How much does a house extension cost in London in 2026?

A straightforward single storey rear extension in London starts at around £80,000 ex VAT for building the new space itself, and most complete extension projects come in from around £100,000 once the knock through, finishes and fittings are included. A two storey side return sits around £185,000 to £190,000, and pairing an extension with a whole house refurbishment typically lands between £210,000 and £250,000.

These are not scraped averages. They are indicative figures from real London projects priced by the same engine that powers the instant estimator on this site, current as of August 2026. Every project is different, so treat everything on this page as a realistic starting point rather than a quotation, and all figures exclude VAT.

Rounded figures from real take offs priced in August 2026, all ex VAT:

• Single storey extension, new space only (around 34 m²): from around £80,000, which is roughly £2,400 per square metre of new space.
• Two storey side return adding a kitchen extension plus a bedroom and shower room above (around 48 m² of new floor area): around £188,000.
• Extension plus full refurbishment of the existing house (around 150 m² overall): £210,000 to £250,000 depending on scope and specification.
• Basement conversions and dig outs (50 to 60 m²): £175,000 to £225,000, or roughly £3,500 to £4,000 per square metre. Below ground work is its own world and deserves its own conversation.

Per square metre, new above ground space in London generally runs £2,400 to £2,800 for a high standard of finish, before VAT and professional fees.

Five things drive the spread more than anything else:

• Structure and openings: the wider the opening into the existing house and the more steel it needs, the more you spend before anything looks different.
• Wet spaces: kitchens and bathrooms concentrate cost. An extension that adds a kitchen and a shower room will always outprice one that adds a plain living room.
• Specification: the difference between a good standard and a premium standard of finish can move the total by a five figure sum on the same floor plan.
• The existing house: extending is the moment most owners also rewire, replumb or refinish. That is often the right call, but it is the refurbishment that grows the budget, not the extension.
• Access and condition: tight sites, conservation areas and period fabric all add genuine cost. Worst ground conditions are priced when they are found, which is why honest estimates carry a range.

The figures above are calibrated to inner west London, the Fulham, Chelsea and Wandsworth belt where most of our projects run. As a working rule from our regional pricing research: prime central London projects often price 8 to 25 percent above these figures, inner north, east and south London typically sits a little below, and outer London commonly runs 10 to 20 percent below. The same extension is not the same price in Mayfair and in Croydon, and any estimate that ignores that is guessing.

Averages end where your project begins. The instant estimator on this site prices your extension from your own description: tell it what you want to build, answer a short set of tailored questions, and a realistic priced range for your project arrives on screen and in your inbox within minutes. It is the fastest honest way to know your number before you speak to anyone.