House Extension Costs UK — Full Pricing Guide for Builders and Homeowners (2026)
House extensions remain the most popular way for UK homeowners to add space and value without the cost and disruption of moving. Ongoing planning reform — including the expansion of permitted development rights and the Neighbour Consultation Scheme — has made it easier to build than at any point in the last decade. But costs have risen sharply: materials and labour inflation has pushed average extension costs up 25–40% since 2021. This guide gives builders and homeowners accurate 2026 figures, a full trade-by-trade cost breakdown, and a clear picture of the planning, building regs, and Party Wall obligations involved.
What does a house extension cost in the UK?
The figures below are complete project costs — all trades, all materials, architect fees, structural engineer, building regulations, and scaffolding. They are what the end client pays for a fully finished, signed-off extension in 2026.
| Extension type | Cost per m² | Typical project cost |
|---|---|---|
| Single-storey rear (standard) | £1,800 – £3,200 | £40,000 – £70,000 (20m²) |
| Single-storey rear (premium) | £2,500 – £4,000 | £55,000 – £90,000 (20m²) |
| Double-storey extension | £1,600 – £2,800 | £70,000 – £120,000 (40m²) |
| Side return (terraced / semi) | £1,900 – £3,200 | £35,000 – £65,000 (18m²) |
| Wrap-around (rear + side) | £2,000 – £3,500 | £70,000 – £150,000+ |
| Garden room / modular | £1,000 – £2,000 | £20,000 – £50,000 |
UK averages 2026. London and South East: add 30–50% to the figures above. Costs exclude VAT where applicable.
Extension types and cost per m²
Single-storey rear extension
The most common extension type. A 20m² footprint is the benchmark — typically kitchen-diner, open-plan living, or utility extension. Standard build spec (blockwork cavity walls, tiled or flat roof, uPVC or aluminium windows) comes in at £1,800–£3,200/m². Premium spec with bifolding glazing, lantern roof, underfloor heating, and high-end kitchen fit-out runs £2,500–£4,000/m². Fully finished including all trades and decoration: £40,000–£70,000 standard, £55,000–£90,000+ premium. The largest cost variables are the glazing package and whether a kitchen or bathroom is included.
Double-storey extension
More cost-efficient per m² than single-storey because foundations, external walls, and roof are shared across two floors. A typical 40m² combined floor area (20m² ground, 20m² first) comes in at £1,600–£2,800/m² — a full project cost of £70,000–£120,000. Almost always requires full planning permission. Structural complexity is higher and the programme longer (typically 14–20 weeks). On a cost-per-m² basis, this is usually the best value extension type.
Side return extension (terraced and semi-detached)
Fills in the narrow side alley that runs alongside the property — typically 2–3m wide and 6–9m long. Often combined with a rear extension to create an L-shaped ground floor. Cost: £1,900–£3,200/m². A pure side return rarely delivers enough floor area on its own but transforms a kitchen when combined with rear extension space. Usually requires planning permission as it extends beyond the side elevation. London market pricing sits consistently at the top of or above the national range.
Wrap-around extension (rear + side combined)
Combines a rear extension with a side return to create a large open-plan ground floor. Structurally complex — the junction between the two extensions requires careful engineering to maintain the structural integrity of the existing house. Cost: £2,000–£3,500/m². A 30–50m² wrap-around project runs to £70,000–£150,000+ depending on specification. Planning permission almost always required. This is one of the most aspirational domestic extension types and typically attracts clients with significant budgets.
Garden room / modular structure
A detached outbuilding or modular structure built in the garden. Cost: £1,000–£2,000/m². Faster to build, often permitted development, and no disruption to the main house. However: limited use (not habitable space in most cases for planning purposes), cannot provide additional bedrooms or bathrooms for the main house, and modular quality varies significantly. Best suited to home offices, studios, and games rooms.
Full cost breakdown by trade
The figures below are what you should expect to pay subcontractors and suppliers for a typical 20m² single-storey rear extension in 2026. These are subcontract and materials costs — add your project management margin (15–25%) on top.
Planning permission vs permitted development
Whether a project needs a planning application depends on the extension type, property type, and location. Getting this wrong can mean enforcement action and the cost of demolishing unauthorised work.
Single-storey rear — permitted development
Single-storey rear extensions fall within permitted development rights up to 3m depth on an attached (terraced or semi-detached) house and 4m depth on a detached house, provided the extension does not exceed 4m in height. Larger extensions — up to 6m on attached and 8m on detached — are possible under the Neighbour Consultation Scheme (notifiable permitted development). Under this scheme the local authority notifies neighbours and has 42 days to raise objections before PD rights are confirmed.
Double-storey extensions
Double-storey rear extensions generally require full planning permission. PD rights for two-storey extensions are very limited — the extension must be no closer than 7m from the rear boundary, no taller than the existing roof, and must not include balconies. In practice, most double-storey projects go through full planning. Programme: allow 8–13 weeks for the application and decision.
Side extensions
Side extensions are not permitted development on the flank elevation facing a highway. In most other cases a single-storey side extension may fall within PD if it is no wider than half the width of the original house — but in practice most side and wrap-around projects require planning permission. Always confirm with the LPA before advising clients. Planning application fee in England (2026): £258.
Conservation areas and listed buildings
PD rights are withdrawn or significantly restricted in conservation areas, Areas of Outstanding Natural Beauty, and for listed buildings. Any extension to a listed building requires listed building consent in addition to planning permission. Do not assume PD applies without checking. Always advise clients to obtain a Lawful Development Certificate for any permitted development works — this provides a formal record that the extension was lawful at the time of construction and prevents disputes on sale.
Building regulations requirements
Building regulations approval is required for all extensions regardless of whether planning permission was needed. There is no route around this. A completed extension without building regs sign-off will create serious problems on sale and may require costly remediation.
- →Structural. Foundation design, beam and lintel specifications, and load-bearing wall openings must be signed off by building control. A structural engineer's calculations are required for any opening in an existing structural wall — even a single-storey extension with a wide opening into the existing house typically requires a steel or concrete lintel specified by an engineer.
- →Thermal performance (Part L). The extension must meet current U-value requirements: external walls 0.18 W/m²K, roof 0.15 W/m²K, floor 0.22 W/m²K, windows and doors 1.6 W/m²K. This drives insulation specification across the entire extension and affects fabric costs materially.
- →Fire safety. If the extension creates a new kitchen or changes the layout in a way that affects means of escape, fire safety provisions may need to be upgraded elsewhere in the property. Confirm with building control at the application stage, not on the day of inspection.
- →Drainage and services. Any changes to drainage, soil stacks, or gas or electrical services require building control notification. Extensions that include a kitchen or bathroom trigger plumbing and drainage inspections. Building regulations fees: £700–£1,500 for a standard domestic extension.
Party Wall Act obligations
The Party Wall etc. Act 1996 applies whenever work is carried out on or close to a shared wall or boundary. For extensions, this means:
- →Work on a shared (party) wall. Any work affecting a wall that is shared with a neighbouring property — including cutting in for beams or joists, raising the wall height, or underpinning — requires a Party Wall Notice to be served on the affected neighbour.
- →Excavation within 3–6m of a neighbouring structure. New foundations within 3m of the neighbour's foundation depth (or within 6m if the new foundations go deeper than a 45-degree line drawn from the neighbouring foundation) also trigger the Act.
- →Timescales. Party Wall Notice must be served at least one to two months before work starts (two months for work on a shared wall; one month for excavation). If the neighbour consents, work can proceed. If they dissent or do not respond within 14 days, a surveyor must be appointed. The client pays the costs.
- →Costs. Party Wall surveyor fees run to £700–£3,000 per affected neighbour. On a semi-detached property with one shared boundary, this is a predictable cost. On a mid-terrace with two neighbours, budget for two sets of fees. Always flag this in writing to the client before work starts and include the notice timescales in your project programme.
What drives extension costs higher
- →Unusual soil conditions. Clay soils prone to shrinkage, high water table, made ground, or proximity to mature trees can push foundation costs from the £5,000 end to £12,000+ — and in some cases trigger piling at £15,000–£30,000 on a domestic extension. Always include a soil investigation recommendation in your pre-quote advice.
- →Tree proximity. Trees with large root systems close to the extension footprint require root barrier installation or deeper foundations to avoid future structural movement. Structural engineers will flag this; the cost of not doing so is paid in subsidence claims later.
- →Premium glazing. Bifolding doors for a typical 3m opening run to £3,000–£8,000 supply and fit. Sliding aluminium systems sit at the top of that range. French doors for the same opening: £1,500–£3,000. A roof lantern over an open-plan extension adds £2,500–£6,000. Clients who specify premium glazing often do so after the quote is agreed — use provisional sums and get sign-off on the specification before ordering.
- →Kitchen or bathroom in the extension. Any wet trade installation — kitchen, utility room, WC, or shower room — adds plumbing, drainage, and potentially soil stack relocation. Budget an additional £2,000–£5,000 for a utility or WC, and £5,000–£15,000+ if a full kitchen fit-out is included.
- →Underfloor heating. A wet UFH system on a 20m² extension slab adds £2,000–£4,000 including manifold and connection to the boiler. Electric UFH is cheaper to install (£500–£1,500) but more expensive to run. This must be specified before foundations are laid — it cannot be retrofitted into a concrete slab.
- →Complex roof form. A simple flat or monopitch roof is the cheapest option. A pitched tiled roof to match the existing house costs more. A large roof lantern or structural glazing element is at the premium end. Roof form is one of the biggest specification variables on a rear extension — get it agreed in writing before pricing.
- →Smart home integration. Wiring for smart lighting, AV systems, and CCTV is straightforward to install at first fix but adds £1,500–£5,000+ depending on the system specified. Like glazing, this is often a client decision made after the quote is signed off. Define what is and is not included.
How to quote an extension
Extensions are high-value, long-programme jobs where underquoting is most likely to result in a loss-making project. Here is how to structure the quoting process to win work at the right price.
Site visit and soil assessment
A site visit is mandatory before any quote. Walk the footprint of the proposed extension, look for signs of tree root activity or poor ground, check existing drainage runs that may need diverting, and assess access for machinery and materials. If ground conditions are uncertain, recommend a soil investigation (desk study or trial pit) before committing to a fixed foundation price. A soil investigation typically costs £500–£1,500 — trivial relative to the cost of underquoting foundations.
Structural engineer input
Engage a structural engineer before issuing a detailed quote on any extension that involves an opening in a structural wall or a complex roof form. The engineer's calculations determine beam sizes, padstone requirements, and foundation specification. Quoting without this is guessing at your largest cost items. Structural engineer fees: £500–£1,500. Some builders charge this back to the client as a pre-contract fee; others absorb it as a cost of winning the work. Either approach is acceptable — do not produce a fixed price without it.
Breaking out prelims
Preliminaries (prelims) are the site-running costs that are real but often invisible in a trade-by-trade breakdown: scaffolding hire, site setup, welfare facilities, plant hire, skips, and general labourer time. On an 8–12 week extension these add up quickly. Present prelims as a separate line item in your quote rather than spreading them invisibly across trade costs. This makes your quote more transparent and makes it harder for clients to cherry-pick trade costs or supply materials themselves on your programme.
Payment stage structure
A domestic extension running for 10–16 weeks requires a milestone payment schedule. Five stages is standard for the industry:
Include the payment schedule as part of the written quote, not as a separate document issued after acceptance. Clients who see a clear payment programme before signing have fewer disputes about when payments fall due. Never start on site without at least the Stage 1 deposit received and cleared. For domestic clients, retention is uncommon — but a defects liability period of 12 months on practical completion is standard. Include this in your contract terms.
Track which channels bring in your extension enquiries
Extensions are your highest-value domestic jobs. Trade2Base tracks where every lead comes from — Google, referral, Checkatrade, direct — so you know exactly where to invest your marketing budget to win more of them.
Start free trial