Prefab Extension Kits
Factory-made extension shells in set sizes, with the price of each laid out below. Pick a footprint and we find a supplier who delivers to you.
Get a quote3 x 3 m
Kit £11,000 to £16,000
A home office or a utility room
4 x 3 m
Kit £14,500 to £21,500
A dining table for six by the doors
5 x 4 m
Kit £24,000 to £36,000
A kitchen with an island
6 x 4 m
Kit £29,000 to £43,000
Kitchen, table and a sofa
Seen from above against the back wall of the house, all to one scale. Kit prices are for the shell only, national: a guide, not a quote.
Panels are cut to order, so other sizes are no problem. Price any size
What comes on the lorry
A prefab kit is the shell of the room, made in a factory to your drawings: insulated wall panels, the roof structure, fixings and a set of assembly drawings. Most suppliers add windows and doors to the order. It arrives flat and goes up in days.
The concrete base, the roof covering, electrics, plaster and the opening into your house are not in the box. Budget for them from the start.
The kit is roughly half the bill
Take the 4 x 3 m size. Standing on its foundations, wired, plastered and joined to the house, the finished room costs a little under twice the kit. Both are national 2026 ranges; the finished figure includes VAT.
- Kit for a 4 x 3 m room
- £14,500 to £21,500
- Finished 4 x 3 m room
- £25,000 to £39,500
Most small kits need no planning application
English permitted development lets you add a one-storey room to the back of a house without applying, provided it sticks out no more than 3 m (attached houses) or 4 m (detached ones) and tops out below 4 m. So the two 3 m deep sizes fit behind almost any house. The 4 m deep sizes suit a detached house, or need the council's prior approval on a semi or terrace, which costs £249 from April 2026 and brings a decision within six weeks.
Want a bedroom above as well? Two floors bring different rules, covered on our two-storey kits page.
Flats and listed buildings fall outside these rules. Scotland draws its own lines, and there a building warrant must be granted before anyone digs. Every kit, wherever it goes, needs building regulations approval.
From order to a room you can use
Fix the size
Measure the back of the house and choose a footprint. The supplier draws the kit to suit your wall, doors and roof.
Get approval
Building control passes the design. Sort planning now too if the kit is deeper than the limits allow.
Pour the base
A groundworker digs and casts the foundations and slab while the panels are being made.
Build and finish
The shell goes up in days. Roofing, electrics, plaster and the opening into the house follow.
Before you order
Can I put a kit up myself?
Yes, if you have building experience and a second pair of hands, because the panels are heavy. A registered electrician must sign off the wiring and building control inspects the work as it goes. Most buyers pay a team for the assembly and the roof.
Which prefab extension kits are best in the UK?
The best kit is the one whose price covers the most. Line up quotes on the same footprint and check each for wall and roof insulation that meets building regulations, structural calculations, windows and doors, a delivery date and who unloads. A cheap kit that leaves those out only moves the cost elsewhere.
Are these prices fixed?
No. They are typical national ranges for 2026, worked out per square metre. Finishing costs in London and the South East usually land 20 to 40% above them. A supplier prices your kit from your own drawings.
Is there VAT on a kit?
Yes, at 20%. An extension to an existing home is standard rated, and a homeowner cannot claim the VAT back. Suppliers quote kits both with and without it, so check which before comparing.
Get a quote
Postcode, then four tap-to-choose questions on size and timing, then your name and number. Free, and done in a minute or two.
Prefab Extension Kits does not make kits. We match your footprint with one supplier covering your area, give you its name first, and only send your details when you give the word.
Thanks, that has come through.
- Next, we match your footprint to a supplier whose delivery area includes you.
- Before the close of the next working day you hear from us which supplier that is.
- If you are happy, it gets your answers and prices your kit. If not, nothing more happens.
No newsletters, no list, no calls from anyone else.