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

A house extension is one of the biggest things most people will ever pay a tradesperson to do — and one of the hardest to price, because no two are the same. In 2026, a typical UK single-storey extension costs somewhere between £1,500 and £2,750 per square metre to build, before kitchens, bathrooms or fancy glazing. That range is wide on purpose: the same 20 m² footprint can come in at £35,000 or £65,000 depending on the specification, the ground, and who you hire.

This guide gives you honest 2026 ballpark figures by extension type, explains what actually moves the number, and shows you how to avoid overpaying — without cutting the corners that matter.

Typical UK extension costs in 2026

These are build-cost ranges for a reasonable mid-market finish. London and the South East sit at the top of each range (often 10–25% higher); the North and Scotland sit nearer the bottom.

Extension type	Per m² (build)	Typical total
Single-storey rear (15–25 m²)	£1,500–£2,500	£30,000–£55,000
Side-return (kitchen)	£2,000–£2,750	£40,000–£70,000
Double-storey	£1,800–£2,800	£50,000–£100,000
Loft conversion (dormer)	—	£30,000–£60,000
Wrap-around (side + rear)	£2,000–£2,750	£60,000–£110,000

Figures are 2026 guide ranges for the building work. Kitchens, bathrooms, structural glazing (large sliding or roof glass) and tricky ground conditions are extra — see below.

What actually moves the price

• Finish, not floor area. The shell is the cheap part. Kitchens (£8,000–£30,000+), bathrooms, underfloor heating, bi-fold or sliding doors (£2,000–£8,000 per opening) and roof lanterns are where budgets run away.
• The ground. Deep foundations, a high water table, trees nearby (bigger footings), or drains to divert can add £3,000–£10,000 before you see a single brick.
• Roof type. A flat roof is cheaper than a pitched, tiled roof. A glass lantern looks great and costs accordingly.
• Access. No side access means everything goes through the house by hand — slower, dearer.
• Where you live. Labour rates in London and the South East are materially higher than the rest of the UK.

Costs people forget to budget for

• Architect / designer drawings: £1,500–£5,000+
• Structural engineer calculations: £500–£1,200
• Planning application (if needed): £258 householder fee in England (2026)
• Building Regulations: £500–£1,000+ in fees and inspections
• Party Wall agreement (if you share a boundary): £700–£2,000+ per neighbour surveyor
• VAT: most domestic building work is charged at 20% — check whether quotes include it.

The single easiest way to pay less

Here is the uncomfortable truth about extensions: two competent builders can quote the exact same job and be £15,000 apart. Not because one is cutting corners — because one is busy, one isn't, one priced the risk high, one knows the supplier who'll do your windows cheaper. The only way to see that gap is to get more than one proper quote.

Most homeowners get one or two quotes from whoever a friend recommended, then feel they have to accept. Getting three or four comparable quotes is the difference between paying the market price and paying someone's "I'm not that bothered" price.

Frequently asked questions

How much does a 20m² extension cost in 2026?

For a single-storey rear extension, budget roughly £35,000–£55,000 for the building work to a good standard, plus kitchen or bathroom fit-out on top. A side-return in London can push past £60,000.

Is an extension cheaper than moving?

Usually, yes — once you count stamp duty, estate agent fees, legal costs and removals, moving to a bigger home often costs £20,000–£40,000 in fees alone, none of which adds a room. An extension turns that money into space.

Does an extension add value?

A well-built extension typically adds more to a home's value than it costs, especially a kitchen-diner or an extra bedroom — but only if the work is sound and documented. Unpermitted or poorly built work can reduce value and stall a sale.

Related reading: Why getting multiple quotes actually saves you money · How to describe a job so tradespeople can quote accurately · The one piece of paper that protects you.