How much does a side return extension cost per square metre in 2026?

Indicatively £3,100–£4,200 per square metre in London and £2,400–£3,300 in the Midlands, the North and Wales, for construction only. Professional fees, VAT and the kitchen sit on top.

Is VAT charged on a house extension?

Yes. Building an extension to an existing dwelling is standard rated, so VAT at 20% applies. Reduced rates exist for a few cases, such as a property empty for over two years, and they rarely apply to an extension.

How long does a side return extension take?

On site, typically 14–20 weeks. End to end from first drawing to finished room, closer to 40 weeks once design, planning, party wall and tender are included.

Cost9 minute read

What does a side return extension cost in 2026?

A worked example for a 24 square metre side return, with every line the quote usually leaves out.

The short answer

Expect £89,000–£125,000 all in for a 24 square metre side return kitchen extension in London, or £74,000–£104,000 outside the South East. That is construction, professional fees, statutory costs, contingency and VAT. A builder's quote for the same job typically reads £14,000–£24,000 lower, because it excludes fees, party wall, building control and the kitchen.

01The rate per square metre, and what it buys

London, good domestic standard	£3,100–£4,200 per sqm
South East, outside London	£2,800–£3,700 per sqm
Midlands, North, Wales	£2,400–£3,300 per sqm

Indicative construction rates for a single-storey rear or side return extension, excluding professional fees, VAT and the kitchen itself. Based on typical 2026 domestic tender levels. A rate is a starting point, not a price: the same 24 sqm can vary by £40,000 on glazing and drainage alone.

Rates per square metre are useful for one thing only. They tell you whether a number you have been given is in the right postcode, before you spend money finding out properly.

02A worked example: 24 square metres in south London

Construction, 24 sqm at £3,100–£4,200	£74,400–£100,800
Structural engineer and drawings	£3,500–£6,000
Planning or lawful development certificate	£300–£900
Building control	£800–£1,400
Party wall award, one neighbour	£900–£3,200
Drainage survey and build-over agreement	£600–£1,500
Contingency at 10%	£8,100–£11,400
All in, rounded, VAT included	£89,000–£125,000

Indicative. The construction line is the rates above applied to 24 sqm. Professional and statutory costs total £6,100–£13,000, which is roughly 12% of construction. Contingency is 10% of everything above it. The lines add to £88,600–£125,200, rounded here. Domestic extension work is standard rated, so VAT at 20% is included in every line. Kitchen units and appliances are excluded, and the party wall range assumes one neighbour, an agreed surveyor at the low end and two surveyors at the high end.

The gap between the construction line and the all-in line is the part homeowners are never shown. It is £14,600–£24,400 in this example, and it is not padding. It is work that has to happen.

03What actually moves the number

—Glazing. A three-panel aluminium slider is indicatively £6,000–£12,000. The same opening in slim steel-look framing can be £25,000 or more. This is usually the single biggest choice you make.
—Drainage. A side return on a Victorian terrace often crosses a shared drain. A diversion and a build-over agreement add around 4 weeks and, indicatively, £2,000–£6,000. It is almost never in an early quote.
—Structure. One goalpost frame is ordinary. Removing a chimney breast, a second opening, or a wide flush beam needs deeper steels, larger padstones and sometimes underpinning.
—Ground conditions. London clay with a mature tree within ten metres can turn ordinary trench fill into a designed foundation.
—Roof and rooflights. A warm flat roof with two rooflights is standard. A structural glass panel is not.
—The room you knocked into. Making good, decorating and re-flooring the existing kitchen or dining room is real money and reads as a rounding error on a quote.
—Kitchen. Units, worktops and appliances are usually supplied by you. Indicatively, £8,000 buys a decent kitchen and £40,000 buys a nice one, and neither figure is in the builder's number.

Every figure in this list is indicative, at 2026 London supply and fitting levels. They are here to show which decisions carry the weight, not to price your job.

04Why three quotes for the same house come back £90,000 apart

Because they are not quotes for the same job. One assumes you supply the kitchen and the other fits one. One carries the drainage diversion as a provisional sum and the other excludes drainage entirely. One has 12% contingency inside the number and the other has none.

None of that is dishonest. It is the predictable result of three firms interpreting the same vague description in three reasonable ways, unpaid, on a Saturday morning.

Common questions

How much does a side return extension cost per square metre in 2026?: Indicatively £3,100–£4,200 per square metre in London and £2,400–£3,300 in the Midlands, the North and Wales, for construction only. Professional fees, VAT and the kitchen sit on top.
Is VAT charged on a house extension?: Yes. Building an extension to an existing dwelling is standard rated, so VAT at 20% applies. Reduced rates exist for a few cases, such as a property empty for over two years, and they rarely apply to an extension.
How long does a side return extension take?: On site, typically 14–20 weeks. End to end from first drawing to finished room, closer to 40 weeks once design, planning, party wall and tender are included.

Rules described here apply in England and Wales, and were last checked on 29 August 2026. Permitted development limits and fees change. Check your council’s current guidance before you rely on a figure.