SIPs extension kits

A structural insulated panel is two sheets of board glued to a core of rigid foam. The panel is the wall and the insulation in one piece, so a SIPs extension kit arrives closer to finished than a timber frame and gives you a thinner wall for the same warmth.

Get a quote

In the kit

Wall panels cut to size with door and window openings formed
Roof panels, where the design uses them
Jointing splines and sealant
Sole plates and fixings
Any ridge beam or posts the panels need
Panel calculations and a numbered layout drawing

Not in the kit

Breather membrane and battens
Windows and doors
Cladding and roof covering
Internal lining and service battens
Lifting gear for large panels

Kit, supply only, per m²: £1,000 to £1,400£20,000 to £28,000 for a 20 m² room
Finished extension, per m²: £2,400 to £3,000£48,000 to £60,000 for the same room

Why people pick SIPs

The main reason is wall thickness. Permitted development limits are measured to the outside of the extension: 3 m from the back of a terraced or semi-detached house in England, 4 m from a detached one. Every millimetre of wall comes out of the room. A SIPs wall can meet the insulation target at a thickness where a stud wall would still need an extra layer, and across three walls that is a visible gain in floor.

The second reason is air tightness. Panels are joined with glued splines, so there are few gaps for draughts. The third is the roof: SIPs roof panels span from wall to ridge beam without trusses, which gives a vaulted ceiling with no extra carpentry.

Panel thickness and the insulation target

UK panels are commonly made 142 mm and 172 mm thick. New extension walls in England need a U-value of 0.18 W/m²K and roofs 0.15. As a rule, a 142 mm panel falls just short of 0.18 by itself and is brought up with a thin insulated lining on the inside. A 172 mm panel generally meets it without help. The core material matters as much as the thickness, so do not compare two kits on millimetres alone.

Ask each supplier for the U-value calculation of the complete wall and roof, with your cladding and lining included. Building control will want it anyway.

Where SIPs go wrong on site

Water. The boards on a SIPs panel are oriented strand board. It copes with a shower but not with standing wet. Panels should stay wrapped and off the ground until the day they go up, and the shell should be membraned and roofed within days, not left for the roofer's next free week.

The base. Panels are rigid and cut square. There is less scope to absorb an uneven base than with studwork, so the sole plate has to be set level and to the drawing before anything else happens.

Late changes. Moving a window in a stud wall is an afternoon with a saw. Cutting a new opening in a SIPs panel changes how the wall carries load and needs the panel engineer's agreement. Fix every opening before the kit is ordered.

Stale air. A room this airtight needs its ventilation designed in from the start. Background vents in the window frames are the minimum, and a hob or a shower needs a fan that runs to the outside.

Paperwork a SIPs kit should come with

Building control officers and warranty providers ask for third-party certification of the panel system, such as a BBA certificate or an equivalent from another accredited body. A supplier who cannot show one is asking you to take the structure on trust. You also need the structural calculations for your design, not a generic span table, and details for the joints at the base, the corners and the junction with the house.

What a SIPs extension kit costs

National guides give one range for flat-pack extension kits, supply only: £1,000 to £1,400 per m² of floor. They do not split it by system, and SIPs quotes tend to sit in the upper part of it. A finished SIPs extension runs at £2,400 to £3,000 per m², a little above timber frame. For a 20 m² room that is £20,000 to £28,000 for the kit and £48,000 to £60,000 finished.

Try other sizes in the kit price guide.

Handling and putting up

A standard 1.2 m wide wall panel can be lifted by two people. Full-height or roof panels several metres long cannot, and a kit made from large panels needs a telehandler or a small crane for a day or two. Ask the supplier which panel sizes they plan to use and whether your access allows the machine. If the only way in is a side passage, say so at the start and the kit can be drawn in smaller panels.

Where our information comes from

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

SIPs kits: common questions

Is a SIPs extension kit harder to build than timber frame?

The assembly is simpler, because there are fewer pieces and no insulation to fit afterwards. It is less forgiving, though. The base has to be right, the panels have to stay dry, and you cannot alter the design on site.

Can I fix shelves and kitchen units to a SIPs wall?

Light loads fix to the board with the right screws. Kitchen wall units and anything heavy want a timber batten or a ply lining behind the plasterboard, which most people fit anyway as a zone for cables.

Do SIPs extensions need planning permission?

Planning looks at size, position and appearance, never at what the walls are made of. A SIPs room within the permitted development limits, finished to look like the house, usually needs no application in England. A lawful development certificate confirms it. Scotland has its own limits and always requires a building warrant.

Keep reading

Timber frame kitsLighter, cheaper to alter on site Steel frame kitsFor long spans and wide glazing Kit pricesKit price against finished price

Get a quote for your kit

A postcode, four short questions about the kit, then your phone and email. Most people finish in under a minute.

Extension Kits is a free introduction service, not a kit maker. We find a kit supplier that covers your area and send you its name first. Nothing is passed on until you say yes.