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

A straightforward single storey rear extension in London, about 20 m² of new floor, flat roofed with no rooflights, plus making good the room it joins, priced at around £57,000 ex VAT on our estimator, and our minimum for a complete extension project, once finishes and fittings are included, is around £85,000, because even a small extension carries almost the same baseline of work as a larger one. A two storey side return priced at around £128,000, and pairing a 20 m² extension with a full refurbishment of a 130 m² house came to between £165,000 and £195,000.

These are not scraped averages. The job figures are indicative, from typical London jobs priced by the same engine that powers the instant estimator on this site, current as of October 2026 unless marked otherwise. 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 typical London jobs priced on our estimator in October 2026, all ex VAT:

• Single storey rear extension, about 20 m² of new floor, flat roofed with no rooflights, plus making good the room it joins: around £57,000, which is roughly £2,900 per square metre of new floor.
• Two storey side return adding a kitchen extension plus a bedroom and shower room above (around 48 m² of new floor area): around £128,000. Our guide to two storey extension costs explains why it prices lower per square metre than a single storey.
• A 20 m² extension plus full refurbishment of a 130 m² house (around 150 m² overall): £165,000 to £195,000 depending on scope.
• Basements: about £181,000 for a new basement of around 40 m² dug under an existing house, and about £202,000 for lowering and converting a cellar of around 55 m², or roughly £3,700 to £4,500 per square metre, with tanking, underpinning and the dig all priced. Below ground work is its own world and deserves its own conversation.

Per square metre of new floor, counting the whole cost of each job, new above ground space in London runs about £2,650 to £2,900 at a standard specification, before VAT and professional fees. The single storey figure covers building the new space and making good the room it joins. Across the extension figures, windows, doors, any rooflights, sanitaryware and tiles are priced for fitting only, with the products supplied by you, and kitchen units, worktops and appliances are not included.

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.