suppliers offer a fuller package with windows, doors, insulation and cladding boards. The price per m² rises accordingly, and you still arrange foundations, roofing and everything inside.
Can I buy a kit for a two storey extension?
Yes, and suppliers make them routinely. Have the supplier’s crew put it up: first-floor panels and roof trusses need lifting plant and a scaffold, and that is not a weekend job.
More to read
Get a quote for a frame kit
Three short parts: a postcode, four questions about the job, your contact details. A minute of your time, and free.
We are an introduction service, not a builder. We look for one timber frame firm that covers your postcode, tell you which firm it is, and hand over your details only once you say yes.
That has reached us. Thank you.
Next, we look for a timber frame firm that takes on work at your postcode. Expect an email or a call from us within a working day, naming the firm.
The firm sees nothing of yours until you have agreed to it. After that they will want to see the house and any drawings before pricing. We never send marketing.