What a cost per square metre actually tells you

I went looking for a figure to sanity-check our own estimates against, and found something more interesting than a number.

Build It's benchmark report prices a 200m² new build, average quality, brick and block, managed by a main contractor, at £257,040. That is £1,285 per square metre. It is a real figure from a real cost model, published by people who do this for a living.

Cost guides published this year put the same house at £1,800 to £3,000 per square metre. Also real, also from people who do this for a living.

Neither is wrong. The gap is almost entirely scope, and once you can see it, £/m² becomes genuinely useful instead of quietly misleading.

What sits inside the number, and what doesn't

The Build It figure is the build: groundworks, shell, roof, first and second fix, plastering, fitting out, decoration, cleaning. It is stated net of VAT. It excludes professional fees, planning and building control charges, service connections, and external works like landscaping and driveways. It also predates 2026, which on its own moves the number.

The higher figures usually fold some of that back in. Sometimes fees. Sometimes VAT. Sometimes the drive and the fence. Every guide draws the line somewhere, and almost none of them tell you where.

So the first question to ask any £/m² figure is not "is that right?" but "what's in it?" A number that answers that is worth ten that don't.

Six things that are usually outside the number

• Professional fees. Architect, structural engineer, planning consultant, building control. Commonly 10 to 15 per cent of the build on anything that needs drawings.
• Service connections. Water, electricity, gas, drainage. Quoted by the utility, not the builder, and quite capable of running into five figures.
• Groundworks risk. Trees, clay, made ground, a high water table. The same house on a different plot is a different price.
• External works. Drives, patios, fencing, planting. Easy to leave out of a build cost and impossible to leave out of the job.
• VAT, which deserves its own paragraph.
• The bit at the end. Blinds, flooring, that light fitting, the skip you did not plan for.

VAT is not a detail

New build housing in the UK is zero-rated. A main contractor building you a house should not be adding 20 per cent, and self-builders can reclaim VAT on materials through the DIY Housebuilders Scheme.

Work on an existing home is different. Extensions, renovations and improvements are standard-rated at 20 per cent. There are reduced rates in specific cases: a property empty for two years or more, or a conversion that changes the number of dwellings, can qualify at 5 per cent.

Which means the same £2,000/m² figure can be £2,000 or £2,400 depending on what you are building, and a guide that does not say which is not much of a guide. Rules change and the detail matters, so check the current position with HMRC or your accountant rather than a blog.

Where you are changes it more than the spec

Every 2026 guide I looked at put London and the South East 20 to 40 per cent above the national figure. That is a bigger swing than most specification choices. Porcelain instead of ceramic will not move your total the way a postcode does.

So what is a £/m² figure good for?

Not a budget. It has no idea about your access, your ground, your drawings or the state of your existing walls.

It is good for one thing, and it is a valuable thing: telling you whether your number is in the right region of reality. If you have priced a single-storey extension at £900/m² and the guides say £1,700 to £3,000, you have not found a bargain. You have left something out, and it is worth ten minutes finding out what before you go any further.

That is exactly how we use it. Our own estimates get checked against these bands, and when a draft lands outside one, the app says so and states what the band includes. It is a sanity check, not a verdict.

The short version

A cost per square metre is a shape, not a price. Ask what is in it, add the fees and the VAT and the connections and the drive, remember where you live, and then treat the answer as the beginning of the conversation with a builder rather than a substitute for it.

The number that matters is the one with your name on it, and nobody can give you that from a table.