House Extension Cost UK 2026: Every Type, Every Size — Real Figures
Quick Answer
House extension costs in the UK range from £1,500–2,500/m² for a standard single-storey rear extension to £2,500–4,000/m² for a premium contemporary finish. A typical 20m² kitchen extension costs £30,000–50,000 for shell and basic fit-out. Factors that move the price: glazing specification, roof type, structural opening into the house, planning fees, and ground conditions.
A house extension is the largest investment most homeowners make in their property — and it's the one where a vague budget causes the most damage. I've been pricing and building extensions for 32 years. The question I hear constantly is: "Why is this so much more than I expected?" The answer is almost always the same: the shell costs were underestimated, and the fit-out was forgotten entirely.
This guide covers every main extension type — single storey rear, double storey, wrap-around and side return — with cost per m² ranges for 2026, real-world examples, and clear explanations of what drives the price beyond the basic build: planning, structural engineer, party wall, glazing specification, ground conditions and fit-out costs.
If you're planning an extension in 2026, read this before approaching any builder or architect. Having a realistic budget before you start the design process saves weeks of wasted redesign and disappointing quotes.
House Extension Cost by Type — UK 2026 Per m² Rates
The per-m² rates below are for the structural shell — foundations, external walls, roof, windows and external doors, first-fix services routed in. They exclude fit-out (kitchen, bathroom, flooring, decoration) and professional fees. For further detail on single-storey extensions, see our dedicated single-storey extension cost guide.
| Extension Type | Cost Per m² | Notes |
|---|---|---|
| Single-storey rear — standard build | £1,500–£2,500 | Most common type; pitched or flat roof; standard glazing |
| Single-storey rear — high-spec | £2,500–£4,000 | Full-width bi-folds, lantern roof, underfloor heating |
| Double-storey rear | £1,800–£3,000 | Per m² of ground floor area; more economical per m² than single storey |
| Wrap-around (side and rear) | £2,000–£3,500 | L-shaped plan; additional complexity at the corner; usually requires planning |
| Side return | £1,800–£2,800 | Filling in the side return of a Victorian terrace; often includes glazed roof |
Per-m² rates fall as the extension gets larger because the fixed costs (scaffolding, groundworks mobilisation, structural openings into the house) are spread over more floor area. A 10m² extension is more expensive per m² than a 30m² extension — this is why it rarely makes sense to build a very small extension if you're going through the disruption and planning process regardless.
Real Cost Examples — Extension Build Costs 2026
The table below shows what specific extension sizes actually cost at current UK trade rates. These are shell-only costs — fit-out is additional.
| Extension | Footprint | Standard Build | High-Spec |
|---|---|---|---|
| Small single-storey rear | 12m² (3m x 4m) | £18,000–£30,000 | £30,000–£48,000 |
| Medium single-storey rear | 20m² (4m x 5m) | £30,000–£50,000 | £50,000–£80,000 |
| Large single-storey rear | 30m² (5m x 6m) | £45,000–£75,000 | £70,000–£120,000 |
| Double-storey rear | 20m² ground + 20m² first floor | £60,000–£90,000 | £90,000–£140,000 |
| Side return (Victorian terrace) | 10–15m² | £20,000–£40,000 | £35,000–£60,000 |
To these shell costs, add: professional fees (architect and structural engineer, £3,000–£8,000 depending on complexity), planning application (£206 fee plus architect's drawings), Building Control (£500–£1,500), and fit-out costs for the new space. A kitchen extension with a quality kitchen fit-out adds £8,000–£25,000 to the build cost; a new family room with flooring and decoration adds £3,000–£8,000.
Beyond the Shell: Extension Costs People Forget
Professional Fees
An architect or architectural designer is needed for planning drawings and Building Control full plans. Fees range from £1,500–£4,000 for a straightforward single-storey extension, higher for complex designs or design-and-manage appointments. Don't skip this — poorly drawn plans lead to building control rejections and on-site problems that cost far more than the fee would have.
Structural Opening into the Existing House
Unless you're planning to leave the extension as a separate room accessed from outside (unlikely), you'll need to create an opening in the existing rear wall. This typically involves installing a steel beam (RSJ) to carry the load above the opening, knocking through the wall, and finishing the reveal. This costs £2,000–£5,000 depending on the size of the opening and the load it carries. It's not always included in extension quotes — check explicitly.
Underfloor Heating
Underfloor heating is almost the standard choice in new extension floors — it works well with the concrete slab or beam-and-block floor construction typical of extensions, and it frees up wall space compared to radiators. A wet UFH system in a 20m² extension costs £2,000–£4,000 for the underfloor element including manifold, connected to the existing boiler. An electric system is cheaper to install (£800–£1,500) but more expensive to run. Get a heat loss calculation before specifying the system size.
Ground Conditions
If your garden has made ground, trees nearby, or poor bearing capacity, standard strip foundations won't be sufficient. You may need to go to engineer-specified depths, or use pile foundations in the worst cases. An unexpected foundation upgrade can add £3,000–£15,000 to the groundworks cost. Always get a quote from a structural engineer before finalising your budget — ground conditions are the most common source of cost overruns in extension projects.
How RenoCalc Quotes a House Extension
RenoCalc reads your floor plan and produces a full extension cost breakdown — shell, structural opening, fit-out and professional fees — in around 3 minutes. The RenoCalc Spreadsheet contains 40,000+ live formulas and a current UK material and labour price library.
Planning Permission and Permitted Development
Understanding what needs planning permission and what doesn't will save you weeks and money on the design process.
Permitted Development — Single Storey Rear Extensions
Under standard permitted development rights in England, a single-storey rear extension up to 4m deep (detached house) or 3m deep (semi-detached or terraced) is allowed without planning permission. The Prior Approval scheme extends this to 8m (detached) and 6m (semi/terrace) subject to a neighbour consultation. Conditions apply: the extension must not exceed 4m in height, must not cover more than half the garden, and materials must be similar in appearance to the existing house.
Permitted development rights are removed for: properties in conservation areas, listed buildings, and some properties where rights have been removed by a planning condition. Check before assuming PD applies to your property.
Planning Application — When Required
You need a full householder planning application for: extensions that exceed PD limits, side extensions, double-storey extensions, wrap-around extensions, and any external alteration in a conservation area. The planning fee in England is £206. Your architect prepares the required drawings — location plan, existing and proposed floor plans, elevations, and site plan. Allow 8–13 weeks for the planning decision.
| Extension Type | Planning Required? | Notes |
|---|---|---|
| Single-storey rear (within PD limits) | Usually not required | Prior Approval needed above 4m/3m standard limits |
| Single-storey rear (exceeds PD limits) | Yes | Full householder application, £206 fee |
| Double-storey rear | Yes | Always requires planning permission |
| Side extension | Yes | Side extensions are not permitted development |
| Wrap-around | Usually yes | Side element triggers planning requirement |
Structural Engineer and Party Wall Agreement
Structural Engineer
A structural engineer is required for all extensions — they produce calculations for the foundation design, the beam over any structural opening into the existing house, and the roof structure. This is a Building Regulations requirement. Cost: £500–£1,500 for a straightforward single-storey extension, £1,000–£2,500 for a double-storey or more complex design. Get this commission in place early — you need the structural specification before the foundation concrete is poured.
Party Wall Act
The Party Wall etc. Act 1996 requires you to serve written notice on neighbours before starting work that affects a shared (party) wall, or involves excavation within 3 or 6 metres of neighbouring foundations (depending on the depth of excavation). A rear extension along a shared boundary typically triggers the Act. Serving notice is free; you can use the government's model notices. If the neighbour dissents or fails to respond within 14 days, a surveyor must be appointed — budget £700–£1,500 per surveyor for a straightforward award.
Glazing and Roof Specification — The Biggest Cost Variable
In a contemporary rear extension, the glazing specification is often the single biggest cost variable after the foundation and superstructure. Getting this decision right early — before the structural design is finalised — avoids expensive revisions to lintels and openings later.
French Doors vs Bi-Fold Doors vs Full-Width Glazing
A pair of French doors costs £800–£1,500 supply and fit. A bi-fold door set (3 or 4 panels) costs £2,500–£5,000 supply and fit. A full-width aluminium sliding or bifold system for a 4m opening costs £4,000–£9,000. A structural glass wall system from a specialist glazing contractor starts at £8,000 and rises steeply. The structural implications are different too — a full-width opening requires a significant steel beam above, which affects cost and programme.
Flat Roof vs Pitched Roof vs Lantern/Rooflight
A flat roof (GRP or EPDM) is the cheapest roof option for a rear extension and is well suited to contemporary designs. A pitched tiled roof matches the existing house better and often gets planning permission more easily in traditional settings but costs more to build. A glazed lantern or large rooflight over a flat roof adds light dramatically but costs £1,500–£5,000 for the rooflight unit alone, plus structural and waterproofing considerations. Specify this early — changing the roof design after foundations are in place is an expensive redesign.
Get a House Extension Estimate Before You Meet Your Builder
Upload your floor plan to RenoCalc and get a full extension cost breakdown — shell, structural opening, glazing, fit-out and professional fees — in around 3 minutes. The RenoCalc Spreadsheet contains 40,000+ live formulas and a current UK material and labour price library. Cover letter, schedule of works and method statements included.
Start Your Free EstimateFrequently Asked Questions
How much does a house extension cost in the UK?
A single-storey rear extension costs £1,500–£2,500 per m² at standard build, or £30,000–£50,000 for a typical 20m² extension. Double-storey extensions cost £1,800–£3,000 per m² of ground floor area. High-spec builds with full-width glazing or green roofs can exceed these ranges. Add professional fees (£3,000–£8,000), planning (£206 if required), and fit-out costs separately.
Do I need planning permission for a house extension?
Single-storey rear extensions up to 4m deep (detached) or 3m deep (semi/terrace) are usually permitted development in England. The Prior Approval scheme extends this to 8m and 6m respectively. Double-storey extensions, side extensions, and extensions in conservation areas require a planning application. The fee in England is £206. Always check with your local planning authority — PD rights can be removed for individual properties.
What does a structural engineer cost for a house extension?
A structural engineer for a house extension typically costs £500–£1,500 for a straightforward single-storey rear extension. More complex designs cost £1,000–£2,500. The structural engineer produces calculations for foundations, the beam over the structural opening, and roof structure — all required by Building Control before work begins.
Do I need a party wall agreement for a house extension?
You need to serve a Party Wall Notice if your extension involves work on or near a shared boundary wall, or excavation within 3 or 6 metres of a neighbour's foundations. Serving notice is free; if the neighbour dissents, a surveyor must be appointed — typically £700–£1,500 per surveyor for a straightforward award.
How long does a house extension take to build?
A standard 20m² single-storey rear extension takes 12–20 weeks from groundworks to completion. This covers foundations, blockwork and roof, weatherproofing, windows and doors, first-fix trades, insulation, plastering, second-fix and finishing. Planning and structural approval add time before groundworks can begin — allow 3–6 months from initial design to start of construction in a straightforward case.
How much does a 4x5m extension cost in the UK?
A 4m x 5m (20m²) single-storey rear extension at mid-spec costs approximately £30,000–£50,000 for the shell and structure in 2026. Add professional fees (£3,000–£6,000), Building Control (£500–£1,500), and fit-out costs — a kitchen adds £8,000–£25,000, a simple family room adds £3,000–£8,000. In London, add 20–30% to build costs.
Questions About House Extension Cost UK
Q: How much does a 3m rear extension cost in the UK?
A 3m deep single-storey rear extension on a typical 4m-wide semi-detached house (12m² footprint) costs approximately £18,000–£30,000 for the shell in 2026. Add architect drawings (£1,500–£3,000), structural engineer (£500–£1,200), and fit-out costs depending on the room use. A 3m extension qualifies under standard permitted development for a semi-detached or terraced house without needing the Prior Approval prior notification process, simplifying the planning route.
Q: What is the planning fee for a house extension in England?
The householder planning application fee in England in 2026 is £258 (updated from £206 in late 2023). Extensions within permitted development limits may qualify for the Prior Approval/Larger Home Extension route, which has a lower fee of £120. Extensions that require a full planning application — double-storey, in conservation areas, listed buildings — use the standard householder fee. Check the Planning Portal for current fees and eligibility.
Q: How much do bi-fold or sliding doors add to an extension cost?
Full-width bi-fold doors for a 4m rear extension opening cost £4,000–£10,000 supply and fit depending on the number of panels, glazing specification (double vs triple glazed), and the quality of the frame (aluminium vs PVC vs timber-clad). Roof lanterns add £2,500–£7,000 depending on size. These premium glazing elements are the single biggest variable in a high-specification extension budget — a standard extension can become a high-spec one largely through the glazing choices.
Q: Does a house extension add value proportional to its cost?
In most UK markets, a well-designed single-storey rear extension returns 50–80% of its cost in added property value at the time of completion — meaning a £50,000 extension may add £25,000–£40,000 to the market value. The return depends on the local market ceiling, the quality of the work, and whether the extension creates genuinely usable space (an open-plan kitchen-diner is typically more valuable than an equivalent utility room extension). High-end finishes above the local market ceiling rarely return full cost at resale.
Q: What Building Regulations apply to a house extension?
A house extension requires Building Regulations approval in virtually all cases. The relevant approved documents include: Part A (structure), Part B (fire safety), Part C (site preparation), Part F (ventilation), Part J (combustion appliances if a new heating appliance is installed), Part L (conservation of fuel and power — the extension must meet current insulation standards), and Part P (electrical safety). Building Control must inspect the foundations, wall construction, roof structure and insulation before the work is covered up. See GOV.UK Building Regulations for the full list of approved documents.
Q: Is a double-storey extension better value per m² than a single-storey?
Yes — double-storey extensions are more economical per m² of usable floor area because the foundations, roof structure and external walls are shared across two floors. A double-storey extension at £1,800–£3,000/m² of ground-floor area effectively doubles your new floor area at marginal additional cost per m². The constraints are planning — double-storey extensions almost always need full planning permission, and the impact on neighbours and outlook is greater — and the structural complexity of integrating two new floors with the existing house.
