A build cost per square metre is a benchmark, not a budget. Published rates cover the building works only, which means they leave out external works, professional fees, contingency and often VAT. Apply one to your floor area and the number you get is typically 20–30% below what the project actually needs.
That gap is where most budgets go wrong, and it has nothing to do with the rate being wrong. The rate is usually fine. It is answering a narrower question than the one being asked.
Where the published figures come from
Cost per square metre rates are derived from tender prices on completed projects, sorted by building type, size and procurement route, then indexed to a point in time and a national average location. In the UK the reference set most of the industry uses is published by BCIS, the cost data service run by RICS.
Two things follow from how that data is built, and both matter more than the headline number.
First, the figures are historic by construction. They come from tenders already returned, so they describe the market as it was when those prices were submitted, not as it is on the day you read them. Second, and more importantly, they measure building works. That is a defined scope, and it is a lot narrower than "the cost of the project".
You will see figures quoted for UK new build housing in a wide band, commonly from around £1,800 per square metre for a basic developer specification up to well over £4,000 for a bespoke architect-designed house in London. Those numbers circulate widely, and they are indicative rather than authoritative — the underlying BCIS data is a subscription service, and anyone quoting it for free is quoting it second hand. Treat a figure you found online as a sense check on your own build-up, never as a substitute for one.
What a rate per square metre leaves out
This is the part that catches people, so it is worth being specific. A building works rate generally excludes:
- The land, and everything associated with acquiring it
- External works — drives, paths, drainage runs, landscaping, boundary treatments, service connections
- Professional fees — architect, structural engineer, building control, planning, surveys
- Statutory and planning costs, including any community infrastructure levy
- Contingency for the things nobody has found yet
- VAT, which is not a simple add-on — see below
- Abnormals — poor ground, demolition, a sloping site, a tricky access
On VAT specifically, the treatment differs by work type rather than being a flat percentage. New build residential and conversions are treated differently from repairs and extensions, and the rules are set out on gov.uk. Getting this wrong on a domestic project is a large error in absolute terms, and it is entirely avoidable.
Worked example: from floor area to a real budget
Take a two-storey house with a gross internal area of 140 m². Gross internal area, or GIA, is the floor area measured to the inside face of the external walls, on every storey, including circulation. It is not the same as the footprint and it is not the same as usable room area, and using the wrong one is a common way to be 15% out before you start.
Apply a rate of £2,200 per m²:
- Building works: 140 × £2,200 = £308,000
- External works and service connections, at 10%: £30,800
- Professional fees, at 10%: £30,800
- Contingency, at 5%: £15,400
- Total: £385,000
Divide that back out and the real figure is £2,750 per m², against a headline rate of £2,200. The published rate was not wrong. It was 25% short of the answer, because it was never trying to answer that question.
The percentages above are illustrative and will move considerably with the site. External works on a tight urban plot with an existing connection might be 4%; on a rural site needing a new drainage run and a long drive, 20% is not unusual. That variability is exactly why a rate per square metre cannot carry them.
Why the same house costs different amounts
Two houses with identical floor areas can differ by 30% or more, and the reasons are structural rather than mysterious.
Shape drives cost more than size does. A simple rectangle has less external wall per square metre of floor than an L-shape or anything with projections and bays. External wall is expensive: it carries the insulation, the windows, the weatherproofing and most of the detailing. More corners means more cost for the same accommodation.
Storeys change the rate. A 140 m² bungalow costs more per square metre than a 140 m² two-storey house, because the single-storey version needs twice the roof and twice the foundation to enclose the same floor area. This surprises people who assume building up is the expensive option.
Ground conditions sit outside the rate entirely. A standard foundation on decent ground and a piled solution on made ground are not variations of the same item. If a site investigation has not been done, that risk is uncosted rather than absent.
Specification compounds. The difference between standard and high specification is not one line. It runs through the windows, the heating, the finishes, the ironmongery and the sanitaryware simultaneously, which is why specification alone can move a rate by a third.
When a rate per square metre is the right tool
It has a proper job, and it does it well. Early on, when all you have is a location, an approximate size and an intended use, a rate per square metre tells you whether a scheme is worth spending design fees on. That is genuinely valuable, and it is the basis of an order-of-cost estimate.
It stops being the right tool the moment there are drawings. Once the design exists, the work can be measured, and a measured estimate is a different order of accuracy. Continuing to rely on a rate per square metre after that point is choosing a rougher answer when a better one is available.
The signal to switch is straightforward: if someone can draw it, someone can measure it.
How to sanity-check a figure you have been given
- Is it building works only, or does it claim to include external works and fees?
- What area basis is it applied to — GIA, footprint, or something undefined?
- What date and location is the underlying data indexed to?
- Does it assume a specification, and is that yours?
- Has anything been assumed about the ground, and on what evidence?
- Is VAT in, out, or unstated?
Any figure that cannot answer those six is a number without a scope attached, which makes it impossible to compare against anything. That is worth knowing before you take it to a lender.
Who this is for
- Developers
- Builders
- Architects
- Private clients
- Financiers
FAQ
How much does it cost to build a house per square metre? Published UK figures for new build housing commonly run from around £1,800 per m² for a basic specification to over £4,000 for a high-specification London build. Those cover building works only, so the all-in project figure is typically 20–30% higher once fees, external works and contingency are added.
What is GIA? Gross internal area: the floor area measured to the inside face of the external walls, across every storey, including circulation space and internal walls. It is larger than the sum of the room sizes and it is not the footprint. Cost rates are normally expressed against GIA.
Does the rate include VAT? Usually not, and VAT treatment depends on the type of work rather than being a flat addition. New build residential, conversions and repairs are treated differently. Check the current rules before adding a percentage.
Why is a bungalow more expensive per square metre than a house? Because roof and foundation are the expensive elements and a single-storey building needs roughly twice as much of both to enclose the same floor area. Building up spreads those costs over more floor.
How accurate is a cost per square metre estimate? Accurate enough to decide whether a scheme is worth developing, and not accurate enough to hold a contractor to. Once drawings exist, a measured estimate built from quantities is a substantially better instrument.
Can I use a rate per square metre for an extension? With care. Extensions carry a disproportionate amount of expensive work — foundations, roof, structural openings into the existing house — across a small floor area, so rates per square metre tend to run higher than for a new build and vary more.
Where to start
If you are at the stage where all you have is a site and a size, a feasibility study will tell you whether the scheme stacks up before you commit design fees. If you already have drawings, skip the rate entirely and get the work measured — send them over and we will come back with a fixed fee and a turnaround date for a full estimate and cost plan. It is also worth reading how to read a bill of quantities if you want to understand what a measured estimate is actually giving you.