By Rob Allison, founder of Thames Design & Build. Rob started as an apprentice carpenter in 2005 and has run Thames Design & Build in Thames Ditton since 2011.
“How much per square metre?” It’s the first question everyone asks, and the honest answer is that a single number will mislead you. So we’ve done something most builders won’t: we’ve gone back through 19 of our completed projects across Surrey and South West London and worked out what actually drives extension costs in Surrey.
One thing to understand before any numbers: almost every extension project also involves work to the existing house — knocking through, refurbishing the rooms the new space opens into, redecorating whatever’s affected. So when we talk about cost per square metre across a whole project, we mean a blended rate across the new extension space and the refurbished existing space together. That’s how real projects work, and it’s why blended rates look lower than extension-only rates.
In short: based on 19 completed projects, a small standalone extension in Surrey typically costs £2,500–£3,000 per square metre, while larger projects mixing extension and renovation come in at around £1,650–£1,850 per square metre blended across the whole project area. The gap between those two numbers tells you most of what you need to know.
Those figures cover the construction work: structure, roofing, glazing, plumbing, electrics, heating, plastering and decoration. They exclude the things you choose and buy yourself — kitchen units, sanitaryware, flooring, light fittings. More on that below.
Our projects range from well under £1,500/m² for lighter-touch work to over £3,000/m². Same builder, same standards, same region. The difference comes down to three things that a headline rate hides completely.
How cost advice sharpens as the design develops
One thing we always explain at the first meeting: cost advice refines alongside the design, in three stages.
At the initial meeting, before any drawings exist, we can give a figure based on measurements alone — accurate to around 20% either way. Once initial schemes are drawn but there’s no detail or specification yet, that tightens to roughly plus or minus 10%. Then, with structural design and a full specification in place, we spend a proper amount of time estimating — typically 8 to 16 hours — and give a fixed price based on an agreed scope.
So a builder who quotes you a precise figure on a first visit, before anything’s been drawn, is guessing. The honest version of early cost advice is a range that narrows as decisions get made.
Why do small extensions cost more per square metre?
This is the clearest pattern in our data. Every project we’ve completed under 70m² cost more than £2,000/m². Every project over 150m² came in at around £1,750/m² or less.
The reason is straightforward: a good deal of building cost is fixed regardless of size. Scaffolding, site welfare, skips, structural openings and project management — a 30m² extension carries much the same fixed costs as an 80m² one, spread over less than half the area.
Take a real example. Earlier this year we built a small rear extension — around 12m² of new space, opening into an adjoining room we refurbished as part of the same job. The total came to £70,000 + VAT. Divide that by the extension alone and it looks like an eye-watering £6,000/m². Divide it by everything the money actually touched and it’s nearer £3,000/m². Both numbers are right, which is precisely why per-square-metre rates mislead, and why small projects always feel expensive for their size.
If you’re weighing up “shall we do a bit now and more later?”, doing it all in one go is almost always better value. And if someone quotes a small extension at a big-project rate, treat it with caution.
New space costs more than refreshed space
Extension space — new foundations, walls, roof and glazing — consistently costs more per square metre than renovating existing rooms. In our data, extension-dominant projects average around £2,000/m², while renovation-dominant projects average closer to £1,750/m². A light renovation can be considerably cheaper again, while a deep renovation to a high standard can approach extension money.
So when budgeting, don’t apply one rate to your whole floor plan. Think of it as new space at a higher rate and existing space at a lower one, adjusted for how far you’re taking the renovation.
The costs that don’t scale with size
Kitchens, bathrooms, en-suites, underfloor heating and heating systems don’t scale with floor area — they’re significant costs in their own right. Two similar-sized extensions can end up £30,000+ apart depending on what’s inside: one might include a fitted kitchen, a new bathroom, underfloor heating, a structural opening into the existing house and high-spec glazing, while the other is a simpler shell with none of those. Same square metres, very different money.
This is also why we quote the way we do. You supply the kitchen and bathroom fittings, because a kitchen can cost £8,000 or £80,000 and that decision should be yours, not ours. Who fits them varies from project to project: on roughly half our jobs we install the kitchen ourselves, and on the other half the kitchen company installs their own, which we coordinate alongside our trades. Either way, your quote sets out exactly where the line sits.
Structural work: the other hidden variable
Knocking through to open up the ground floor means steel beams, engineering and temporary works. One of the larger extensions in this dataset — a wraparound at £200,000 + VAT — cost what it did for two reasons: substantial structural openings into the back and side of the house at the same time, and large-format glazing that ran to tens of thousands before anyone had picked up a trowel. Neither was a problem; the clients wanted a big, bright, open space and got exactly that. But it shows how two projects that both get called “an extension” can be £130,000 apart. (And extensions are only part of what we do — full house renovation contracts run to double that and beyond.)
So list the rooms and systems you’re adding, not just the square metres. And if your plans involve removing walls, expect the structural work to be a meaningful line in the budget.
How much does a loft conversion cost?
In practice, loft conversions range from £60,000–£120,000 + VAT, and the spread comes down to structure. At the lower end sits a rear dormer on a mid-terraced house — the roof largely stays as it is, and the dormer does the work. At the top end, a detached house where the whole roof is restructured to create the space: more steel, more roof work, more of everything. Every loft carries certain costs regardless of size — the new staircase, access, scaffolding — but it’s the structural approach that decides which end of the range you’re at. Combining a loft with a ground-floor extension in one project saves on shared overheads, which is why we often price them together.
The awkward truth about costs
The most common issue we see isn’t people with unrealistic expectations — it’s people with expectations that were realistic six or seven years ago.
Construction costs have risen substantially since 2020. Materials jumped during Covid and never came back down, and labour costs have followed. A project that cost £60,000 in 2019 is realistically £85,000–95,000 today. If your budget is based on what a friend paid for their extension in 2018, or a magazine article from a few years back, it needs updating before you start designing.
We’d rather have that conversation at the first meeting than after you’ve fallen in love with a design you can’t build.
How to budget for an extension
- Start with the shape of your project, not a single £/m² rate: how much new space, how much renovated space, and how far the renovation goes.
- Add the lump sums: kitchens, bathrooms, heating, structural openings.
- Expect small projects to cost more per m². That’s physics and logistics, not a builder trying it on.
- Don’t forget VAT. Builders quote plus VAT, and 20% on a £100,000 project is another £20,000 on top.
- Compare quotes on scope, not rate. Check what’s included, what’s client-supply, and whether fitting is covered.
- Get a realistic estimate early, before you’ve spent anything on design. We use our own completed-project data to give a realistic picture of extension costs in Surrey at the first meeting — accurate to around 20% either way, tightening as the design develops.
If you’d like a straight answer on your specific project, we offer a free consultation. We’ll tell you what your budget can realistically achieve — even if the answer is “not quite what you’re hoping for yet.”
Thames Design & Build is a design and build company based in Thames Ditton, Surrey, specialising in home extensions, loft conversions and renovations across Elmbridge and the surrounding areas. Figures are based on our completed projects; every home is different, and we’re happy to give you a project-specific estimate.