Thinking about adding space rather than moving? With stamp duty, removal fees and mortgage rates all adding up, a well-planned extension is often the better value route. But what does a house extension cost in the UK in 2026? This guide breaks down realistic prices per square metre, typical costs by extension type, the “hidden” fees most budgets miss (planning, building regulations, party wall surveyors, VAT) and the latest 2026 changes to fees and build-cost inflation – so you can budget with confidence before you speak to a builder.
In 2026, a house extension in the UK typically costs £1,500–£2,500 per square metre for a standard single-storey build, so a 20m² rear extension usually lands between £30,000 and £55,000. Two-storey extensions cost less per m², while London and premium specifications can push prices above £3,000 per m².
Those figures are for the build itself. Once you add professional fees, VAT (if your builder is VAT-registered), finishes such as a new kitchen, and a contingency, the all-in cost of an extension is often 25–40% higher than the headline build price. The Federation of Master Builders’ March 2026 guidance puts the average single-storey extension at £1,800–£3,000 per m² once specification and location are factored in, and notes that over 20% of enquiries to its members in 2025 related to extensions.
The table below gives realistic 2026 ranges for the most common extension types. Figures are indicative build costs (excluding VAT, professional fees and kitchen/bathroom fit-out) for a mid-range specification outside central London.
| Extension type | Typical size | Cost per m² (2026) | Typical total build cost | Typical build time |
|---|---|---|---|---|
| Single-storey rear extension | 15–25m² | £1,500–£2,500 | £25,000–£60,000 | 10–14 weeks |
| Side return extension (terraced) | 8–15m² | £2,000–£3,000 | £25,000–£50,000 | 10–14 weeks |
| Wrap-around extension | 25–40m² | £2,000–£3,000 | £60,000–£110,000 | 14–20 weeks |
| Two-storey rear extension | 30–50m² (both floors) | £1,200–£2,000 | £50,000–£100,000 | 16–24 weeks |
| Over-structure (storey on existing extension) | 10–20m² | £1,500–£2,500 | £25,000–£50,000 | 10–16 weeks |
| Premium glazed/architect-led extension | 20–30m² | £2,800–£3,500+ | £60,000–£105,000+ | 14–20 weeks |
Why is a two-storey extension cheaper per square metre? Because the most expensive elements – foundations and the roof – are shared across two floors of usable space. Smaller projects, by contrast, often look expensive per m² because fixed costs such as scaffolding, site set-up, drainage and professional fees don’t scale down.
If you are also weighing up going upwards instead of outwards, compare these figures with our guide to loft conversion costs.
Simple rectangular footprints with a flat or mono-pitch roof are the most cost-efficient. Complex rooflines, wrap-around corners and vaulted ceilings add structural steel, labour and time.
Labour and materials cost more in London and the South East. Restricted access – for example a mid-terrace with no side access where everything must come through the house – can add thousands to groundworks and skip/grab-lorry costs.
Removing a rear wall to create an open-plan space usually needs steel beams and structural calculations. Clay soils, nearby trees, shallow drains or a public sewer running under the footprint can require deeper or piled foundations and a build-over agreement with the water company. Before planning an open-plan layout, read our guide on how to tell if a wall is load-bearing.
Standard doors and windows keep costs down. Large sliding or bi-fold doors, roof lanterns and structural glazing look spectacular but need bigger steels and specialist installers.
Many extensions house a new kitchen, which is usually priced separately from the shell. See our breakdown of how much a new kitchen costs in the UK to complete your budget.
These are the costs most often left out of early budgets. Together, professional and compliance fees commonly add 15–20% to the build price.
| Cost item | Typical 2026 cost | When you need it |
|---|---|---|
| Measured building survey | £500–£1,500 | Before design, to produce accurate existing drawings |
| Architectural design & drawings | £2,000–£8,000+ | Planning and building regulations drawings |
| Structural engineer calculations | £500–£2,000 | Steels, removed walls, foundations |
| Householder planning application (England) | £548 + Planning Portal service charge | If outside permitted development |
| Lawful Development Certificate | £274 (50% of householder fee) | Recommended for permitted development builds |
| Building control fees | £800–£2,000 | Almost always required |
| Party wall surveyor(s) | £700–£2,000 per adjoining owner | Work on or near a shared wall/boundary |
| VAT | 20% of labour and materials | If your contractor is VAT-registered |
| Contingency | 10–15% of build cost | Always – for unforeseen issues |
Fee ranges are indicative and vary by region and complexity; always obtain written quotes.
Not always. Under householder permitted development rights, a single-storey rear extension can usually project up to 3 metres from the original rear wall of a semi-detached or terraced house (4 metres for a detached house) without a planning application, provided it meets the height, materials and 50% curtilage rules. Using the prior approval route, these limits increase to 6 metres and 8 metres respectively, according to the Planning Portal’s guidance on extensions.
Permitted development does not apply to flats and maisonettes, and it is restricted in conservation areas, National Parks and on listed buildings. Even where you don’t need planning permission, a Lawful Development Certificate gives buyers’ solicitors proof that the work was lawful when you come to sell.
2026 update: English planning application fees increased by 3.8% on 1 April 2026, in line with the September 2025 CPI rate, as confirmed by the Planning Portal (January 2026). A householder application now costs £548. Despite earlier proposals to extend householder permitted development rights, no changes to the rear-extension limits have been implemented in 2026.
Almost every extension needs building regulations approval, covering foundations, structure, fire safety, drainage, ventilation and energy efficiency. Your building control body will inspect at key stages and issue a completion certificate at the end – a document you will need when you sell.
If you are building on or near a boundary, digging foundations within 3 metres of a neighbour’s building (or 6 metres in some cases), or cutting into a shared wall, the Party Wall etc. Act 1996 is likely to apply. You must serve notice on your neighbours, and if they dissent, a party wall surveyor (or two) will prepare an award. As the building owner, you normally pay these fees. Our guide to party wall rules for extensions explains the process, and Survey Merchant’s party wall surveyors can serve notices and prepare awards quickly and at a fixed fee.
After a period of relative stability in 2024–25, construction costs are rising again. According to the BCIS construction industry forecast (July 2026), the General Building Cost Index rose 1.4% between the first and second quarters of 2026, giving annual growth of 3.8%. BCIS attributes this partly to higher energy-intensive material costs linked to oil prices above $100 a barrel, and forecasts building costs to rise 13.1% and tender prices 15.5% over the five years to 2031.
For homeowners, that means:
A well-designed extension usually adds value, but not always pound-for-pound. The uplift depends on your local “ceiling price”, what the extra space is used for (an extra bedroom or a large kitchen-diner tends to add most) and the quality of finish. Before you commit, it is sensible to get an independent view from a suitably qualified valuer on the likely post-works value – Survey Merchant’s property valuation service can help you judge whether the numbers stack up, or whether to adjust the specification.
An extension is one of the largest sums most homeowners spend on their property. Involving a suitably qualified, accredited surveyor – regulated or accredited by recognised bodies such as RICS, CIOB or the RPSA – helps you avoid the costly mistakes: undiscovered structural defects, boundary disputes, overlooked party wall duties or an extension that costs more than the value it adds. A pre-works building survey can flag issues such as subsidence, damp or defective drainage before your builder breaks ground.
Survey Merchant helps homeowners across the UK plan, cost and protect their extension projects from day one:
Planning an extension? Speak to our party wall team about serving notices early, or contact Survey Merchant today for a fast, fixed-fee quote.
Survey Merchant provides vetted RICS surveyors across 100+ UK locations at fixed fees:
→ Level 2 Home Survey (HomeBuyer Report)
→ Level 3 Building Survey (full structural survey)
→ RICS Red Book property valuations
→ Party wall surveyors — notices, awards & schedules of condition
→ Expert witness surveyors — CPR Part 35 reports for property disputes
