How to Price an Oak Frame Extension: Structural Frame, Cladding and Finishing
Quick Answer: A green oak frame extension in the UK runs £3,200–£4,800/m² for a fully finished structure including frame, encapsulation, glazing and internal finishes — significantly above a standard masonry extension at £1,800–£2,800/m². The frame itself is £900–£1,600/m² supply and erect; the rest is the wraparound build (SIPs encapsulation or oak-and-glass infill, foundations, roofing, glazing and internals). All visible green oak structural members must be designed to BS EN 1995-1-1 (Eurocode 5) and certified by a structural engineer at TRADA-equivalent standard.
Summary
Oak frame extensions are a premium product. The price is driven by the frame itself — typically air-dried green oak from a UK or French frame company (Carpenter Oak, Border Oak, Oakwrights, English Heritage Buildings) — and by the necessary care taken with the wraparound construction. A green oak frame moves as it dries (shrinks 5–10% across the grain over 5–7 years), so the encapsulation must allow that movement without compromising thermal envelope or weather seal.
The pricing trap on oak frame projects is that the frame supplier is often quoted on a "supply and erect" basis with the wraparound work assumed by a separate main contractor. This works only if the contractor understands oak — the joints between SIPs panels and oak shoulders, the lead flashings around the corner posts, the secret-fix encapsulation that hides the membrane behind a feature stop bead — these details add significant labour cost over a standard timber-frame or block job. Many builders price an oak job as if it were standard frame and lose 15–25% on margin once they reach the cladding stage.
The other cost driver is glazing. Most oak frame extensions are designed around full-height glazing infill panels — direct glazed onto the oak posts and beams, typically with structural silicone or rebated glazing beads. This is specialist glazing work, not a standard window installation, and the unit cost is significantly higher than equivalent area of timber-frame glazing.
Key Facts
- Oak frame supply and erect — £900–£1,600/m² (covers green or air-dried oak posts, beams, braces, pegs)
- Total cost (frame + envelope + finish) — £3,200–£4,800/m² for fully finished, including foundations
- Premium oak species — French oak £80–£140/ft³; UK oak £60–£100/ft³
- Frame design — usually CAD-designed by frame company, structural calcs by their engineer
- SIPs encapsulation — £180–£280/m² for 142mm or 172mm panels (R-value 5.5–6.8 m²K/W)
- Oak-and-glass infill — £900–£1,800/m² for glazed bay between oak posts
- Roof — slate or clay tile — £140–£220/m² covering, plus £180–£260/m² structure
- Foundations — strip £140–£220/m run, raft £160–£250/m² depending on ground
- Building Regs — Part A (structure), L (insulation), B (fire), F (ventilation), L1B (existing dwelling)
- Planning — most over 30m² need permission; PD allowance for rear extensions varies (3m semi-detached, 4m detached, larger temporary allowances may apply)
- Listed buildings — listed building consent required; Article 4 area also restricts PD
- Glazing — direct-set to oak — £600–£1,100/m² supplied and fitted (not standard window pricing)
- Lead flashings — £40–£80/m run including dressing and pointing
- Underfloor heating — £55–£90/m² supplied and fitted (most common heat emitter for oak frame extensions)
- Lead time — 12–20 weeks for frame manufacture and delivery
- Erection — typically 3–7 days on site for an average single-storey frame
- Completion — typically 16–24 weeks from frame arrival to handover
Quick Reference Table — Oak Frame Cost by Component
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Try squote free →| Component | £ per m² of GIA | Notes |
|---|---|---|
| Frame (supply + erect) | £900–£1,600 | Includes oak, joints, pegs, design |
| Foundations | £180–£320 | Strip or raft, varies with ground |
| Floor (concrete + UFH + finish) | £180–£280 | Insulated screed common |
| Wall encapsulation (SIPs) | £180–£280 | Includes external sheathing |
| Roof structure + covering | £320–£480 | Tile, slate, or zinc on insulated rafters |
| Glazing — direct set | £600–£1,100/m² of glass | Glass infill bays |
| Glazing — bifold/sliding | £900–£1,800/m² | Standard high-spec doors |
| Internal finishes | £400–£600 | Lime plaster, lime render or boarding |
| Mechanical & electrical | £180–£320 | UFH, lighting, sockets |
| External cladding (where used) | £80–£200 | Oak feathered, larch, render |
| Total (typical 30m² extension) | £3,200–£4,800 | Includes all of above |
Detailed Guidance
Frame design — pre-engineered vs bespoke
Most UK oak frame companies offer two routes:
- Pre-engineered range — standardised frame designs from a catalogue (e.g. Border Oak's "Standard" range). Lower design cost, faster lead time, typical £900–£1,200/m² for the frame.
- Bespoke design — frame designed from scratch by the company's design team. £1,200–£1,600/m². Necessary for unusual plans or where the frame is the architectural feature.
Pre-engineered frames suit kit-style rear extensions; bespoke frames suit large open-plan rooms with feature trusses or unusual roof shapes. Always get written confirmation from the frame company that their structural engineer has certified the design under Eurocode 5 (BS EN 1995-1-1) — this is required for building control sign-off.
Encapsulation — SIPs vs oak-and-glass
The two main approaches to filling between the oak structural members:
SIPs panels (Structural Insulated Panels): 142mm or 172mm sandwich panels (OSB / PIR or PUR / OSB), fixed externally to the oak frame on a rebated edge or to a timber sub-frame fitted to the oak shoulders. Gives a continuous insulation envelope (typical U-value 0.18–0.22 W/m²K), no thermal bridging at the frame, full vapour control.
Cost: £180–£280/m² supply and fix. Requires careful detailing where SIPs meet oak — most frame companies provide drawings showing how to seal the junctions. Don't skimp on the tape and gasket (Pro Clima Tescon Vana or equivalent) — this is where vapour leaks happen.
Oak-and-glass infill: Frameless glass bays between exposed oak posts and beams, structurally silicone-set or rebated and gasket-glazed. Gives the iconic "oak frame extension" look but is significantly more expensive.
Cost: £900–£1,800/m² of glazed area, including specialist installer.
Most projects mix both — solid SIPs walls on the back and side, glass infill on the principal elevation.
Foundations — strip vs raft
Most oak frame extensions sit on:
- Strip foundation — concrete strip 600–900mm deep × 600mm wide, blockwork up to oversite. £140–£220/m run including excavation and backfill.
- Raft foundation — 200–250mm reinforced slab on engineered fill. £160–£250/m² of slab. Used on poor ground (clay, made ground) or where minimum disturbance is required (close to existing trees).
Building Control will normally require a structural engineer's design for an oak frame extension because of the loadings at the corner posts (point loads), which differ from masonry's distributed load.
Roof — slate, tile, zinc
Oak frame roofs are typically:
- Clay or natural slate — £140–£220/m² covering, classical aesthetic, 80+ year life
- Concrete tile — £80–£140/m², budget option, 50–70 year life
- Zinc or copper standing seam — £200–£300/m², modern aesthetic, 60–100 year life
- Sedum/green roof — £140–£240/m² supply and fit, plus structure upgrade for load
Insulation between rafters or above (warm roof) — typically 200–250mm PIR or 250–300mm wood fibre. Wood fibre adds £30–£60/m² over PIR but performs better against summer overheating, increasingly specified.
Glazing — direct-set vs aluminium frame
Direct-set glazing (glass fixed straight to oak posts and beams with structural silicone) is the iconic look. Cost £600–£1,100/m² of glass. Requires a specialist glazier and oak detailing that allows for movement.
Aluminium-framed glazing (slim-frame fixed windows or bifolds) is cheaper at £600–£900/m² for fixed lights or £900–£1,800/m² for opening doors. More forgiving of frame movement.
The price difference is significant on a project with 20m² of glazing — direct-set adds £6,000–£10,000 over aluminium-framed.
Movement allowance — why oak isn't standard timber
Green oak frame shrinks across the grain by 5–10% over 5–7 years as it dries from ~30% moisture content to ~15% (UK ambient). The detailing must allow this:
- Slip planes between oak and SIPs (typically a Z-bar or rebate with sliding gasket)
- Sliding head detail at internal walls fixed to oak posts
- No rigid finishes (e.g. no plaster) hard against oak — use shadow gap detail
- Metal flashings dressed to allow movement (lead aprons, not rigid clips)
Customers should be warned about visible movement: typical 25–40mm shrinkage on a 200mm post over 5 years, with shake (longitudinal cracks) appearing in the first 2 years. This is normal and structurally insignificant but visually surprising — set the expectation at quote stage.
Frequently Asked Questions
How much is an oak frame extension in 2026 UK pricing?
£3,200–£4,800/m² for a fully finished single-storey rear extension. A 30m² extension typically £96,000–£144,000 all-in. A 50m² extension £160,000–£240,000.
Can I get an oak frame extension for under £100k?
Possibly for a small (15–20m²) extension with minimal glazing and standard finishes — £60,000–£90,000 range. Larger or glazing-heavy projects rarely come in under £100,000.
Do I need planning permission for an oak frame extension?
Same rules as any extension. Single-storey rear extensions under PD limits (3m semi-detached, 4m detached) up to 4m height don't need planning. Larger Prior Approval extensions (up to 6m semi/8m detached) need a Prior Approval application. Listed buildings or Article 4 areas need full planning.
Why is oak frame so expensive vs standard extension?
Three reasons: oak material itself (£60–£140/ft³), the labour skill required for the frame and its detailing, and the high specification of the wraparound (glazing, finishes) that customers typically demand on what is a feature project.
How long does an oak frame extension take?
12–20 weeks frame lead time, then 16–24 weeks on site. Total programme 28–44 weeks from order. Factor 8–12 weeks for design and planning before that.
Regulations & Standards
Building Regulations Approved Document A — structural safety; oak frame designs require Eurocode 5 calcs
Building Regulations Approved Document L1B — energy efficiency for extensions to existing dwellings
Building Regulations Approved Document B — fire safety; typically 30 minutes' fire resistance on exposed oak structural members
Building Regulations Approved Document F — ventilation; MVHR or trickle vents
BS EN 1995-1-1 — Eurocode 5: design of timber structures
BS 5268-2:2002 — structural use of timber (legacy, still cited on retrofits)
BS 8417:2011+A1:2014 — preservation of wood (relevant for cladding, not frame)
TRADA Wood Information Sheets — industry guidance on oak frame design and detailing
Town and Country Planning (General Permitted Development) Order 2015 — extension PD rights
TRADA — Oak Frame Construction Guide — UK timber industry guidance
Carpenter Oak — UK frame manufacturer technical resources
Border Oak — UK frame manufacturer technical and pricing examples
Building Regulations Approved Document A — structural requirements
Planning Portal — Extensions — PD rules for extensions
side-return extension pricing for terraced and semi-detached properties