Summary
Timber frame is the fastest-growing residential extension method in the UK, driven by three factors: speed of weather-tight (typically 1–2 weeks vs 4–6 weeks for masonry), thermal performance (better airtightness, often exceeding Part L), and predictable factory-controlled quality (closed-panel especially). The headline £/m² figure for shell-and-core is similar to or slightly lower than masonry, but the convergence happens at turn-key: kitchen, bathroom, MEP, and finishes cost the same regardless of structural system.
The big variables are: (1) open-panel vs closed-panel vs SIPS (structural insulated panels) — closed-panel and SIPS are factory-assembled and quicker on site but cost more per m² in supply; (2) foundation design — timber frame is lighter than masonry, so often allows simpler trench-fill, raft, or insulated screed-slab foundations; (3) cladding choice — render, timber, brick slip, fibre cement, masonry skin all change the external appearance and price; (4) warranty provider — NHBC and Premier Guarantee charge 1.0–1.6% of build value, structural warranty is essential for resale and any future mortgage.
Where customers get confused: "timber frame" and "oak frame" are NOT the same. Oak frame (post-and-beam green oak) is a specialist premium product (see oak frame extension pricing guide) at £2,800–£4,500/m². Timber frame as covered here is stick-built or panelised structural softwood (typically C24 grade) with insulation, breather membrane, and external cladding — a mainstream construction system used by NHBC volume housebuilders.
This guide covers shell-and-core and turn-key pricing for stick-built and SIPS extensions, with breakdown of foundation, frame, MEP, and finish line items.
Key Facts
- Shell-and-core price (single-storey) — £1,800–£2,500/m² (frame supply, erect, weather-tight, no internal finishes)
- Turn-key price (single-storey) — £2,500–£3,500/m² (full finish, kitchen, bathroom, decoration)
- Two-storey premium — add 10–15% per m² (additional foundation depth, scaffold, structural design)
- SIPS shell-and-core — £2,000–£2,800/m² (premium for factory-built closed panel)
- Open-panel frame supply — £400–£600/m² (cheapest frame supply, requires more site labour for insulation/membrane)
- Closed-panel frame supply — £550–£800/m² (factory-insulated, faster on site)
- SIPS panel supply — £600–£950/m² (highest insulation, simplest on-site assembly)
- Erection labour — £250–£450/m² (frame crew on site for 5–10 days typical)
- Foundation (trench-fill strip) — £130–£220/m² of footprint
- Foundation (raft / insulated slab) — £200–£320/m² of footprint
- Roof structure — £180–£320/m² (warm roof composite over timber frame)
- External cladding (render) — £85–£140/m² of wall
- External cladding (timber, Siberian larch) — £120–£200/m² of wall
- External cladding (brick slip system) — £150–£250/m² of wall
- External cladding (full masonry skin) — £180–£290/m² of wall
- Internal plasterboard + skim — £25–£40/m² of wall
- MEP first fix — £180–£320/m² of floor area
- MEP second fix + commissioning — £120–£240/m² of floor area
- Kitchen (mid-range supply + fit, 15–20 unit) — £8,000–£18,000
- Bathroom (mid-range supply + fit) — £6,000–£12,000
- Building Regulations Part L 2021 — Max U-value walls 0.18 W/m²K, roof 0.15, floor 0.18, windows 1.4; air permeability ≤5 m³/h.m² @ 50Pa
- Building Regulations Part A — Structural design and sign-off (engineer SE letter typical)
- Eurocode 5 (BS EN 1995-1-1:2004+A2:2014) — design of timber structures
- NHBC / LABC / Premier Guarantee fee — 1.0–1.6% of build value (structural warranty)
- Building Control fee — £600–£1,800 for typical single-storey extension
- Planning permission — required for most extensions; permitted development applies under GPDO 2015 Class A (rear extension up to 4m single-storey detached, 3m semi/terrace, height limits apply)
- Party Wall Act 1996 — notice required if working within 3m or 6m of neighbour structure
- VAT — 20% standard rate on labour and materials for extension to existing dwelling (no DIY housebuilders refund scheme)
Quick Reference Table
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Try squote free →| Build type | Size | Shell-and-core (£/m²) | Turn-key (£/m²) | Total turn-key |
|---|---|---|---|---|
| Single-storey stick-built (basic spec) | 20m² | £1,800–£2,000 | £2,500–£2,900 | £50,000–£58,000 |
| Single-storey stick-built (mid spec) | 30m² | £2,000–£2,300 | £2,800–£3,200 | £84,000–£96,000 |
| Single-storey closed-panel | 30m² | £2,200–£2,500 | £2,900–£3,300 | £87,000–£99,000 |
| Single-storey SIPS (high spec) | 30m² | £2,400–£2,800 | £3,100–£3,500 | £93,000–£105,000 |
| Two-storey stick-built (mid spec) | 50m² (25 + 25) | £2,200–£2,500 | £2,900–£3,300 | £145,000–£165,000 |
| Two-storey SIPS (high spec) | 50m² | £2,600–£3,000 | £3,300–£3,800 | £165,000–£190,000 |
| Foundation type | Suitability | Cost per m² (footprint) |
|---|---|---|
| Trench-fill strip | Standard ground, no trees | £130–£190 |
| Reinforced strip | Slightly weak ground | £160–£240 |
| Raft foundation | Reactive clay, sloping site | £200–£280 |
| Pile and beam | Trees within 5m, poor ground | £280–£450 |
| Insulated raft slab (e.g. KORE, Isoquick) | Passivhaus / high airtightness | £240–£340 |
| External cladding | £/m² installed | Aesthetic | Maintenance |
|---|---|---|---|
| Rendered EWI | £85–£140 | Smooth modern | Repaint 8–12 yrs |
| Siberian larch (vertical) | £120–£200 | Contemporary | Oil 2–3 yrs (or weather grey) |
| Cedar shingle/shake | £140–£220 | Traditional | Minimal |
| Brick slip on rail | £150–£250 | Brick appearance | Re-point 30+ yrs |
| Full masonry skin (cavity tie) | £180–£290 | Indistinguishable from masonry build | Minimal |
| Fibre cement board (Cembrit, Marley Eternit) | £110–£180 | Industrial / modern | Repaint 15+ yrs |
Detailed Guidance
Open-panel vs closed-panel vs SIPS
Open-panel (stick frame, factory-cut) — Timber studs (typically 140×38mm or 140×47mm C24) assembled in factory or on site; insulation, membrane, plasterboard installed on site. Cheapest in supply, most labour on site. Suitable for self-build, smaller jobs, customer wanting flexibility on insulation/finish.
Closed-panel (factory-insulated) — Panels arrive on site with insulation, breather membrane, and sometimes external sheathing already fitted. Faster erection (typically 3–5 days for a 30m² extension). Quality-controlled in factory. Mid-price.
SIPS (Structural Insulated Panels) — Sandwich panels with OSB skins and rigid insulation (PIR or EPS) core, no studs. Best airtightness, fastest install (1–3 days for shell). Highest material cost. Excellent for Passivhaus or near-Passivhaus performance.
For most domestic extensions, closed-panel is the sweet spot: factory quality, manageable site labour, good thermal performance, deliverable through a local timber frame contractor.
Foundation strategy
Timber frame is roughly 30–50% lighter than masonry per m² of wall. Practical implications:
- Lower bearing pressure = shallower or narrower foundations often acceptable
- Sensitive to differential settlement — small ground movement shows as cracks in plasterboard
- Raft and insulated raft slab popular with timber frame (continuous slab eliminates differential)
- Pile foundations required if trees within 5m (NHBC Chapter 4.2) — but timber frame piles can be smaller diameter
Always engage a structural engineer (SE) early. Foundation design + Part A sign-off is non-optional.
Insulation and Part L 2021 compliance
Part L 2021 (the 2025 update notwithstanding) requires the following maximum U-values for new extensions:
| Element | Max U-value (W/m²K) | Typical timber frame achievement |
|---|---|---|
| External wall | 0.18 | 140mm PIR between studs + 50mm internal = 0.16 |
| Roof (warm) | 0.15 | 150mm PIR rafter + 50mm = 0.14 |
| Floor (insulated slab) | 0.18 | 150mm EPS under slab + screed = 0.16 |
| Windows (whole unit) | 1.4 | Triple-glazed timber/alu units = 1.0 |
| Doors | 1.4 | Insulated composite/timber = 1.2 |
Airtightness target: ≤5 m³/h.m² @ 50Pa for extensions; new dwellings typically ≤8 (2021) tightening to ≤5 under FHS. SIPS and closed-panel easily achieve ≤3.
Airtightness layer (vapour control layer or VCL) MUST be continuous around perimeter; lapped, taped, and protected. This is the most common build failure on timber frame extensions.
Cladding selection
The external skin sits on a battened cavity (50mm typical) outside the breather membrane. Cavity must be vented top and bottom. Options:
- Through-coloured render on EWI carrier board — modern look, good thermal performance, popular
- Larch/cedar timber — contemporary; weathers to silver-grey if untreated; needs oil maintenance to retain colour
- Brick slip system (e.g. Eurobrick, Tilebrick) — looks like brick from outside but lightweight; £150–£250/m²
- Full masonry skin — single skin brick or block + render on stainless cavity ties to frame; most expensive but matches original house
- Mixed elevation — common: timber on top + render on bottom, or brick at low level + render above
Heating system selection
For new extensions Part L requires consideration of low-carbon heat. In practice:
- Extend existing gas central heating — add 1–4 radiators to existing system; check boiler is sized adequately (use combi vs system vs conventional to right-size); £800–£2,500 typical
- Air source heat pump (ASHP) — if customer is replacing whole-house heating; £6,000–£12,000 (BUS grant £7,500 available); pairs well with underfloor heating in extension
- Underfloor heating (UFH) — typically wet UFH in insulated screed; £55–£90/m² supply and install; pairs well with ASHP
Planning permission and Building Regs
Permitted development (single-storey rear, GPDO 2015 Class A):
- Detached: up to 4m from rear wall, max 4m ridge height
- Semi/terrace: up to 3m from rear wall
- Larger Home Extension (Prior Approval): up to 8m (detached) / 6m (semi) — neighbour consultation required
- Not within 2m of boundary if eaves >3m
Always requires planning permission:
- Two-storey extension (mostly)
- Front extension on any property type
- Listed building or conservation area
- Article 4 Direction area
Building Regulations approval — always required for any extension:
- Full Plans application (preferred) or Building Notice
- LA Building Control or Approved Inspector
- Plan check fee + inspection fee = £600–£1,800 typical
- Structural calcs sign-off (Part A)
- Thermal calcs sign-off (Part L)
Structural warranty
Mortgage lenders typically require structural warranty for newly built work. Options:
- NHBC — most lender-recognised; 1.2–1.6% of build value; high quality standards
- LABC Warranty — backed by local authority building control; 1.0–1.3% of build value
- Premier Guarantee — competitive pricing; 0.9–1.4%; good for one-off projects
- Protek — specialist self-build / extension warranty; flexible
Warranty is separate from Building Regs sign-off. Don't conflate the two.
Comparison vs masonry build
| Factor | Timber frame | Masonry |
|---|---|---|
| Shell-and-core cost | £1,800–£2,500/m² | £2,000–£2,700/m² |
| Turn-key cost | £2,500–£3,500/m² | £2,500–£3,500/m² |
| Time to weather-tight | 5–14 days | 4–8 weeks |
| Thermal performance (typical) | U=0.14–0.18 | U=0.18–0.25 |
| Airtightness (typical) | 1–5 m³/h.m² | 5–10 m³/h.m² |
| Wet trades on site | Few (foundations, screed) | Many (bricklayer, plasterer, screed) |
| Sound transmission | Lower mass = needs care on acoustic detailing | Higher mass = naturally better |
| Customer perception | "Lightweight, lower value" myth | "Solid, traditional, higher value" perception |
| Resale value impact | Same (when warrantied) | Slightly preferred in some markets |
Timber frame wins on speed, thermal performance, and predictability. Masonry wins on perceived solidity and acoustic mass.
Pricing breakdown for a typical job
A 30m² single-storey rear extension, closed-panel timber frame, rendered EWI cladding, turn-key including new kitchen and bathroom:
| Item | Cost |
|---|---|
| SE design + Building Regs drawings | £3,500 |
| Planning fee (householder) | £258 |
| Building Control (full plans) | £950 |
| Party Wall Act surveyor | £1,200 |
| Foundation (trench-fill, 30m² footprint × £170/m²) | £5,100 |
| Drainage connection | £2,200 |
| Closed-panel timber frame supply (30m² × £680/m²) | £20,400 |
| Frame erection labour (6 days × 3 crew @ £320) | £5,760 |
| Roof structure + warm roof insulation | £6,800 |
| Rendered EWI cladding (60m² wall × £115/m²) | £6,900 |
| Windows (3× triple-glazed + 1× bi-fold) | £8,500 |
| Internal plasterboard + skim (90m² × £32/m²) | £2,880 |
| MEP first + second fix (£500/m² × 30m²) | £15,000 |
| Kitchen supply + fit | £12,000 |
| Bathroom supply + fit (if included) | £7,500 |
| Flooring + internal doors | £4,200 |
| Decoration | £2,800 |
| NHBC Warranty (1.4% of £95k build value) | £1,330 |
| Skip + waste removal | £950 |
| Site welfare + scaffold | £2,400 |
| Subtotal | £110,628 |
| Markup at 15% on supply, 25% on labour (already in figures) | — |
| Contingency 7% | £7,744 |
| Total to customer (turn-key) | £118,372 |
Round to £118,000 — at £3,945/m² this is upper-mid-range for high-spec turn-key. Strip out kitchen/bathroom (£19,500) and you have shell-and-finished at £98,500 / £3,283/m². Strip out further to shell-and-core only (omit MEP £15k, finishes £7k, decoration £2.8k) and you reach ~£74,000 / £2,466/m².
Frequently Asked Questions
Is timber frame really cheaper than masonry?
For shell-and-core, yes — typically 10–20% cheaper. For turn-key, the saving narrows to 0–10% because the kitchen, MEP, and finishes cost the same regardless of structural system. The bigger benefit is speed (weather-tight 2–3 weeks faster) and thermal performance (Part L compliance easier).
How long does a timber frame extension last?
Properly designed and built to BS EN 1995 with treated timbers per BS 8417 Use Class 2 and adequate moisture protection: 60–100+ years. The frame itself outlives the cladding, roof covering, and internal finishes. NHBC/Premier Guarantee provides 10-year structural warranty; well-built timber frame extensions are common in the UK from the 1970s onwards still standing well.
Do I need an architect for a timber frame extension?
You need: (1) Building Regs drawings + structural calcs (architect OR architectural technologist OR design-and-build contractor — £1,500–£5,000), (2) Structural engineer sign-off (£800–£2,500), (3) Planning application drawings if planning is needed (£600–£2,000). A timber frame company can do all of this in-house for design-and-build packages; otherwise you assemble the team yourself.
Can I do the work myself and just buy the frame?
Yes — self-build timber frame kits are widely available (e.g. Frame Tech, MBC Timber Frame, Custom Build). Customer-supplied frame + customer-arranged trades is common. As contractor, be cautious about taking on responsibility for someone else's frame design — get the supplier to confirm spec in writing and only quote your portion (foundations, MEP, finishes).
What's the VAT rate for an extension?
Standard 20% VAT on labour and materials for any extension to an existing dwelling. The DIY housebuilders VAT refund scheme applies only to new builds and conversions of non-residential buildings — not extensions. Confirm with your accountant for any specific job.
Regulations & Standards
Building Regulations Part A — Structure (load calculations, foundation design)
Building Regulations Part B — Fire safety (compartmentation, cavity barriers in timber frame)
Building Regulations Part C — Site preparation and resistance to moisture
Building Regulations Part E — Resistance to passage of sound (between dwellings/rooms)
Building Regulations Part F — Ventilation (MVHR or trickle vents)
Building Regulations Part L 2021 — Conservation of fuel and power (U-values, airtightness)
Building Regulations Part M — Access to and use of buildings (level threshold, door widths)
Building Regulations Part P — Electrical safety
BS EN 1995-1-1:2004+A2:2014 — Eurocode 5: Design of timber structures
BS 8417:2011+A1:2014 — Preservation of wood — Use Classes
BS 5268-2:2002 (withdrawn but referenced) — Structural use of timber: permissible stress design
NHBC Standards Chapter 6.2 — External timber framed walls
NHBC Standards Chapter 4.2 — Building near trees (foundation depth)
STA (Structural Timber Association) Technical Guidance — industry good practice
TRADA Technical Guidance — Timber design and detailing
Town and Country Planning (General Permitted Development) (England) Order 2015 — extension limits
Party Wall etc. Act 1996 — neighbour notification requirements
CDM 2015 — Construction (Design and Management) Regulations 2015
Approved Document L: Conservation of fuel and power (2021) — gov.uk
Approved Document A: Structure — gov.uk
Planning Portal: Extensions — Planning Portal
Structural Timber Association — Technical Guidance — STA industry body
TRADA — Timber Frame Construction — industry technical guidance
NHBC Standards — NHBC warranty technical standards
oak frame extension pricing guide — pricing oak-frame (post-and-beam) extensions
single storey extension pricing guide — single-storey extension pricing (masonry + timber frame)
double storey extension pricing guide — two-storey extension pricing
external wall insulation pricing guide — EWI render system pricing
foundations — foundation type selection for extensions
permitted development rights — permitted development guidance