Extension kit cost

The figures behind the guide

The guide uses two national ranges. A flat-pack extension kit, supply only, is £1,000 to £1,400 per m² of floor: £25,000 to £35,000 for a 25 m² room. That range is not split by system, so the kit figure stays the same whichever type you pick.

A finished extension, with every trade paid, is £2,200 to £2,800 per m² in timber frame and £2,400 to £3,000 in SIPs. We have found no national guide that publishes a finished figure for steel frame, so the guide shows none.

These are typical ranges drawn from published UK builder and contractor price guides. They are a guide, not a quote. A supplier prices from your drawings, and your quote may fall outside the range.

The other half of the bill

The kit comes to a bit under half of the finished cost. Some of the rest can be priced in advance:

Not every job needs every line. A party wall surveyor is only needed if you build at the boundary or dig near a neighbour's footings and the neighbour does not consent. Planning fees only apply if the extension is not permitted development, and a lawful development certificate is optional proof that it is.

We do not print figures for the opening into the house, windows and doors, roof covering, electrics, heating, plastering or floor finishes, because they depend on your choices more than on floor area. They are all inside the finished per m² range above.

Kit against brick and block

The same guides put a finished brick and block extension at £1,800 to £2,200 per m². That is lower than timber frame and SIPs. A kit does not make the building cheaper to produce: factory panels cost more than blocks and mortar.

What changes is who does the work and for how long. A bricklayer builds the walls of a 25 m² room in two to three weeks, weather allowing. A kit stands in days, in any weather, and can be put up by you.

Where a kit saves money

Your own labour. Putting up the frame yourself removes the largest single labour item from the structure. Insulating and boarding an open panel kit is also within reach of a careful amateur.

No main contractor. If you hire the groundworker, roofer, electrician and plasterer directly, nobody adds a margin for managing them. You are paying for that with your evenings instead.

Time on site. Scaffolding, skips and a portable toilet are hired by the week. A structure that is weathertight in a fortnight needs them for less time.

A kit does not save anything on the ground, the opening into the house or the fit-out. Brick, timber or steel walls make no difference to those bills.

VAT

Work to extend an existing home is standard-rated, so 20% is added to the kit and to every VAT-registered trade. The refund scheme for DIY housebuilders does not cover extensions. Price guides and adverts are not consistent about whether VAT is in the figure. Ask each supplier to quote including VAT and delivery, so the numbers you compare are the numbers you would pay.

London and the South East

All figures here are national. Guides suggest adding 20 to 40% in London and the South East, where labour costs more. A kit is made in a factory and its price moves less with your postcode than the trades that follow it, though delivery distance does count.