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

“How much will it cost?” is the first question on any extension project — and in London the honest answer is “it depends”. Cost is driven by the size and type of extension, the condition and access of your property, the specification you choose and current building-cost inflation. The indicative 2026 ranges below are a realistic starting point for budgeting; a designer can give you a figure tailored to your house.

Indicative 2026 extension costs in London

These are broad build-cost ranges for a good-quality finish in London. They exclude professional fees, VAT and fitting-out (kitchens, bathrooms, furniture), which we cover below.

• Side return extension: from around £70,000 — a cost-effective way to widen a period kitchen-diner.

• Small / single-storey rear extension: roughly £65,000–£95,000 depending on size and spec.

• Larger rear extension: from around £80,000, rising with glazing and structural spans.

• Wraparound extension: from around £100,000 and upwards.

• Double-storey extension: from around £110,000 — often better value per square metre as you extend up as well as out.

Basement digs, poor ground conditions, restricted access and high-end glazing or finishes can all push costs well above these ranges. Two similar-looking extensions frequently differ in price because of what happens below ground and behind the walls.

What drives the price up or down

• Size and structure: large clear spans need more steel and bigger foundations.

• Ground and access: a terraced house with no side access costs more to build than one you can reach easily.

• Specification: glazing, roof lights, heating, joinery and finishes are where budgets swing most.

• Below ground: drainage diversions and deeper foundations are common hidden costs.

Don’t forget the costs beyond the build

The headline build figure is only part of the picture. Professional fees, structural calculations, party wall matters, planning or lawful-development fees, surveys and fitting-out all add up. We break these down in our companion guide, Additional Costs to Consider.

Making your budget go further

There are sensible ways to get more for your money without cutting corners — combining an extension with a loft conversion or refurbishment, value-engineering the design, and choosing where to spend and where to save. See our guide, How to Save Money on Your Extension.

Get a figure for your project

For a cost range tailored to your home, book a free consultation with Studio MASS: email info@studio-mass.com, call 0208 638 6727, or use the Talk to Us button above.

Last reviewed: July 2026. Building costs move over time — treat these figures as indicative and get a current quote for your project.

About the author

Robert McBride BSc(Hons) MFPWS is an architectural technologist and director of Studio MASS, a London practice specialising in residential extensions, conversions and new-build homes.