Timber Frame and SIPs Extensions Explained
Should you build your extension with timber frame or SIPs? Costs, build speed, insulation, and how they compare to traditional masonry.
Most UK extensions are built with traditional masonry - concrete block inner walls, brick outer walls, cavity insulation between. It's the construction method every builder knows. But timber frame and SIPs (Structural Insulated Panels) offer significant advantages in speed, insulation, and environmental impact.
Here's how they compare, what they cost, and when to consider them.
The Three Construction Methods
Traditional Masonry
The UK default: concrete block inner leaf, cavity insulation, brick or block outer leaf.
- Wall build time: 4–6 weeks (weather-dependent)
- U-value: 0.25–0.30 W/m²K (meeting Building Regs minimum)
- Cost: £1,500–£2,500/m² (complete extension)
- Pros: Every builder can do it, wide choice of finishes, proven track record
- Cons: Slow, weather-dependent, heavy foundations needed, thermal bridging at junctions
Timber Frame
A structural timber skeleton (typically 140mm or 190mm studs at 600mm centres) with insulation between the studs and an external cladding layer (brick, render, timber boarding, or tile hanging).
- Wall build time: 1–2 weeks (erected from pre-cut or panelised components)
- U-value: 0.18–0.25 W/m²K (better than standard masonry)
- Cost: £1,400–£2,200/m² (complete extension)
- Pros: Faster build, lighter structure (smaller foundations), better insulation, less waste, drier construction
- Cons: Needs specialist builder, external cladding adds cost, careful moisture detailing required
SIPs (Structural Insulated Panels)
Factory-manufactured panels - rigid insulation (typically PIR or EPS) sandwiched between two structural boards (OSB or cement particle board). Delivered as wall-sized panels and assembled on site.
- Wall build time: 2–5 days
- U-value: 0.15–0.20 W/m²K (exceptional - passive house territory)
- Cost: £1,600–£2,500/m² (complete extension)
- Pros: Fastest erection, best insulation, minimal thermal bridging, factory precision, very airtight
- Cons: Higher material cost, limited on-site modification, needs specialist installer, heavier panels need crane access
Cost Comparison: 20m² Single-Storey Extension
| Component | Masonry | Timber Frame | SIPs |
|---|---|---|---|
| Foundations | £5,000–£8,000 | £4,000–£6,500 | £4,000–£6,500 |
| Structure (walls + roof) | £12,000–£18,000 | £10,000–£15,000 | £12,000–£17,000 |
| External cladding | Included (brick) | £3,000–£6,000 | £3,000–£6,000 |
| Insulation | £1,500–£2,500 | Included in frame | Included in panels |
| Windows and doors | £3,000–£6,000 | £3,000–£6,000 | £3,000–£6,000 |
| Internal fit-out | £6,000–£10,000 | £6,000–£10,000 | £6,000–£10,000 |
| Professional fees | £3,000–£6,000 | £3,500–£6,500 | £3,500–£6,500 |
| Total | £33,500–£56,500 | £32,500–£56,000 | £35,500–£58,000 |
The overall costs are surprisingly similar. Timber frame saves on structure but spends more on cladding. SIPs cost more in materials but save on labour and programme time. The real differences are in speed, insulation performance, and on-site disruption.
Speed: The Biggest Advantage
| Phase | Masonry | Timber Frame | SIPs |
|---|---|---|---|
| Foundations | 1–2 weeks | 1–2 weeks | 1–2 weeks |
| Walls | 4–6 weeks | 1–2 weeks | 2–5 days |
| Roof | 1–2 weeks | 1 week | Included in panel erection |
| Weathertight | 6–10 weeks | 3–5 weeks | 2–3 weeks |
| Internal fit-out | 4–6 weeks | 4–6 weeks | 4–6 weeks |
| Total | 12–18 weeks | 9–14 weeks | 8–12 weeks |
Getting to weathertight (roof on, walls closed) is where timber frame and SIPs shine. A masonry extension sits open to the weather for 6–10 weeks. A SIPs extension is weathertight in 2–3 weeks. This reduces weather delays, protects the existing house, and compresses the overall programme.
Insulation Performance
| Method | Typical U-value | EPC Impact | Heating Cost Saving |
|---|---|---|---|
| Masonry (standard) | 0.25–0.30 | Meets minimum regs | Baseline |
| Masonry (enhanced) | 0.20–0.25 | Above regs | 10–15% saving |
| Timber frame | 0.18–0.25 | Above regs | 15–25% saving |
| SIPs | 0.15–0.20 | Significantly above regs | 25–40% saving |
SIPs have virtually no thermal bridging - the insulation is continuous across the entire panel, with no studs creating cold spots. This makes SIPs the construction of choice for near-passive house performance. The Energy Saving Trust promotes SIPs as one of the most effective ways to achieve low-energy buildings.
If you're pairing your extension with a heat pump, SIPs or well-insulated timber frame maximises the heat pump's efficiency - the better the insulation, the lower the flow temperature needed, and the higher the COP.
External Cladding Options
Timber frame and SIPs need an external weather layer. Common options:
| Cladding | Cost/m² | Look | Maintenance |
|---|---|---|---|
| Brick slip | £60–£100 | Matches existing brick | None |
| Full brick outer leaf | £80–£120 | Identical to masonry | None |
| Cement render | £40–£70 | Smooth modern finish | Repaint every 8–12 years |
| Silicone render | £50–£80 | Self-cleaning, modern | Virtually none |
| Timber boarding (larch/cedar) | £40–£80 | Natural, contemporary | Stain/oil every 3–5 years (or leave to silver) |
| Zinc/metal cladding | £80–£150 | Industrial/contemporary | None |
| Tile hanging | £60–£100 | Traditional, period homes | Minimal |
Brick slip is the most popular choice when matching an existing brick house - thin brick tiles bonded to an insulation-backed carrier board create a genuine brick appearance without building a full brick wall.
When to Choose Each Method
| Scenario | Best Method | Why |
|---|---|---|
| Standard rear extension, brick house | Masonry | Builder familiarity, matches existing |
| Speed is critical (minimal disruption) | SIPs | Weathertight in days |
| Maximum energy performance | SIPs | Best U-values, airtightness |
| Loft conversion dormer | Timber frame | Light weight, fast erection at height |
| Garden room or outbuilding | SIPs or timber frame | Speed, insulation, no wet trades |
| Contemporary design with mixed cladding | Timber frame | Flexible cladding options |
| Listed building requiring traditional appearance | Masonry | Authentic construction expected |
| Upper-floor extension or additional storey | Timber frame | Lighter than masonry - reduced foundation loads |
Finding a Specialist
Timber frame and SIPs require specialist knowledge:
- Timber frame: Look for builders experienced with panel or stick-built timber frame. The Structural Timber Association (STA) maintains a directory of certified companies.
- SIPs: Use an STA-registered SIPs manufacturer and installer. Key UK suppliers include Kingspan TEK, SIP Build UK, and SIPS@Clays.
- Architects: Ensure your architect has experience designing for timber frame or SIPs - the detailing is different from masonry.
- Structural engineers: Must be comfortable with timber engineering, not just steel and concrete.
Find quality-assured contractors through TrustMark or the Federation of Master Builders.
Next Steps
- Consider your priorities - speed, insulation, cost, or appearance?
- Talk to your architect - discuss alternative construction methods early in the design
- Get comparative quotes - ask for masonry, timber frame, and SIPs prices for the same design
- Check Building Regulations - all methods must comply, but the details differ
- Visit a completed project - see and feel the difference before committing
- Get a cost estimate - use our free calculator for a starting budget
- Check our glossary for terms like SIPs, OSB, U-value, thermal bridge, and vapour barrier
Frequently Asked Questions
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