Structural insulated panels (SIPs) have moved from a niche product used mainly in Scandinavian-style new builds to a mainstream option for UK house extensions. 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:
| Panel Thickness | Insulation Core | Approx. U-value (W/m²K) | Typical Use |
|---|---|---|---|
| 114 mm (EPS) | 100 mm EPS | 0.28 | Minimum/budget |
| 142 mm (EPS) | 130 mm EPS | 0.22 | Standard wall |
| 172 mm (EPS) | 150 mm EPS | 0.18 | Current Regs target wall |
| 172 mm (PIR) | 130 mm PIR | 0.15 | High-performance wall |
| 224 mm (EPS) | 200 mm EPS | 0.14 | Low-energy/Passivhaus |
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).
SIPs Extension Costs (2026)
Supply-Only Panel Cost
| Panel Type | Supply Cost (per m² of panel) |
|---|---|
| 114 mm EPS SIP | £35–£55 |
| 142 mm EPS SIP | £45–£70 |
| 172 mm EPS SIP | £55–£90 |
| 172 mm PIR SIP | £75–£120 |
| Roof panels (200+ mm EPS) | £65–£110 |
Panels are factory-cut to your specific layout, with apertures pre-cut for windows and doors. The design and cutting is included in most suppliers’ quoted price for orders above a threshold (typically 30–40 m²).
Full Extension Cost Breakdown (25 m² single-storey rear, 2026)
| Element | Typical Cost |
|---|---|
| Groundworks and foundations (concrete slab) | £8,000–£14,000 |
| SIPs panels (wall and roof, supply) | £7,000–£14,000 |
| Erection (specialist team, 1–2 days) | £3,000–£6,000 |
| Structural timber sole plate and ridge beam | £1,500–£3,500 |
| Roofing membrane, battens, tiles/slate | £3,500–£7,000 |
| External cladding or render (on SIPs OSB) | £3,000–£8,000 |
| Windows and doors | £4,000–£12,000 |
| First and second fix (electrics, plumbing) | £4,000–£10,000 |
| Internal finishes (boarding, skimming, flooring) | £4,000–£10,000 |
| Architect/technologist drawings and Regs | £2,000–£5,000 |
| Total | £40,000–£89,500 |
| Per m² finished | £1,600–£3,580 |
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
| Factor | SIPs | Blockwork (cavity) | Light Timber Frame |
|---|---|---|---|
| Programme (weathertight) | 1–3 days shell erection | 4–10 weeks brickwork | 1–3 weeks |
| Thermal performance | Excellent out-of-box | Requires significant insulation thickness | Good with modern spec |
| Airtightness | Excellent if taped correctly | Moderate; requires added membranes | Good with membranes |
| Self-build suitability | Moderate — specialist tool needed | High | Moderate |
| Trades availability | Specialist erector needed | All builders can do it | Widely understood |
| Design flexibility | High (factory-cut to spec) | Unlimited | High |
| Fire performance | OSB chars; core must be protected | Inherently non-combustible | Requires protection |
| Approximate structural cost | £130–£200/m² | £90–£160/m² | £100–£175/m² |
| Typical total (finished, mid-spec) | £2,200/m² | £2,400/m² | £2,100/m² |
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 airtightness below 3 m³/h/m² at 50 Pa, which puts them comfortably within Part F requirements and approaching Passivhaus-territory at better specified junctions. The critical workmanship points are:
- Panel-to-panel joints — must be taped internally with a VCL (vapour control layer) tape
- 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
- 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 performance
- Condensation (Part C) — SIPs are vapour-open on the warm side of the insulation by design; no separate vapour barrier is typically needed if the panel manufacturer’s specification is followed
- Thermal (Part L) — well-specified SIPs panels achieve current targets without difficulty; confirm U-values with your supplier at the design stage
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 faces
- OSB facing is moisture-sensitive during construction; panels must be protected from extended rain exposure before cladding
- Fewer contractors to quote means less competitive pricing in some areas
Finding SIPs Suppliers and Builders
There are around a dozen major SIPs suppliers in the UK, including Kingspan TEK, UK SIPs, and Premier SIPs. 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 will require for sign-off.
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. The total panel supply price should typically represent 15–25% of the total extension cost — if it’s significantly higher, the other package elements may be underpriced and subject to overrun.
