House Extension Quote Calculator

How this estimate is put together

The demo prices an extension from its footprint and how it is built, the way most builders do their own first pass: a rate per square metre for a single-storey rear extension, covering foundations, structure, roof, windows and a standard first fix; a lower rate per square metre for a double-storey extension, because the ground and first floor share the same foundations and roof, so the extra floor costs meaningfully less per square metre than building single-storey twice over; and a premium of 10 to 15 percent on the single-storey rate for a wrap-around, since an L-shaped structure means more foundations, more roof junctions and more valleys than a straight run against the back wall. Shell only strips 30 to 40 percent off the build cost for a bare structural envelope with none of the fit-out, and high spec adds 25 to 40 percent back for a considered kitchen, bathroom and finish throughout. A kitchen fit-out, a bathroom, bifold or sliding doors and a knock-through with structural steel are then added as their own lines, since they are usually decided, and priced, separately from the shell. The rates themselves are illustrative but sanity-checked against published UK extension cost guides, and are cited in the calculator's own code comments for anyone who wants to check them. None of these numbers is special: when we build a calculator for a real building firm, the rates, the ranges, the questions and the rules are all theirs, entered once and changed whenever they like.

The point is not the arithmetic, which is deliberately simple. It is that the homeowner gets an honest range in under a minute, alongside a separate, honest line about what it does not cover: VAT and the architect, structural engineer and building control fees that typically add 10 to 15 percent on top, so nobody mistakes a build estimate for the full cost of the project. The builder gets a lead that already contains the extension type, the size, the finish level and the extras the customer wants, before a single measurement is taken on site.

Why a public price filters dreamers from buyers

Extension enquiries are chronically unqualified on budget. A homeowner who has seen a beautiful open-plan kitchen extension online rarely has a feel for what it costs to build, and a contact form asking them to "get in touch for a quote" does nothing to close that gap before a builder has driven out, measured up and spent an evening putting a proposal together for nothing. A public ballpark changes the shape of that funnel: someone who was budgeting forty thousand pounds and sees a range starting well above that figure rules themselves out before they ever pick up the phone, and the enquiries that do land already understand roughly what they are buying. That is not a smaller pipeline, it is a cleaner one, and every site visit that follows starts from a customer who has already agreed, in their own head, that the number is in the right region.