How Much Does a House Extension Cost in 2026?

Guide Costs Per Square Metre in 2026

For a quality single storey extension in our four counties and the Home Counties, a sensible planning figure is £2,200 to £3,200 per square metre of new floor area for the build itself, at a good standard of finish. A straightforward 30 square metre kitchen extension therefore starts somewhere around £66,000 to £96,000 before fees and VAT.

Different extension types carry different economics. Double storey extensions are the value play: two floors share one set of foundations and one roof, so a double storey extension typically costs 25 to 40 percent more than the same footprint built single storey, while doubling the space.

Loft conversions typically come out between £50,000 and £85,000 for a quality dormer conversion including a bathroom, with hip-to-gable and mansard designs above that. Wrap-around extensions price like large single storey schemes with more steel, basement conversions are the most expensive space you can add at £3,000 to £5,500 per square metre, and barn conversions vary more than any other category because the existing structure sets the agenda.

What Moves the Number Most

Structure. Steel is the silent budget-setter. Removing the back wall of a house to create one open space, or holding up two storeys over a wide opening, is engineering, and it is priced accordingly.

Glazing. Sliding and bifold door sets, rooflights and structural glass are frequently 10 to 20 percent of an extension budget on their own. Beautiful, worth it, and worth deciding early.

Kitchens and bathrooms. Most published per square metre figures, including ours, exclude the kitchen itself. A kitchen can be £15,000 or £60,000, and only you know which end of that range you are on.

Site, access and specification. A tight terraced plot where everything arrives by hand costs more than an open driveway. Underfloor heating, smart wiring, air source heat pumps and premium finishes all add real money, and all cost less when decided at design stage rather than on site.

The Costs People Forget

The build cost is not the project cost. A realistic budget also carries VAT at 20 percent, because unlike new builds, extensions are standard-rated. Professional fees for design, structural engineering, building control and planning typically add 8 to 15 percent of build cost.

Where you share boundaries, party wall costs add a few hundred to a few thousand pounds. Surveys appear where ground conditions, trees or drains demand them. And we recommend holding a 10 percent contingency: on a well-prepared project you may not spend it, but you will be glad it exists.

Why Prepared Projects Cost Less

The biggest cost risk in extending is not the headline price. It is variation: decisions made on site, surprises in the structure, and specification chosen under time pressure. This is why our four-stage process front-loads the thinking: an Opinion of Probable Cost before design, a feasibility check on planning and structure, then a fully coordinated technical package before anyone breaks ground.

The build that follows is calmer and, in our experience, meaningfully cheaper than the reactive version.

Get a Figure for Your Project

Every home is different, which is why generic tables only take you so far. Our instant extension calculator shapes a guide figure to your extension type, size, finish level and location in under a minute, and a free project appraisal turns it into a proper Opinion of Probable Cost for your actual house.

Extending versus moving is ultimately a numbers decision. Once stamp duty, agent fees and moving costs are counted, a well-planned extension in the right street is very often the stronger answer. If you are weighing the same question for a new build, our guide to the cost of building a house in our four counties covers that side.

An extension priced properly at the start is almost always cheaper than one priced optimistically.