£/m² is a genuinely useful shorthand once you understand what drives it and its limits. Used as a single number to budget an entire project, it's one of the most reliable ways to end up badly disappointed.
What the number is actually measuring
A £/m² figure is simply the total build cost divided by the floor area created. It tells you nothing on its own about what's inside that cost — a bare-shell single-storey box with a flat roof and a basic kitchen will produce a wildly different number to a two-storey extension with a vaulted glazed roof, underfloor heating and a bespoke kitchen, even though both are 'an extension'.
The figure is a ratio, not a specification. Two projects can share an identical £/m² rate while being completely different builds, purely by coincidence of area and total cost lining up.
The factors that move the number most
Single-storey vs two-storey changes the economics significantly, because foundations and roof — the most expensive elements per square metre — are spread over more floor area in a two-storey build.
Beyond storeys, the biggest swings come from groundworks difficulty (sloping sites, poor ground, high water table), roof complexity, glazing area and type, structural spans requiring steel, and finish specification from builder-grade to bespoke joinery and stone.
- Single vs two-storey (area spread over the same roof/foundation cost)
- Ground conditions and site access
- Roof form and glazing area
- Structural spans and steelwork
- Kitchen, bathroom and finish specification
- M&E extent — underfloor heating, home automation, extra circuits
Why online averages are close to meaningless for your job
A national average figure is built from a huge range of projects across regions with very different labour and material costs, on properties with very different access and ground conditions, to very different specifications. Averaging all of that produces a number that describes no real project.
Kent adds its own variables too — a site in a Wealden village with a narrow lane and no mains drainage nearby will cost differently to build than an identical extension on an estate with easy access and services already at the boundary.
A better way to use £/m²
Use it as a sense-check on a quote you've already had surveyed and priced properly, not as a way to generate a budget from scratch. If a genuine, itemised quote for your specific project comes back at a rate wildly outside what similar recent local jobs have cost, that's worth asking about — it might mean something's missing from scope, or it might mean the specification is unusually high or low.
It's also useful for very early, rough conversations before a design exists — 'roughly what does a single-storey kitchen extension of this rough size tend to cost around here' — provided you treat the answer as an order-of-magnitude starting point, not a number to commit a budget against.
Watch for what's included and excluded
Some quoted £/m² figures include fees, VAT, kitchens and fitted furniture; others cover shell-and-core building work only and exclude all of that. Comparing two figures without checking what sits inside each is comparing two different things wearing the same label.
Ask specifically what a quoted rate includes: is it VAT-inclusive, does it cover fitted kitchen and bathroom suites, external works, and design and approval fees, or just the construction labour and materials.
Where a proper quote does better than any average
A written, itemised quote based on a site visit and a defined specification for your actual property will always beat a generic £/m² figure, because it accounts for your ground conditions, your access, your roof shape and your chosen finishes rather than an average of thousands of unrelated jobs.
We give free site visits precisely so that the number you're working from reflects your project rather than a national statistic that happens to have your rough floor area attached to it.
Common questions
Why do £/m² figures for extensions vary so much online?
Because they're calculated from very different specifications, storey heights, regions and inclusions. A bare single-storey box and a fully glazed two-storey extension with bespoke joinery can both be labelled 'extension cost per m²' despite being nothing alike.
Is two-storey cheaper per square metre than single-storey?
Usually yes, because the most expensive elements — foundations and roof — are spread across more floor area. If you need the extra space anyway, going up rather than out is often better value per square metre, though it costs more overall.
Should I budget my whole project using an average £/m² figure?
No. Use it only as a rough early sense-check before a design exists. Once you have a defined scope, a proper itemised quote from a site visit will always be more reliable than any average figure.
Does a quoted £/m² rate usually include VAT and fees?
It varies enormously between quotes, so always ask specifically. Some include fitted kitchens, fees and VAT; others cover only the shell construction. Comparing two rates without checking this is comparing different things.
What Kent-specific factors change the number most?
Site access down narrow lanes, distance from mains services, sloping or clay ground common in the Weald, and conservation area or listed status requirements for materials all move the real cost independently of floor area.