The short answer
Both are timber-based and both go up quickly. SIPs give a thinner wall for the same insulation, better airtightness and open roof spaces, at a similar price per m² once finished. Timber frame is often cheaper on simple designs, far more widely understood by builders and easier to alter on site. For a small, simple extension the difference is modest; for a vaulted roof or a tight plot, SIPs pull ahead.
On this page
People often treat SIPs as a kind of timber frame. They are cousins, not twins. Both use wood, both are made off site and both are finished with the same cladding and plasterboard. The difference is in what holds the building up.
Two ways to make a wall
Timber frame is a skeleton. Vertical studs, typically 140 mm deep and 600 mm apart, carry the load. A sheathing board braces them, insulation fills the spaces between, and membranes go on each side. Panels arrive either “open”, with insulation fitted on site, or “closed”, insulated and sometimes glazed in the factory.
SIPs have no skeleton. Two boards bonded to a foam core form a solid panel that carries load across its whole width. Our guide to what SIPs are goes through the make-up.
That one difference drives nearly everything else.
Side by side
| Timber frame | SIPs | |
|---|---|---|
| What carries the load | Studs at 600 mm centres | The whole bonded panel |
| Insulation | Between studs, often with an extra layer | Continuous foam core |
| Wall thickness for 0.18 W/m²K | Roughly 190 to 230 mm before cladding | Roughly 170 to 200 mm before cladding |
| Airtightness | Good, depends on membranes and taping | Very good, few joints |
| Roof | Trusses or rafters; room-in-roof costs extra | Panels span to a ridge beam; open by default |
| Finished cost per m², including VAT | £1,400 to £2,400 | £1,300 to £2,400 |
| Changes on site | Possible, with a saw and an engineer’s say-so | Costly; panels are remade |
| Builders who know it | Many | Fewer |
The cost figures are typical national ranges for 2026 for finished extensions, including VAT. London and the South East usually cost 20 to 40% more. They are a guide, not a quote, and the bands overlap: a well-specified timber frame costs more than a plain SIPs build.
Where SIPs do better
Thin walls. With a urethane core, a SIP reaches a given U-value in less thickness than studs packed with mineral wool. Along a six-metre wall, 40 mm saved is a quarter of a square metre of floor. On a narrow side return that is worth having.
Heat loss through the timber. Every stud is a strip of solid wood running from inside to outside, and wood insulates far worse than foam. Designers allow for this by assuming around 15% of a stud wall is timber. A SIP wall has solid timber only at joints and openings, so its real performance sits closer to the headline figure.
Airtightness. Fewer, larger pieces with sealed splines leak less than a frame that relies on a membrane being taped perfectly around every socket box.
Roofs. This is the clearest win. SIP roof panels span from wall to ridge with nothing in between, so a vaulted kitchen ceiling or a bedroom in the roof comes as standard. See SIPs roof panels for how that works.
Where timber frame does better
Price. Studs, mineral wool and sheathing board are cheap, common materials, and plenty of factories make frames. The typical finished ranges are close, so on a real job the difference comes down to specification and the firm, and timber frame often wins on simple designs.
Flexibility. A joiner can frame a new opening, add a noggin for a heavy basin or adjust a wall that does not quite meet the old house. With SIPs those decisions have to be right on paper.
Familiarity. Any competent carpenter can put up a timber frame, and every building control officer has inspected dozens. That makes it easier to get three comparable quotes locally.
Sound. A stud wall filled with dense mineral wool usually blocks airborne noise a little better than a foam-cored panel of the same thickness.
What stays the same
Foundations are similar. Both are light, both sit on a sole plate above a damp-proof course, and in both cases the depth is set by the ground. Our foundations page covers the options.
Both need a breather membrane, a ventilated cavity and cladding outside, and plasterboard inside for fire protection. Both must be kept dry. Both are treated as non-standard construction by some lenders, and both need an engineer’s calculations for building control. Planning rules do not distinguish between them at all.
Speed is close too. A timber frame extension shell takes about a week to stand; a SIPs shell three to five days. Over a three-month project that difference disappears.
Which to choose
Pick SIPs if the roof is vaulted or holds a room, if the plot is tight and wall thickness costs you floor space, or if very low heat loss and draught-free rooms are high on your list.
Pick timber frame if the extension is a simple box under a flat or trussed roof, if you want the lowest price from a local builder, or if you expect to adjust details as the work goes on.
Plenty of projects mix the two: timber frame walls under a SIP roof, or SIP walls with a conventional flat roof. A good designer will price both.
If SIPs look like the right fit, the cost calculator gives a budget by floor area, and the quote form below is the quickest way to get a real price for a SIPs extension at your address.