SIPs Extension Cost: Are Structural Panels Worth It?

Structural insulated panels (SIPs), a North American system that has been certified in the UK since about 2000, have moved from a niche product used mainly in self-build new homes to a mainstream option for UK house extensions.¹ The appeal is straightforward: a SIPs shell can be erected in a day or two for a typical single-storey extension, reaches weathertight faster than blockwork, and delivers excellent thermal performance from the outset. Whether that speed premium translates to genuine cost savings depends on your project’s specifics.

What Are SIPs?

A structural insulated panel consists of a rigid foam insulation core, typically EPS (expanded polystyrene) or PIR (polyisocyanurate), bonded under pressure between two structural facing boards, usually OSB/3 (oriented strand board). The result is a composite sandwich panel that acts as both structure and insulation simultaneously.

Standard panel thicknesses for UK residential extensions:

The EPS figures are one maker’s published values for panels with OSB facings: its 172 mm panel, with 150 mm of EPS, is rated at 0.21 W/m²K.² Graphite-enhanced EPS and urethane cores do better for the same thickness. Kingspan rates its 142 mm TEK panel, a 112 mm urethane core between two 15 mm OSB/3 facings, at a worst-case whole-wall U-value of 0.20 W/m²K with no additional insulation, and its 172 mm panel at 0.17 W/m²K.³ For a new wall in an extension, Approved Document L sets a limiting U-value of 0.18 W/m²K, so most EPS panels need extra insulation, usually in the service void, to comply.⁴

For roof panels, thicknesses of 172–300 mm are common; roofs typically need better U-values than walls (target 0.15 W/m²K under Part L 2022).⁴

SIPs Extension Costs (2026)

Supply-Only Panel Cost

Panels are factory-cut to your specific layout, with apertures pre-cut for windows and doors. Design and cutting are often included in the quoted price above a minimum order size. Uncut stock panels cost less: one maker lists a 172 mm urethane-core panel at £150.00 inc. VAT for a 2440 × 1220 mm sheet, about £50 per m², before design, cutting, splines and fixings.⁵

Full Extension Cost Breakdown (25 m² single-storey rear, 2026)

The wide range reflects the difference between a budget-spec (render finish, uPVC windows, vinyl flooring) and a high-spec fit-out (brick slip cladding, aluminium bifolds, engineered timber floor).

SIPs vs Blockwork vs Timber Frame

SIPs are not inherently cheaper to build than other methods, the panel supply cost is higher than buying bricks and blocks, but on projects with tight programmes (rental properties, limited disruption, winter builds) the speed advantage can meaningfully reduce overall cost through reduced scaffolding time, faster thermal envelope closure, and shorter contract periods.

Thermal and Airtightness Performance

SIPs extensions can readily achieve air permeability below 3 m³/h/m² at 50 Pa when the joints are sealed properly. Kingspan says its TEK system can achieve air leakage levels as good as about 1 m³/h/m² at 50 Pa.⁶ The critical workmanship points are:

1. Panel-to-panel joints, must be taped internally with a VCL (vapour control layer) tape
2. Sole plate junction, the joint where the panel meets the floor is the most common air leakage point; a continuous bead of acoustic sealant and VCL tape is standard
3. Window and door reveals, factory-cut reveals make this easier than stud frame; still requires careful taping and air-sealing at the frame perimeter

Planning and Building Regulations

SIPs extensions are assessed identically to any other extension under planning, the system does not distinguish by material. Building Regulations requirements to note:

• Structural calculations, required; the SIPs supplier typically provides these for their panel system; a structural engineer sign-off is expected by building control
• Fire (Part B), SIPs using OSB facings require protection if within 1 m of a boundary; internally, OSB is typically covered by plasterboard before occupation which provides the necessary fire performance⁷
• Condensation (Part C), the inner OSB facing and taped joints resist vapour on the warm side, so makers such as Kingspan say no separate vapour control layer is needed if their jointing and ventilation guidance is followed, though a condensation risk analysis to BS 5250 is still recommended³
• Thermal (Part L), well-specified SIPs panels achieve current targets without difficulty; confirm U-values with your supplier at the design stage⁴

Pros and Cons Summary

Pros:

• Very fast to weathertight
• Excellent thermal performance from the factory
• Reduced wet trades compared to blockwork
• Lower on-site waste than masonry
• Consistent quality (factory-manufactured, not site-built)

Cons:

• Less flexible for changes once ordered; amendments to panel designs are expensive
• Requires a specialist erection team; not all local builders are familiar with SIPs
• Penetrations (electrics, plumbing) need careful planning, chasing is not an option in the panel faces³
• OSB facing is moisture-sensitive during construction; panels must be protected from extended rain exposure before cladding³
• Fewer contractors to quote means less competitive pricing in some areas

Finding SIPs Suppliers and Builders

The Structural Timber Association has counted 15 of its members operating in the SIPs market, from Kingspan’s TEK system to smaller makers.¹ Most offer a full design service, structural calculations, and supply-only or supply-and-erect packages. Many also carry BBA (British Board of Agrément) certification for their panel system, which building control bodies commonly look for as evidence that the system is fit for purpose.¹ UKAS suspended the BBA’s accreditation on 26 February 2026, so while that stands the BBA cannot issue new accredited certificates. A certificate issued before that date remains valid, and an equivalent from another UKAS-accredited certification body, or a European Technical Assessment, carries the same weight with building control.⁸

Ask for references from completed extensions (not just new builds) and check whether the price includes panel cutting, fixing hardware, and tape/sealant or whether these are extras. In the breakdown above the panels are about 16–18% of the total extension cost (£7,000 of £40,000 and £14,000 of £89,500); if a quote’s panel share is far higher, the other package elements may be underpriced and subject to overrun.