Extension Kits
The structural shell of an extension, made to your drawings and delivered flat, for you or your builder to put up.
- Wall panelsCut to your drawings and numbered
- Roof structureJoists or rafters, and the deck
- Sole plates and fixingsBrackets, straps, nails and screws
- Breather membraneAlready on most wall panels
- Structural calculationsFor the frame, from an engineer
- Assembly drawingsA plan showing where each panel goes
Not in a kit: foundations, the opening into the house, windows, roof covering, wiring and plaster. See the full list.
Get a quoteThe kit is the shell. You run the rest.
A kit supplier makes the walls, the roof structure and the calculations that prove they stand up. Groundworks, glazing, roofing, electrics and plastering are yours to organise, or your builder's.
That split is where the saving comes from: you take on the managing, and often the hammering too. If you would rather not, most suppliers will send a crew to put the frame up. Say so on the form.
What a kit costs, and what the room costs finished
About 5 m by 5 m
- The kit, delivered
- £25,000 to £35,000
- The extension, finished
- £55,000 to £70,000
Footings, glazing, roofing, services and finishes make up the rest: about £30,000 to £35,000 here.
A guide, not a quote. Figures are national; how they are worked out.
Three ways to frame it
| Kit | What arrives | Suits | Finished, per m² |
|---|---|---|---|
| Timber frame | Stud wall panels two people can carry | A first build, and anyone who may change their mind on site | £2,200 to £2,800 |
| SIPs | Insulated panels, structure and warmth in one piece | Tight plots, where a thinner wall buys floor space | £2,400 to £3,000 |
| Steel frame | Light galvanised frames, or posts and beams | Wide glazed openings and long spans | Priced per job |
From order to weathertight
Drawings and approval
Building regulations approval always, planning permission sometimes. The kit is made from the approved drawings, so these come first.
The base
Footings and slab go in while the kit is being made. The supplier sets how level and square it must be.
Delivery and frame
One lorry, as a rule. A single-storey shell is usually standing within a week.
Closing it in
Roof covering, then windows and doors. Only then is the house wall opened up.
Before you ask for prices
Can I put up an extension kit myself?
Yes. No law requires a builder for the structure, and a timber or light steel kit is designed for two competent people with ordinary tools. Gas work must be done by a Gas Safe engineer and new electrical circuits must be certified.
Do extension kits need planning permission?
Planning cares about the size and look of the room, not how it was built. Most rear kits in England are kept within permitted development: 3 m deep on a semi or terrace, 4 m on a detached house, finished to match. A lawful development certificate puts that in writing before the kit is made. Scotland has its own limits and always needs a building warrant.
Will a kit pass building regulations?
A kit is not approved in advance; your extension is. You apply, and the supplier's calculations and drawings go in with the application. Building control then inspects the work as it is built.
Is VAT included in kit prices?
Ask every supplier, because adverts differ. Extensions to existing homes carry VAT at 20%, and the refund scheme for DIY housebuilders covers new homes only.
Get a quote for your kit
A postcode, four short questions about the kit, then your phone and email. Most people finish in under a minute.
Extension Kits is a free introduction service, not a kit maker. We find a kit supplier that covers your area and send you its name first. Nothing is passed on until you say yes.
Thank you. Your request has reached us.
- We look for a kit supplier that delivers to your postcode and makes the system you chose.
- Within one working day we email or phone you with that supplier's name.
- If you are happy with them, we hand over your request and they price the kit. If not, nothing is passed on.