UK House Extension Cost Guide 2026: Price by Type, Size, and Region

How much does a house extension cost in 2026?

A 2026 UK house extension for a typical 3-bed semi-detached house will cost in the range of £30,000 to £90,000 for a single-storey rear extension, £55,000 to £110,000 for a double-storey rear extension, and £80,000 to £150,000+ for a wraparound or large side-return scheme in London or the South East. The headline figures from major 2026 UK pricing guides cluster around these bands.

The cleanest way to compare 2026 prices is by square metre. A mid-range single-storey extension in most UK regions lands at £1,800–£2,500/m² build cost, with a basic finish at the lower end and a high-spec finish at the upper end.

A 25 m² single-storey rear extension at the national mid-range of £2,000/m² gives a £50,000 build cost, before professional fees, VAT, and any kitchen fit-out. That is the typical reference project for a 3-bed semi.

A well-designed single-storey rear or kitchen extension typically adds around 10–15% to property value in the short term, with a medium-term ROI of 70%+ over five years as the market appreciates.

Single-storey vs double-storey extension costs

A double-storey extension costs roughly 1.5× to 1.8× a single-storey extension for the same footprint, not double. The reason is that the second storey shares the foundations, groundworks, roof structure, and some of the first-fix M&E, so the marginal cost of adding a second storey is much less than the cost of building a second equivalent single-storey extension beside it.

Single-storey extension

• 2026 build cost (national, 3-bed semi): £1,800–£3,000/m².
• Typical total build cost for a 25 m² rear extension: £45,000–£75,000.
• Typical total build cost for a 15 m² side return: £25,000–£55,000.

Double-storey extension

• 2026 build cost (national, 3-bed semi): £1,500–£2,400/m² of total floor area (both floors counted).
• Typical total build cost for a 40 m² double-storey rear (20 m² × 2 floors): £60,000–£100,000.
• GetMaster’s 2026 data puts a 30–50 m² double-storey rear at £45,000–£90,000+ depending on spec and location.

A double-storey extension almost always needs a structural engineer’s design for the new first-floor joists, the additional load on the foundations, and the beam positions over the bifold doors below. The structural engineer’s fee is typically 1–2% of build cost.

Rear, side, wraparound extension costs

The position of the extension changes the cost more than the size, because the structural work and Party Wall exposure differ.

Rear extension (most common)

A rear extension projects from the back wall of the house, usually into the garden, typically 3–5 m deep and 4–8 m wide. It is the most straightforward type of extension to build because:

• It usually does not affect a party wall (depending on terrace/semi/detached layout).
• It has only one or two external walls to construct.
• The roof pitch can match the existing house or be a simple flat roof.

For a 25 m² rear extension on a 3-bed semi in 2026, expect:

• National range: £45,000–£75,000 (mid-range finish).
• London / South East: £60,000–£110,000.
• North East / Wales: £35,000–£55,000.

Side return extension

A side return fills the narrow alley between the back of the house and the boundary wall, common on Victorian and Edwardian terraces. It typically creates a 10–25 m² kitchen-diner and almost always requires a Party Wall Award because the work touches the boundary wall.

• 2026 build cost (national): £2,000–£3,000/m² outside London, higher in London.
• Typical total cost (15 m² side return): £25,000–£55,000 nationally; £35,000–£75,000 in London.

Wraparound extension

A wraparound combines a side return and a rear extension into a single L-shaped build, typically delivering 30–50 m² of new open-plan kitchen-diner-family space. It is the most expensive type per m² because the floor plan is irregular, the foundations step, and the roof geometry is more complex.

• 2026 build cost (national, non-London): £2,200–£3,200/m².
• London / high-spec wraparound: up to £5,500/m².

A typical 40 m² wraparound for a 3-bed semi-detached Victorian terrace in 2026 will run £90,000–£130,000 nationally, and £130,000–£220,000 in London or the South East with a high-spec finish.

Extension cost by region

Region shapes 2026 extension costs as much as size and type. The same 25 m² single-storey rear extension that costs £50,000 in the Midlands might cost £70,000 in Outer London and £90,000 in Inner London.

Regional multipliers

| Region | 2026 multiplier (vs UK mid-range) | Indicative build cost per m² | |--------|-----------------------------------:|----------------------------:| | Inner London | ×1.30–1.50 | £2,400–£3,000+ | | Outer London | ×1.15–1.25 | £2,100–£2,600 | | South East (outside London) | ×1.05–1.15 | £1,900–£2,400 | | East of England | ×1.00–1.05 | £1,800–£2,200 | | Midlands (East / West) | ×0.95–1.00 | £1,700–£2,100 | | North West / Yorkshire | ×0.90–0.95 | £1,600–£2,000 | | North East | ×0.85–0.95 | £1,500–£1,900 | | Wales | ×0.85–0.95 | £1,500–£1,900 | | Scotland | ×0.90–1.00 | £1,600–£2,100 |

The London premium reflects higher labour rates, skip and scaffold constraints, parking permits, and tighter Party Wall exposure. The Midlands is a useful "baseline" because it sits close to the national mid-range and most builder pricing guides use it as the reference.

Working it through

For a 25 m² single-storey rear extension at the UK mid-range of £2,000/m²:

• Inner London: £60,000–£75,000.
• Outer London: £52,000–£65,000.
• South East: £47,000–£60,000.
• Midlands: £42,000–£52,000.
• North West / Yorkshire: £40,000–£50,000.
• North East / Wales: £37,000–£47,000.

For a 40 m² double-storey rear extension, multiply by roughly 1.5×–1.8× to account for the second storey.

What's included in an extension quote (and what isn't)

A 2026 fixed-fee extension quote from a builder usually includes:

• Design and structural engineering, including the architect’s planning drawings (if needed) and the structural engineer’s beam and foundation design.
• Building Regulations application (either Full Plans or Building Notice).
• Groundworks, foundations, and drainage, including any new soil-pipe runs and connection to the existing sewer (subject to a build-over agreement if the new foundation crosses a public sewer).
• External walls, insulation, roof structure, and roof covering, designed to the relevant U-value targets in Approved Document Part L.
• Windows, doors, and rooflights, including bi-fold doors if specified.
• First-fix M&E for electrics, plumbing and heating, including any new radiators or underfloor heating loops.
• Plastering, joinery, and decoration to a "builders’ finish".

It typically does not include:

• The architect’s full design fee (most homeowners commission the architect separately to keep the design independent of the builder). Architect fees for a full-service domestic extension run 7–12% of build cost in 2026.

• The topographical survey (see the next section). This is usually commissioned by the homeowner so the architect and structural engineer have an independent baseline.
• The kitchen fit-out (cabinets, worktops, appliances). For a 20–30 m² kitchen space, a mid-range fit-out in 2026 typically runs £15,000–£40,000 depending on cabinet run length, appliances and worktop material.

• Bi-fold doors: a 3-panel aluminium bifold in 2026 typically costs £3,000–£6,000 supply and install, depending on size, finish and thermal spec.
• A replacement kitchen, downstairs WC, or utility if the extension forces a redesign of the existing ground-floor layout.
• External landscaping (patio, turf, planting) to finish the garden side of the extension.

The VAT treatment is the same as for loft conversions: most extension work on an occupied home is standard-rated at 20%, with a zero-rating available only for a residential property that has been empty for two years or more under VAT Notice 708.

The role of a topographical / measured building survey

A topographical survey is to a house extension what a measured building survey is to a loft conversion: the engineering baseline that the architect and structural engineer design from. For an extension, the key inputs are:

• Levels and contours of the garden, the boundary, the existing patio, and the adjacent ground (for setting the finished floor level of the new extension relative to the existing house and the damp-proof course).
• Boundary positions to confirm the extension sits inside the property line and does not encroach on a neighbour.
• Drainage runs to map the existing soil stack, rainwater downpipes, and any underground services that the new extension has to connect to.
• Tree positions and species to comply with BS 5837 (trees in relation to design, demolition and construction), which can constrain the foundation depth and the finished floor level.

A topographical survey for a typical house extension in 2026 costs in the region of £800–£1,200 + VAT for a small residential site, rising to £2,000+ for a larger or more complex parcel.

If the extension sits over or near a public sewer, the topographical survey will also pick up the sewer record from the statutory sewer map. Your builder or architect will then need a build-over agreement from the water authority before starting work.

A measured building survey of the existing house is a separate deliverable and is usually commissioned alongside the topographical survey for a 2026 extension. The measured survey gives the architect the existing floor plans, elevations, and a section through the house, so the new extension can be designed to match the existing floor levels, ceiling heights, and brick coursing. A measured building survey for a typical 3-bed house in 2026 costs £1,200–£2,000.

The right order for pricing a UK extension in 2026 is:

1. Commission a topographical survey and a measured building survey (often the same provider delivers both, in a single site visit).
2. Architect feasibility study against the relevant Class A PD limits (4 m / 6 m / 8 m depending on house type — see the planning-permission-for-an-extension blog).
3. Two or three fixed-fee quotes from extension builders who all price from the same baseline.
4. Submit a householder planning application if the design falls outside PD (£548 from 1 April 2026).
5. Tender acceptance and contract.

icelabz provides RICS-compliant topographical and measured building surveys across London and the South East, with deliverables in 2D CAD and (optionally) Revit BIM and point cloud. Contact us for a fixed-fee quote and a typical 10–15 working-day turnaround.

Hidden costs and contingency budgeting

A 2026 extension quote that looks tight on day one is almost always the one that runs over by 10–15% by completion. The hidden costs that catch homeowners out are not dishonest line items — they are legitimate scope items that a builder cannot price until the ground is opened up, the existing drains are exposed, or the LPA planning officer asks for an additional drawing.

The most common hidden costs on a 2026 UK extension are:

• Abnormal ground conditions. Clay, fill, or a high water table can force piled foundations where strip foundations were assumed. Allow £5,000–£15,000 contingency for a 3-bed semi in a typical suburban area, and £15,000–£30,000 for a London plot with Victorian made-ground.
• Underground services. A new extension often exposes a redundant gas service, an unmarked electrical cable, or a deeper-than-expected soil pipe. Rerouting or capping typically costs £500–£2,500 depending on the service.
• Boundary and party-wall surprises. A neighbour’s extension or a historic underpinning can change the assumed foundation detail. The contingency is built into the Party Wall surveyor’s report if you commission it before signing the build contract.
• Drainage connections. Connecting the new kitchen or utility to the existing soil stack sometimes requires a new manhole, a pump, or a build-over agreement with the water authority. Allow £1,000–£3,000 for a typical connection.
• LPA planning conditions. Conservation-area consents, ecological surveys (bats, newts), and tree-protection orders can each add £500–£2,000 to the design and the fee bill.
• Decorative and finishing scope. The builder’s "ready to decorate" finish assumes you will paint, lay flooring, and fit the kitchen yourself. The cost of decorating a 25 m² new space typically adds £1,500–£3,000 if outsourced.

The rule of thumb used by most 2026 quantity surveyors is to add 10–15% to the builder’s fixed-fee quote as a contingency, held in a separate account and released only against agreed variations. A 15% contingency on a £60,000 extension is £9,000 — enough to absorb one significant hidden cost without derailing the project.

How to read and compare extension quotes

A 2026 extension quote is a legal document, and a £5,000–£10,000 difference between two builders is almost always explained by scope differences, not by one builder being cheaper than the other. The line items to scrutinise when comparing three like-for-like quotes are:

• Foundations depth and type. Is the quote based on strip foundations at 1 m, or does it include a piled or rafted foundation for poor ground?
• External finish. Are the facing bricks specified (and matched to the existing house), or is the quote based on a render-only finish with a brick-slip accent?
• Windows and doors. Are the bi-fold doors a specific brand and size, or a "supply and fit aluminium bifolds to match drawing"? The cheapest bifold is rarely the same product as a 3-panel Schüco or Reynaers.
• Kitchen and utility M&E. Are the electrics, plumbing and heating quoted to first-fix only, or to a "live and working" kitchen specification? A first-fix quote assumes you will pay a separate kitchen fitter to do the second fix.
• Skip and scaffold. Is the cost of skip hire, scaffolding licences and parking suspensions included in the quote, or is it a separate allowance?
• Preliminaries. A 10–15% preliminaries line covers site setup, site management, portable toilet, and insurance. A quote with no preliminaries line is hiding them in the build rates.

If two quotes differ by more than 10%, ask each builder to walk you through their quote line by line. In almost every case, the cheapest quote has either omitted a scope item or assumed a cheaper specification.

UK house extension cost FAQ

Is a house extension cheaper than moving house? For most UK homeowners in 2026, an extension that adds the space they need costs 40–60% of the cost of moving (after stamp duty, legal fees, agent fees, and the price premium for a bigger house in the same area). For a family adding a kitchen-diner or a fourth bedroom, an extension is usually the cheaper and less disruptive option.

How long does a single-storey rear extension take? Most straightforward single-storey rear extensions take 10–12 weeks on site from the start of groundworks to practical completion. Allow an extra 8–12 weeks at the front for design, planning (if needed), and tender, so a realistic end-to-end timeline is 20–30 weeks for a typical PD project.

Do I need planning permission for an extension? Many single-storey rear extensions fall under Permitted Development (PD) rights under GPDO 2015 Schedule 2 Part 1 Class A, with the 4 m / 6 m / 8 m depth limits depending on house type. Side extensions and double-storey extensions frequently exceed the Class A limits and need a full householder application. The full PD rules are covered in the planning-permission-for-an-extension blog.

What is a build-over agreement? A build-over agreement is consent from your water authority (Thames Water, Anglian Water, etc.) to build over or within 3 m of a public sewer. It is required when the new extension sits over a public sewer. The agreement is usually a desk-based review, costing £200–£500 in admin fees and adding 4–8 weeks to the programme.

Do I need Building Regulations approval? Yes. Every extension — PD or not — needs a Full Plans or Building Notice application to Building Control (or an Approved Inspector). Structural integrity (Part A), fire safety (Part B), insulation (Part L), ventilation (Part F), and damp-proofing (Part C) are the main approval areas.

Can I extend a terraced house without a Party Wall Award? Almost never. A side return or rear extension that touches the party wall requires a Party Structure Notice under the Party Wall etc. Act 1996, served at least 2 months before the notifiable works start. A Party Wall Award is only required if a neighbour dissents, but the notice must be served regardless. The Party Wall surveyor fee for a single-neighbour matter runs £1,000–£2,000 (£1,500–£3,000 in London).

Does a topographical survey really matter for a rear extension? Yes. A topographical survey picks up the levels needed to set the finished floor level, identifies any public sewer the extension sits over, and confirms the boundary. Without it, the builder is guessing the foundation depth and the connection to the existing drainage — a recipe for a design that fails Building Regulations or a build-over agreement that is refused retrospectively.