Timber frame extension kits

Kit, supply only, per m²

£1,000 to £1,400£20,000 to £28,000 for a 20 m² room

Finished extension, per m²

£2,200 to £2,800£44,000 to £56,000 for the same room

National guide figures per square metre of floor. A guide, not a quote.

Open panel or closed panel

Almost every timber kit sold for extensions is open panel. Each panel is a frame of studs with a sheathing board on the outside and a breather membrane over that. The inside is left open, so you can see every stud, run cables and pipes through them, then insulate and line the wall yourself. Two people can carry a panel and stand it up without machinery.

Closed panel kits leave the factory with the insulation in and both faces boarded, sometimes with the windows already fitted. They go up faster and reach weathertight sooner. They are also heavy enough to need a crane or telehandler, they cost more, and they leave you no room to fix a base that is a little out of level. On a single-storey extension with ordinary access, open panel is usually the better buy.

What the frame sits on

The panels are nailed to a treated timber sole plate, and the sole plate is fixed to your base over a damp-proof course. Keep the plate at least 150 mm above the finished ground outside, so rain splashing off the patio never reaches timber. That usually means two or three courses of brick or block between the footing and the plate.

The base is your job, not the supplier's, and the kit is cut to the drawing rather than to what was poured. Get the supplier's tolerance for level and square in writing and hand it to whoever pours the base. Then check both diagonals with a tape on the finished base, before anyone fixes a delivery date. A base that is 20 mm out can be packed. One that is 50 mm out of square means recutting panels.

The outside skin, planning and fire

A timber frame can wear almost anything: a brick outer leaf, brick slips on a carrier board, render on a board, fibre cement or timber cladding. The choice affects two sets of rules.

The first is planning. In England an extension only counts as permitted development if its outside materials are similar in appearance to the house. Larch boards on a red brick semi will not pass that test, so you would need a householder application, currently £548. A brick skin or brick slips matched to the house keep you inside the permitted development route, provided the size limits are met too.

The second is fire. A wall within 1 m of the boundary has to resist fire from both sides and can have only very small openings, and combustible cladding there is tightly limited. In practice the boundary wall of a timber kit gets a non-combustible board and a brick or render finish, even if the garden side is clad in timber. Tell the supplier where the boundaries are before the kit is drawn.

Insulation and the 0.18 target

In England, the new walls of an extension have to reach a U-value of 0.18 W/m²K, the roof 0.15 and the floor 0.18. A 140 mm stud packed with mineral wool does not get to 0.18 by itself. Kits reach it by using rigid foam board between the studs and adding a continuous layer across the inside face, often as insulated plasterboard, which also covers the studs so heat cannot track through the timber.

Ask the supplier for a U-value calculation for the whole wall, including your chosen cladding, and hand it to building control with the application. If insulation is not in the kit price, get the thickness and type in writing so you can buy the right boards.

What a timber frame kit costs

National guides put a flat-pack extension kit, supply only, at £1,000 to £1,400 per m² of floor. A finished timber frame extension, with everything done, runs at £2,200 to £2,800 per m². So on a 20 m² room the kit is £20,000 to £28,000 of a £44,000 to £56,000 job. These are national figures. In London and the South East, expect 20 to 40% more on labour-heavy work.

The kit price guide works this out for any floor area and shows what the rest of the money goes on.

Five checks before you pay a deposit

• Who did the engineering. The frame calculations should carry a structural engineer's name and cover wind bracing and holding-down, not just stud sizes.
• Timber grade and treatment. Structural timber is stamped C16 or C24. Sole plates and anything outside the membrane should be preservative treated.
• Membership. The Structural Timber Association runs a quality scheme for frame manufacturers. Membership is not a guarantee, but it is a fair first filter.
• Offloading. Find out whether the lorry has its own crane. If it does not, you need hands or a forklift on the day.
• What happens if a panel is wrong. Get the remake time and who pays for it in the order.

Where our information comes from

• GOV.UK: Permitted development rights for householders, technical guidance
• GOV.UK: Approved Document L, conservation of fuel and power
• GOV.UK: Approved Document B, fire safety
• GOV.UK: Building regulations approval
• GOV.UK: Fees for planning applications
• Structural Timber Association
• TRADA, the Timber Research and Development Association
• Scottish Government: Building standards

Rules quoted are for England unless we say otherwise. Wales follows closely; Scotland and Northern Ireland have their own building standards.