How much does a house extension cost in the UK? (2026)
Verified UK house extension cost per m² for single-storey, double-storey, and side return projects in 2026, plus professional fees, planning, party wall, and what extension quotes typically miss.
A UK house extension in 2026 costs £1,800–£3,000 per m² for a standard single-storey extension, dropping to £1,500–£2,400 per m² for double storey. For a typical 25–30 m² rear extension, that puts the build cost at roughly £45,000–£90,000, plus VAT, plus professional fees, plus planning. The figures below come from cross-referencing four independent UK cost-guide publishers.
Quick answer
A typical UK single-storey rear extension costs £1,800–£3,000 per m² in 2026 (£3,000+ for premium spec; +15–25% in London). Double storey drops to £1,500–£2,400 per m² because foundations and roof amortise. Add 3–15% for architect fees, £258 for planning permission (England householder), £400–£1,200 for Building Control, and £1,000–£2,500 per neighbour for any required Party Wall Award. VAT (20%) is on top of all of this, and most quotes show it separately.
How to read this guide#
Two kinds of figures appear below:
- Headline price ranges (per m² rates, professional fees, statutory fees): cross-referenced against multiple UK cost-guide publishers and, in the case of planning permission, the statutory fee from GOV.UK.
- Practical guidance (typical timelines, regional uplifts, contingency advice, quote red flags): drawn from standard UK construction industry practice. Useful but not cross-referenced figure-by-figure.
Where we could not verify a figure, we have left it out or described it qualitatively rather than publish a number that does not trace to a source.
Headline ranges (verified)#
Build cost per m²#
| Extension type | £/m² (standard spec) | Typical 30 m² total |
|---|---|---|
| Single-storey rear | £1,800 – £3,000 | £54,000 – £90,000 |
| Double-storey rear | £1,500 – £2,400 | £45,000 – £72,000 |
| Side return (8–15 m²) | (£20,000 – £60,000 typical lump sum) | n/a |
Premium specifications (high-end glazing, complex roof forms, bespoke joinery) push single-storey costs to £3,300+ per m² (Homebuilding & Renovating). The London and South-East premium is roughly 15–25% above national rates (HomeOwners Alliance reports £2,300–£3,400 per m² for London single-storey).
These per-m² rates cover the build to second-fix: foundations, walls, roof, glazing, M&E first and second fix, plaster, and floor finishes. They exclude: VAT, professional fees, kitchen fit-out beyond connection points, external works beyond the immediate building footprint, and any bespoke specification upgrades.
Professional and statutory fees#
| Item | Range | Source |
|---|---|---|
| Architect (design only) | 3–7% of build cost | Homebuilding & Renovating |
| Architect (full project management) | 8–15% of build cost | Homebuilding & Renovating |
| Structural engineer (typical extension) | £450 – £1,500 | MyJobQuote, HOA |
| Planning permission (England, householder) | £258 | GOV.UK statutory |
| Lawful Development Certificate (England) | £129 | GOV.UK statutory |
| Building Control (typical extension) | £400 – £1,200 | Homebuilding, HOA |
| Party wall surveyor (per neighbour) | £1,000 – £2,500 | Homebuilding, HOA |
Practical guidance (industry standard)#
The remainder describes how UK extension projects typically run in practice: what is included in a quote, what is usually missing, the sequence of professional engagements, and the common red flags. These are conventions used across the UK domestic-build trade.
What the headline build cost normally covers#
A complete extension build quote should typically cover:
- Site setup, welfare, and protection of existing finishes
- Demolition of any existing structure being removed
- Foundations and groundworks, including drain diversions where required
- Brickwork, blockwork, and steel beams to structural-engineer drawings
- Roof structure and covering, including flashings and gutters
- Windows and doors to the agreed specification
- First-fix electrics and plumbing
- Plasterboarding, plastering, and skim
- Second-fix electrics and plumbing
- Floor screed and finishes (where specified)
- Internal joinery: skirting, architrave, doors
- Decoration to the new extension only
- Building Control sign-off through the appointed inspector
It usually does not cover:
- VAT (20%), almost always shown as a separate line; double-check
- Architect, structural engineer, and party wall fees (engaged separately by the homeowner)
- Planning application fees (paid directly to the local authority)
- Building Control inspector fees (usually invoiced separately)
- Kitchen, bathroom, or bedroom fit-out beyond the connection points
- Decoration to the existing house adjacent to the extension
- External works: patios, paths, drainage to the boundary
- Floor finishes if "specified by client"
- Lighting fittings and switches beyond standard provision
When you compare extension quotes, the easy mistake is comparing a £68,000 quote inclusive of all the above to a £58,000 quote that excludes several items. Read the exclusions section before you compare totals.
How extension quotes are usually staged#
A typical extension contract has staged payments tied to milestones:
- Deposit: 5–10% on signing
- Mobilisation: at site setup
- Foundations complete
- Watertight (walls and roof done)
- First-fix complete
- Plastered
- Second-fix complete
- Snagging and final completion
Never pay materially ahead of work done. A staged structure protects both parties: the builder is paid for completed work, and the homeowner is not exposed to a contractor walking off site with cash already paid.
Planning permission and permitted development#
Many domestic extensions in England fall within Permitted Development Rights, meaning you can build them without full planning permission provided you stay inside the size, position, and material limits set by the Town and Country Planning (General Permitted Development) Order. Single-storey rear extensions on a detached house can be up to 8 m deep without planning, on a semi or terrace up to 6 m, subject to neighbour consultation under the Larger Home Extensions process.
If your project is permitted development, applying for a Lawful Development Certificate (£129) is recommended. It gives written confirmation from the council that the work does not require planning permission, which matters when you sell.
If your project needs full planning, the householder application fee is £258. Decisions take 8 weeks for a straightforward case, longer if neighbours object or the council requests amendments. Listed buildings and conservation areas have additional consents that take longer.
When the Party Wall Act applies and what it costs#
The Party Wall etc. Act 1996 governs work that affects shared walls or that involves excavation close to neighbouring structures. You need a Party Wall Award (and must give formal Party Wall Notices) if you are:
- Working on a wall shared with a neighbour
- Excavating within 3 m of a neighbour's building, at the same depth or deeper than their foundations
- Excavating within 6 m at certain greater depths
Each affected neighbour can either consent to your notice (no surveyor needed) or appoint their own surveyor. If they appoint, a Party Wall Award is drawn up by the surveyor or surveyors. The building owner (you) pays both surveyor's fees, typically £1,000–£2,500 per neighbour. One shared "Agreed Surveyor" is cheaper than each side appointing separately, if the neighbour will agree.
The Party Wall process should start at least 2 months before work begins. Skipping it is a common shortcut and a costly one. Neighbours can stop work if you have not served notice properly.
Regional variation#
Extension costs vary materially by region. The London and South-East premium is the most consistently documented at 15–25% above national average. Other regional adjustments below come from standard UK quantity-surveying practice and apply mainly to the labour and trade portion (materials are nationally priced):
- Inner London: ~20–30% above
- Outer London / M25: ~12–20% above
- South-East (Brighton, Cambridge, Oxford, Guildford): ~5–12% above
- Midlands and East: close to national
- North of England, Wales: ~5–10% below
- Northern Ireland, rural Scotland: ~8–12% below
These are rough adjustments. Actual variation depends on the contractor's location, current trade demand, and access constraints (urban infill sites cost more than open-plot rural ones, regardless of region).
Red flags in extension quotes specifically#
Beyond the standard quote red flags (covered separately), some are extension-specific:
No structural engineer drawings referenced. Any extension involving removal of an external wall, opening a load path through to the existing house, or supporting an upper storey on new walls needs structural calculations. A quote that does not reference SE drawings is either missing that scope or planning to skip it.
No allowance for ground conditions. Quotes that price foundations without mentioning ground conditions assume good ground. Clay soils, made ground, high water tables, or proximity to trees can add £5,000+ in foundation work. Check whether your quote calls out the ground assumption.
No mention of Building Control inspector route. Either the local authority or an Approved Inspector handles Building Control. Approved Inspectors are usually faster but the costs can vary. A quote that doesn't say which is being used has not factored the fees in.
Optimistic timelines. A typical 30 m² single-storey extension takes 14–20 weeks on site once started. Quotes promising completion in 8 weeks are either pricing for an unrealistic schedule or missing scope. Each missing week's allowance comes back as an extra at the end.
Provisional sums for groundworks without a soil survey. "Foundations: provisional £8,000" without a soil survey is a placeholder, not a price. For projects of any size, get the soil tested before you sign.
No allowance for power, water, and welfare. Site setup costs are real: temporary power, water, a portable toilet, fencing, signage. A quote that has the trades but no site setup line is missing it.
Contingency#
For a typical extension, 10–15% contingency on top of the build cost is standard. Older properties (pre-1930), unknown ground conditions, or significant alterations to the existing house warrant 15–20%. Contingency that is not used at the end of the project is genuine upside; you can spend it on the bathroom or kitchen specification you deferred. Contingency you needed and did not budget for becomes a mid-project pressure conversation that almost always favours the builder.
Sequence of professional engagements#
For a typical domestic extension:
- Architect: engaged first, produces the design and applies for planning (if needed).
- Structural engineer: engaged once the design is agreed, produces calculations and drawings for the steel beam and foundation design.
- Party wall surveyor: engaged 2 months before work starts if the Act applies.
- Builder: quoted on the architect and engineer's drawings, appointed once the price and contract are agreed.
- Building Control: appointed at quote stage, usually paid for in stages alongside the build.
Skipping or compressing these (for example, hiring a builder who will "design and build" without separate architect and engineer involvement) sometimes works on small extensions, but for anything over 20 m² or involving structural changes, it materially increases risk.
Comparing your extension quote#
Extension quotes vary widely for the same scope, often by 30%+, because specification and inclusions differ more than the headline numbers suggest. The fastest way to know whether your quote is fair is to compare line items rather than totals. The quote checker on this site parses every line, compares to current UK rates with regional adjustment, flags missing scope, and runs a Companies House check on the contractor. Below the cost of a single architect site visit, and faster than reading the quote yourself.
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Frequently asked questions
- How much does a single-storey rear extension cost in the UK in 2026?
- A single-storey rear extension costs £1,800–£3,000 per m² for a standard specification (Checkatrade, HomeOwners Alliance). For a 20 m² extension that is roughly £36,000–£60,000 plus VAT and professional fees; for 30 m² roughly £54,000–£90,000. London and the South-East run 15–25% above national average. Premium specifications (high-end glazing, bespoke joinery, complex roofs) can push per-m² costs to £3,300+ (Homebuilding & Renovating).
- Why does a double-storey extension cost less per m² than a single-storey?
- Foundations and roofs cost roughly the same whether the extension is single or double storey, so adding a second floor doubles the usable space without doubling those fixed costs. Per-m² costs typically drop from £1,800–£3,000 (single) to £1,500–£2,400 (double) at standard specification. The total cost still goes up because there is more floor area, but each m² is cheaper.
- How much do architect fees add to an extension?
- Between 3% and 15% of the build cost, depending on the level of service. Design-only (drawings up to planning) is typically 3–7%; full project management through to completion is 8–15%. Architectural technologists charge less (2–5%) and can deliver the design portion; full PM service pays back when the project is complex or the homeowner cannot manage it directly. Source: Homebuilding & Renovating, HomeOwners Alliance.
- How much is planning permission for a house extension?
- In England, the householder application fee is £258 in 2026 (statutory, GOV.UK). A Lawful Development Certificate (used when the work is permitted development and you want certainty) is £129. Wales, Scotland, and Northern Ireland have separate fee schedules. Many extensions do not need full planning permission if they fall within Permitted Development limits. Your architect or planning consultant will advise.
- When do I need a Party Wall Award?
- When the extension involves work to a shared wall with a neighbour, excavation within 3 m of a neighbour's structure (and at the same depth or deeper), or excavation within 6 m at certain depths. The Party Wall etc. Act 1996 sets the rules. Surveyor fees are typically £1,000–£2,500 per neighbour, paid by the building owner (you). One Agreed Surveyor for both sides is cheaper than separate surveyors, if the neighbour will agree to it.
- What's commonly missing from extension quotes?
- Common gaps: VAT (some quotes are quoted excluding 20% VAT, so always confirm), making good decoration to the existing house adjacent to the extension, kitchen fit-out beyond plumbing connections, flooring, external works (paving, drainage, landscaping), professional fees if not engaged separately, and Building Control fees. Also frequently missing: temporary protection for existing finishes, scaffolding for higher-level works, and skip hire beyond the initial dig.
- Should I include a contingency budget?
- 10–15% on top of the headline build cost is the conventional contingency for extensions where the property is reasonably new and well-documented. 15–20% is more sensible for older properties (Victorian terraces, 1930s semis, anything with unknown ground conditions or historic alterations). Contingency money you do not spend rolls into upgrades (better glazing, better kitchen) at the back end of the project. Contingency you needed and did not have becomes a difficult mid-build conversation.