What it costs to finish a prefab extension kit

The kit is the walls and roof frame. Turning it into a warm, wired, plastered room joined to your house costs about as much again. This page goes through where that money goes, so the figure on the delivery note is never confused with what the finished room will cost.

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The gap, on a 4 x 3 m room

The kit: £14,500 to £21,500
Finishing it: £11,000 to £18,000
The finished room: £25,000 to £39,500

Typical national ranges for 2026. The finished figure includes VAT; kit quotes may not, so treat the middle figure, our own arithmetic (finished less kit), as rough. Work it out for another size

Foundations: the biggest unknown

A kit is light compared with brick, but it still needs footings taken down to firm ground and a slab or suspended floor on top. What the ground is like decides which kind, and the price spread is wide.

Foundation	When it is used	Effect on the budget
Strip footings	Good bearing soil with no roots nearby	The usual choice and the cheapest
Concrete raft	Weak or backfilled soil that needs the load spread	More concrete and steel mesh than strips
Piles	Clay that shrinks and swells, big trees, or deep poor soil	Designed after a ground investigation; can add several thousand pounds

The structural engineer chooses the foundation, sometimes after a soil test. Nobody can price it properly before that.

Before you order the kit, find out where your drains run and whether any trees stand close to the new wall. A drain under the footprint has to be diverted or bridged, and building control will expect deeper footings near trees on clay.

Everything else between kit and room

Assembly

Standing the panels, fixing the roof structure and making the shell weathertight. Some suppliers include it, some quote it separately, some leave it to you. Panels for anything over a small room are heavy enough to need a team of three or four.

Roof covering

Many kits arrive with the roof structure but not the finish. A flat roof needs a membrane such as EPDM or a GRP layer, plus flashing where it meets the house. A pitched roof needs battens and tiles or slates to match.

The opening into the house

Knocking through the old back wall needs a steel beam or lintel sized by an engineer, temporary props while it goes in, and making good on both sides. A standard beam supplied and fitted typically costs £1,200 to £2,500. On a kitchen extension this is often the single biggest trade cost after the foundations.

Windows and doors

If they are not in the kit price, they are a large separate order. Wide sliding or bifold doors carry long lead times, so order early.

Services

Every new circuit has to be certified as meeting Part P of the building regulations. Radiators or underfloor heating extend the existing system. A sink, WC or shower needs waste pipes with a fall to the drain.

Inside finishes

Plasterboard and skim, flooring, skirting, decoration, and the kitchen if there is one. This part is where homeowners most often save by doing the work themselves.

Fees

£600 to £1,200 for a structural engineer's drawings and calculations
£1,000 to £2,500 for planning drawings by an architect or designer, where you need them
About £700 to £1,100 for building control in England, including VAT; each council sets its own charges
£700 to £1,500 per neighbour for a party wall surveyor, where one agreed surveyor acts for both sides

A simple kit room may need little or no architect's time, because the supplier draws the kit. The engineer and building control are not optional. A party wall surveyor is only needed when the work falls under the Party Wall etc. Act and a neighbour does not consent; if each side appoints its own, you pay for both and the cost rises.

VAT

Building work on an existing home is standard rated at 20%. The finished-room figures on this page already include it. When you compare kit or trade quotes, check whether each includes VAT before you add them up.

Sources

Prices are typical national ranges for 2026 from published UK cost guides; building control charges come from council charge schedules for 2026/27. Rules come from GOV.UK.

Finishing questions

What is the cheapest way to finish a prefab kit?

Do the decorating and flooring yourself, keep the roof flat, put the kitchen or bathroom where the drains already are, and use one firm for groundwork and assembly so you are not paying two lots of travel and set-up. The structural work, roofing, electrics and gas are not jobs to cut corners on.

Can the kit supplier finish the job too?

Many offer to erect the kit, and some offer a full build to a finished room. Others sell supply only. Ask each one exactly where their work stops, because a quote that ends at a weathertight shell leaves you to find every trade after it.

How much contingency should I keep?

Keep 10 to 15% of the finishing budget aside. The surprises are almost always below ground: a drain where the footings need to go, softer soil than expected, or roots that push the foundation design to piles.

Get a quote, kit and finishing

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.