How much does an extension cost in the UK in 2026?

Single-storey rear extension

The most common UK extension type. A typical 4m x 5m single-storey rear (20 m²) costs around £45,000–£65,000 nationally for a mid-range fit-out, rising to £80,000+ in London for the same footprint. The drivers within that range: glazing choice (a wall of bifolds adds £5,000–£15,000+ over a single French door), kitchen integration (most rear extensions are kitchen-led), and whether the existing rear wall stays or comes down.

A simple rear extension keeps the original rear wall as an internal partition. A more involved one knocks the wall through and inserts a steel beam (an RSJ), opening the new room into the existing kitchen. The knock-through adds £4,000–£8,000+ for the steel and structural work alone.

Side return extension

Common in Victorian and Edwardian terraces, where a narrow strip beside the kitchen runs to the rear boundary. Side returns are space-efficient (you're claiming a footprint that was useless garden) but the construction is fiddlier: drainage often runs along the boundary, party walls apply on both sides, and the existing house wall almost always needs underpinning or substantial structural work.

UK rates for side returns run £2,500–£3,500 per m². A typical 8 m² side return (1.5m wide x 5.3m long) lands around £25,000–£35,000 nationally for the shell, with London prices reliably higher. The total project typically lands £40,000–£70,000 once kitchen fit-out, glazing and party wall surveyor fees are added.

Double-storey rear or side

A double-storey extension delivers significantly more floor area for less than double the cost. The upper floor typically costs 50–70% of the ground floor per m², because the foundations, ground works and roof are mostly already paid for. For a 30 m² double-storey (15 m² ground, 15 m² above), expect £60,000–£100,000+ nationally and £120,000–£200,000+ in London.

The constraints: planning permission is more often required (double-storey rear extensions exceed permitted development limits in most cases), visibility from the street pulls in conservation-area or design-led restrictions, and party wall obligations are heavier.

Wraparound extension (rear plus side)

A wraparound combines a single-storey rear with a side return, wrapping around the L-shape of the original house. Wraparounds give the largest footprint addition and the highest value uplift, and they're the most expensive single-storey type. UK national rates are £2,400–£3,300 per m². A typical 25 m² wraparound costs £60,000–£85,000 nationally and £80,000–£130,000+ in London.

Wraparounds typically require full planning permission rather than permitted development, because the side element triggers the "beyond the side wall" restriction. Expect 8–14 weeks for planning approval before work starts.

Kitchen-led extension hybrid

Most single-storey rear and wraparound extensions are kitchen-led: the new floor area becomes the new kitchen, with the old kitchen space repurposed or knocked through. Treating it as "extension plus new kitchen" rather than "extension" alone is more honest about the total cost. The extension shell might cost £45,000. The kitchen fit-out for that space (units, worktops, appliances, integrated lighting) often adds £15,000–£40,000+ depending on finish.

For the kitchen portion specifically, see what a kitchen renovation should actually cost in your area→.

The headline rate covers the shell. These don't.

Party wall surveyor fees. If your extension is within 3 metres of a shared boundary or affects a shared wall, the Party Wall etc. Act 1996 requires you to serve notice on each adjoining neighbour. If they appoint their own surveyor, you pay both yours and theirs, typically £800–£1,500 per surveyor (industry data 2026). On a terrace with party walls on both sides, this can mean £3,000–£6,000+ in surveyor fees alone.

Structural engineer reports. Almost every extension needs a structural engineer for the steel calculations and foundation specification. Typical fees: £1,000–£3,000 (FMB, industry data 2026), with London tilting toward the upper end.

Building Control fees. Required regardless of whether planning permission is needed. Most local authorities charge £900–£1,500 for an extension. Verify with your council before you start.

Drainage and services moves. Soil stack relocations, gas pipe moves, electrical supply reroutes, water main diversions: £2,000–£5,000 typically, more if the connections are old or the route is awkward.

Scaffold across the project duration. Domestic scaffold runs £1,000–£2,500 for a typical extension build, more for higher access or longer durations.

Skip hire across the project. Plan for £600–£1,500 across an 8–12 week build. Most builders include skip hire in the quote, but the line is worth checking.

Interim accommodation. Many extensions are buildable while you live in the house. Some aren't, especially if the work disrupts the kitchen or bathroom for weeks. If you need to move out, factor 6–12 weeks of accommodation costs.

If your extension involves a heat pump, insulation upgrades or significant glazing improvements that lift your EPC rating, you may qualify for grant funding. To check which energy grants you qualify for→, the eligibility check takes about two minutes.

The general 10–15% renovation contingency rule isn't enough for an extension. Extensions have more unknown variables (foundations, party walls, ground conditions) and a longer construction window for things to go wrong. The rule:

15–20% contingency for an extension on a post-1960 property. 20%+ for an extension on a pre-1960 property (Victorian, Edwardian, interwar). 25%+ for a listed building or in a conservation area.

Ringfence the contingency. It's a separate budget line, not part of materials or labour. For the full breakdown of what contingency covers and how to manage it across the project, see how much contingency to keep for your renovation→.

Inflation if the project runs across a financial year. Material prices typically reset each spring, and labour rates rise with general wage inflation. If your project starts in October and finishes the following May, expect a 3–5% effective price rise on quotes that aren't locked.

Fixed-price versus cost-plus contracts. A fixed-price contract makes the contractor responsible for cost overruns within the agreed scope. Variations (genuinely new work) become extras with separately agreed pricing. A cost-plus contract has the contractor charge actual cost plus a percentage, and you carry the overrun risk. Most domestic extensions work on fixed-price with a written variation procedure. Cost-plus suits situations where the scope is genuinely unknowable up front (heritage work, severe ground complications) but it transfers significant risk to you.

Stage payments. A reasonable structure on an extension: 5–10% deposit on contract signing, then payments at agreed milestones (foundations complete, walls up to roof, watertight, first fix, second fix, snagging signed off). Avoid upfront payments above 15% on an extension. Larger upfront amounts leave you exposed before any meaningful work has started, and the milestone structure works better for both sides anyway.