SIPs extensions
Structural insulated panels, or SIPs, are a sandwich of rigid insulation foam bonded between two sheets of oriented strand board. Cut to your drawings in a factory, they form the walls and often the roof of an extension, carrying the load and the insulation in one layer.
Installed, a SIPs extension typically costs £2,400 to £3,000 per m² before VAT. The shell goes up in days and the panels pass through a side gate, so no big crane is needed.
Installed SIPs extension
£2,400 to £3,000 per m²
Single storey, before VAT
Why people choose SIPs
- Very good insulation in a thin wall, so more floor space inside the same footprint.
- Airtight construction that keeps the new room warm and quiet.
- Panels light enough to carry through a side passage: ideal for terraces.
- A vaulted ceiling is easy because the roof panels span without trusses.
- A weathertight shell in days, so internal trades can start sooner.
- Precise factory cutting: openings, angles and roof pitches come out where drawn.
SIPs extension prices
| Item | Typical price, ex VAT |
|---|---|
| Installed SIPs extension Per m² of floor, single storey, shell to finished room | £2,400 to £3,000 per m² |
| 15 m² SIPs extension A 5 x 3 m kitchen extension within permitted development | £36,000 to £45,000 |
| 25 m² SIPs extension A 5 x 5 m family room, likely needing prior approval on a semi | £60,000 to £75,000 |
| Factory timber frame, for comparison Per m² installed | £2,200 to £2,800 per m² |
| Brick and block, for comparison Per m² installed | £1,800 to £2,200 per m² |
Where these figures come from
Typical ranges from published UK contractor and builder price guides, read November 2026, before VAT at 20%. A guide, not a quote: the price for your house depends on a survey, the ground and the specification. Work out your own figure with the cost calculator.
Thinking SIPs? Send us the size and we will find a specialist who builds with them.
Ask for a free quoteHow SIPs compare
SIPs sit at the higher end of the per m² range, £2,400 to £3,000 per m² against £2,200 to £2,800 per m² for factory timber frame. You are paying for the insulation and the factory work up front. In return the walls are slimmer for the same performance, the shell is airtight without much fiddly taping on site, and heating bills in the new room are lower.
Compared with brick and block, SIPs are quicker and lighter on the foundations, but the finish outside is usually render, cladding or brick slips rather than a full brick skin. If the council expects brick to match the house, a brick slip system or a brick outer leaf on its own foundation can be added.
Insulation and building regulations
Extensions in England must meet the fabric standards in Approved Document L. SIPs panels around 150 mm to 200 mm thick usually meet or beat the wall and roof requirements comfortably, which leaves room to use more glass. Ask the supplier for the U-value of the exact panel and the condensation risk calculation for your build-up.
Building control will still inspect the foundations, the drainage, fire separation near boundaries and the junction with the house. Fire rating matters where the wall is close to a boundary: the panel system needs a tested lining for that.
What can go wrong
Moisture is the enemy of any timber-based panel. Good SIPs extensions have the panels lifted above ground level on a sole plate with a damp-proof course, a breathable membrane and a drained cavity behind the cladding, and careful sealing of every joint. Ask how the firm details the base, the roof edge and the junction with your house.
Later changes are harder: cutting a new window into a SIPs wall needs an engineer, and running new cables through panels means using the chases made in the factory. Plan sockets and lights early.
How a SIPs extension is built
Design and panel drawings
Architectural drawings become a cutting schedule for every panel and opening.
Foundations and base
Strip, raft or screw piles, then a level sole plate with a damp-proof course.
Shell in days
Wall and roof panels are lifted into place, splined and sealed, then made weathertight.
Cladding and finish
Membrane, battens and cladding outside; services, plasterboard and floor inside.
Questions people ask
How much does a SIPs extension cost per m²?
Typically £2,400 to £3,000 per m² before VAT, installed. Bare panel kits cost much less, but foundations, roof covering, glazing, cladding and finishes are extra.
Are SIPs better than timber frame?
They insulate better for the same thickness and go up faster. Timber frame is cheaper and more flexible to alter later. Both are good choices when detailed properly.
Do SIPs extensions need planning permission?
The same rules apply as for any extension. In England, up to 3 m deep (4 m detached) can be permitted development, with larger single-storey rear extensions possible through prior approval. Materials must look similar to the house. See the technical guidance.
How long does a SIPs extension last?
Built to current regulations and kept dry, as long as other timber-based construction. Look for third-party certification of the panel system and a structural warranty.
Can SIPs be used for a roof only?
Yes. SIPs roof panels are often used on a brick extension to get a vaulted, well-insulated ceiling without trusses.
Get prices for a SIPs extension
Three short steps: where the house is, what you want to add, and how to reach you. About two minutes.
This is a free matching service, not a modular extension specialist. Once we have found a firm that builds in your part of the country, we tell you its name, and your details stay with us until you agree.
Thanks, your enquiry is in.
- We search for a firm that makes and fits the type of extension you picked, in your part of the country, then email or ring you with its name.
- Your details reach that firm only if you say yes.
- They look at the house and the access, then price the job. No marketing from us, ever.