House Extension Cost Calculator
Build an extension budget in separate lines — construction, VAT, fees and contingency — instead of one number that leaves things out.
Why this calculator gives you six numbers, not one
Most extension calculators multiply your floor area by a rate and show a single total. That number is almost always wrong, and it is wrong in a predictable direction: too low. It quietly leaves out VAT, design fees, the planning application, Building Control, and any allowance for the things nobody can see until the ground is open.
This one does what our house extension cost guide recommends instead. It builds the budget in separate lines so you can see which assumption to change when your project changes — and where a figure genuinely cannot be estimated from a web page, it says so rather than inventing one.
The construction band comes from the Federation of Master Builders’ 2026 guide and Checkatrade’s June 2026 guide, both of which publish £1,800–£3,000 per m² for a single-storey extension, excluding VAT. It is a national benchmark, not a Cambridge quotation.
House extension cost calculator
Your early project budget
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This is a budgeting illustration, not a quotation. A written, site-specific quotation based on current drawings and a defined specification supersedes any figure on this page. Access, ground conditions, drainage, the opening into the existing house, glazing and finishes can all move a real price well outside this band.
Send us your drawings and we will price it properly — or read the full cost guide behind these figures.
What each line is, and what it is not
The construction allowance is the builder’s work: site setup, groundwork and foundations, brickwork and blockwork, the structural opening into the existing house, roof, glazing, first and second fix, plastering and finishes. What any one published rate actually covers varies enormously between sources — some describe a weather-tight shell, others a finished room — which is why a low headline rate and a higher quotation are often describing different jobs. Our guide sets out the full list of items to check a rate against.
The planning fee is statutory and fixed: £548 for a full householder application in England from 1 April 2026, £274 for a proposed Lawful Development Certificate, £249 for prior approval on a larger home extension. Not every extension needs an application at all. The Planning Portal adds its own online service charge to qualifying applications.
VAT is the line most often missing from a headline figure. Most ordinary work to an existing home is standard-rated at 20% when the contractor is VAT-registered. A relief that applies to one item does not reduce the rate on the whole extension.
Contingency is not padding and it is not a fund for upgrades. It is money set aside for what the ground, the drains or the existing structure turn out to be. The FMB recommends at least 10%, and 15% or more on older or structurally involved work.
The four lines we deliberately will not estimate
Architect’s fees, structural engineering, Building Control and Party Wall surveyors all appear as blanks above until you type in a real figure. That is on purpose.
There is no official domestic fee scale for architectural work — a designer may charge a fixed sum, a percentage or an hourly rate depending on how much of the job they take on. Structural engineering depends on what your design actually needs: a single beam over a rear opening is a different piece of work from a frame and new foundations. Building Control has no national fee; the charge depends on the work, the floor area and whether you use the local authority or a registered approver. And a Party Wall surveyor may cost you nothing at all, because whether fees arise depends on the work and on how your neighbour responds to the notice.
Any calculator that fills those four boxes with national averages is guessing, and the guess is what makes the total look reassuring. We would rather show you the gap.
Turning an estimate into a price
A figure from this page is a sanity check, not a budget you can commit to. To price your extension properly we need current drawings, the structural specification if you have it, and a clear idea of what you want included — kitchen, glazing, flooring, landscaping — because those are the lines that separate two quotations that look otherwise identical.
If you do not have drawings yet, send photographs, a description and a postcode. We will tell you whether we can give you an early figure from that or whether a site visit would be more use. We aim to respond to every enquiry within one working day with the most useful next step.
Request a project consultation, read the full house extension cost guide, or look at how we run extension projects and the extensions we have photographed from foundations to handover.
Sources: Federation of Master Builders 2026 guide and Checkatrade June 2026 guide for the £1,800–£3,000 per m² band; statutory planning fees for England from 1 April 2026; FMB contingency guidance; HMRC VAT guidance for building work. Prepared and reviewed by Daryl Edwards, Company Director, Prestige Building and Brickwork Ltd. Last reviewed 1 September 2026.