SIPs Extension Kits

What arrives on the lorry

A kit is delivered flat, with every panel numbered to match the assembly drawings. Wall panels come first off the stack: full-height pieces, usually 1.2 m wide, with door and window openings already cut and timber lining the edges. Roof panels follow, often four to six metres long. With them come the treated sole plates that sit on your damp-proof course, the insulated splines that lock one panel to the next, long screws, expanding foam, sealant and tape.

A wall panel weighs roughly 20 to 25 kg per square metre, so two people can walk one into place. Roof panels are another matter. Their length puts them beyond safe manual handling, and you should budget for a telehandler or a small crane on erection days.

What a kit leaves out

The list is longer than most first-time buyers expect. Foundations and the floor slab. The breather membrane, battens and cladding outside. The roof covering and any roof lights. Windows and doors. Wiring, plumbing, heating, plasterboard, plaster and floor finishes. Scaffolding and lifting. And labour, unless you pay for erection.

That is why a shell at £1,200 to £1,800 per m² and a finished extension at £1,300 to £2,400 per m² are not comparable numbers: the first is panels only, the second a room you can use. Ask any supplier which one a price is. Use the cost calculator to see both for your floor area.

Three ways to buy the same panels

Supply only. The lorry leaves and the panels are yours. Right for experienced self-builders and for builders who have stood SIPs before.

Supply and erect. The supplier's own crew stands the shell, joints it and usually wraps it in membrane. You take over a dry structure. This is the most common choice, because a badly sealed joint is hard to find later and a trained crew is quick: three to five days for a typical single-storey extension.

Full build. One firm does foundations to decoration. You pay more and manage less.

Self build: the parts people underestimate

The base. SIPs forgive nothing underneath them. If the slab is 15 mm out of level, the error climbs the wall and shows at the roof. Ask the supplier for a setting-out drawing and its level tolerance before the groundworker starts, and measure the diagonals before the lorry is booked. Our foundations page covers this in detail.

Rain. OSB copes with a shower. It does not cope with sitting wet for weeks. Store panels flat on bearers, off the ground and under a sheet, and get the membrane on the day the shell is finished.

Paperwork. A kit supplier designs the panels; it does not apply for approval on your behalf unless you agree that. Somebody has to submit the Building Regulations application with the engineer's calculations, and book each inspection. If you direct the trades yourself, read the HSE's notes on domestic clients too, because health and safety duties on site may then rest with you.

VAT. An extension kit carries VAT at 20%. The DIY housebuilders' refund covers new homes and certain conversions, not extensions.

What changes a kit price

Panel thickness and core come first. A 142 mm panel with a urethane core is the usual wall; a 172 mm panel, or a thicker one with an expanded polystyrene core, costs more and insulates better. Roof shape is next: a flat or single-pitch roof is the cheapest to panel, while a vaulted roof needs a ridge beam and longer panels.

Then the openings. Every wide set of doors needs a timber or steel beam over it and posts beside it, and each one is engineered. Delivery distance and whether an articulated lorry can reach your road also show up in the figure.

Ask every supplier which third-party certificate covers its panel system. Building control officers, warranty providers and mortgage lenders all look for one, and the Structural Timber Association keeps a list of member manufacturers.