How Much Does a House Extension Cost in the UK? (2026 Guide)

A house extension is an addition to an existing property — typically a single-storey rear extension, side return, double-storey extension, or wrap-around. In the UK in 2026, extension costs range from £30,000 for a basic single-storey to £100,000+ for a large double-storey, with most single-storey rear extensions costing £1,800–£2,500 per square metre.

Extensions remain one of the most popular ways to add space and value to a UK home. But the costs vary enormously depending on the type of extension, where you live, and the specification you choose. This guide breaks down realistic 2026 prices so you can budget accurately — whether you're a homeowner planning the build or a builder pricing the job.

1 Cost Per Square Metre by Extension Type

The most reliable way to estimate an extension cost is by the square metre. These ranges cover the structural build including groundworks, walls, roof, windows, doors, and basic internal finishing (plastering, electrics, plumbing, and decoration) — but not kitchens, bathrooms, or high-end finishes.

• Single-storey rear extension: £1,800–£2,500 per sqm. This is the most common type. A flat roof or parapet design sits at the lower end; a pitched roof with lantern or large bi-fold doors pushes towards the top.
• Double-storey extension: £1,500–£2,200 per sqm. Per square metre, a double-storey is actually cheaper because you share the foundations and roof across two floors. The total cost is higher, but you get roughly twice the space.
• Side return extension: £2,000–£3,000 per sqm. Side returns are narrow, which makes them less efficient to build. Steelwork to open up the kitchen wall, plus working in tight access, pushes the per-metre cost up.
• Wrap-around extension: £2,000–£2,800 per sqm. Combines rear and side return into an L-shaped or full-width extension. More structural steel but a dramatic transformation of the ground floor.

Tip: These rates apply to most of England and Wales outside London. For London and the South East, add 15–25% to account for higher labour rates and material delivery costs. Scotland and Northern Ireland pricing is broadly similar to the national average.

2 Total Costs by Size

To give you a clearer picture, here's what typical extension projects cost in total at mid-range specification in 2026. These assume a location outside London with reasonable site access.

• Small single-storey (3m x 4m = 12 sqm): £25,000–£35,000. Enough for a new kitchen-diner or utility room. Often falls within permitted development limits.
• Medium single-storey (5m x 4m = 20 sqm): £40,000–£55,000. The most popular size — gives a meaningful open-plan living and kitchen space with room for bi-fold doors to the garden.
• Large single-storey (6m x 5m = 30 sqm): £55,000–£75,000. A substantial addition that can accommodate a full kitchen, dining area, and living space.
• Double-storey (various sizes): £60,000–£100,000+. Adds both ground-floor living space and a bedroom or bathroom above. The total depends heavily on internal specification upstairs.

Remember, these are build costs. You'll also need to budget for professional fees (architect, structural engineer, Building Control), which typically add 10–15% on top. For guidance on how builders should approach pricing these jobs, see our guide on how to quote a building job.

3 Example Quote Breakdown

Here's a realistic itemised breakdown for a 4m x 5m single-storey rear extension (20 sqm) at mid-range specification. This is the kind of detailed quote a good builder should provide — and what homeowners should expect to see. Understanding builder profit margins helps both sides have realistic expectations.

VAT at 20% would add £7,546, bringing the total to approximately £45,276 including VAT. Some extension work on existing residential properties may qualify for a reduced 5% VAT rate — check with your accountant. Tools like VoxTrade can help builders produce detailed, itemised quotes like this directly from a site visit, without spending hours on spreadsheets afterwards.

4 What's Included — and What Isn't

Typically included in extension build costs:

• Foundations and groundworks (excavation, concrete, damp-proof membrane)
• Structural walls (blockwork, insulation, cavity ties, lintels)
• Roof structure and covering (joists, membrane, tiles or flat roof system)
• Windows and external doors (including bi-folds or sliding doors)
• Internal fit-out (plastering, first and second fix electrics, basic plumbing, decoration)
• Flooring (screed and basic finish)
• Skip hire and waste removal

Typically NOT included:

• Kitchen fit-out: A new kitchen can add £5,000–£25,000+ depending on specification. This is usually quoted separately.
• Bathroom fit-out: If the extension includes a bathroom or en-suite, expect an additional £3,000–£8,000.
• Landscaping: Re-laying a patio, garden reconfiguration, or fencing after the build — budget £2,000–£5,000.
• Planning application fees: £258 for a householder application in England (2026), plus architect or planning consultant fees.
• Party wall agreements: If you're building near or on a boundary, surveyor fees can run £700–£1,500 per neighbour.

5 Factors That Affect Your Extension Cost

Two identical-looking extensions can cost very different amounts. Here are the main factors that move the price.

• Ground conditions: Clay soil, high water tables, tree roots, or proximity to drains can mean deeper or more engineered foundations. A standard strip foundation might cost £3,000; a piled foundation on difficult ground can cost £8,000–£12,000.
• Access: If materials have to be carried through the house because there's no side access, labour costs increase significantly. Crane hire for steel beams adds cost too.
• Location: London and the South East are consistently 15–25% more expensive than the Midlands and North. Builder day rates vary from £180–£220 in the North to £250–£350 in London.
• Specification level: Aluminium bi-fold doors cost more than uPVC sliding doors. Underfloor heating costs more than radiators. A zinc-clad flat roof costs more than a rubber membrane. Every upgrade adds up.
• Structural complexity: Removing a load-bearing wall to open up the existing house requires steel beams (RSJs) and temporary support — typically £1,500–£4,000 depending on the span.

Planning Permission vs Permitted Development

Many single-storey rear extensions fall under Permitted Development (PD) rights, meaning you don't need planning permission. The standard PD limits allow you to extend 3 metres from the rear wall for attached houses or 4 metres for detached houses. Under the Larger Home Extension scheme (prior approval process), you can extend up to 6 metres (attached) or 8 metres (detached).

However, you will always need Building Regulations approval, regardless of whether planning permission is required. Building Control checks that the extension meets structural, fire safety, thermal, and drainage standards. The inspection fees are typically £500–£1,000.

You'll almost certainly need a full planning application for double-storey extensions, side extensions visible from the highway, extensions in conservation areas, and any work on listed buildings. Allow 8–12 weeks for the planning process.

How Long Does a House Extension Take?

A typical single-storey rear extension takes 3 to 5 months from breaking ground to completion. A double-storey extension usually takes 4 to 7 months. But the build itself is only part of the timeline.

• Design and planning: 2–4 months (architect drawings, structural calculations, planning application if needed, Building Regs submission)
• Procurement: 2–4 weeks (builder availability, material ordering — steel beams can have 4–6 week lead times)
• Build phase: 3–5 months for single-storey, 4–7 months for double-storey

So from your first conversation with an architect to moving furniture into the new space, expect 6 to 10 months in total. Delays are common — bad weather, material shortages, or discovering unexpected issues during groundworks can all push the timeline out.

Wrapping Up

A house extension is a significant investment, but it's one of the few home improvements that reliably adds more value than it costs — particularly in areas where the price per square foot of living space is high. The key to a successful project is realistic budgeting from the outset, getting detailed itemised quotes from at least three builders, and building in a 10–15% contingency for the unexpected.

If you're a builder quoting extension work, producing fast, professional, itemised quotes is what wins jobs. VoxTrade lets you walk through a site, describe the work by voice, and generate a priced quote before you leave — giving homeowners the detail and confidence they need to say yes.