House Extension Cost Calculator
Build an extension budget in separate lines — construction, VAT, fees and contingency — instead of one number that leaves things out.
Enter your floor area and this calculator builds an early budget for a single-storey extension in separate lines — construction, planning fee, VAT, contingency, and the professional fees you already have — so you can see what is and is not included. The construction band is the national £1,800–£3,000 per m² published by the FMB and Checkatrade in 2026, excluding VAT. It is a benchmark, not a Cambridge quotation.
House extension cost calculator
For single-storey extensions in England. Two-storey work is priced from drawings — this band does not scale to two floors.
Your early budget, line by line
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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.
Why this calculator gives you separate lines, not one number
A single headline total leaves out the lines that decide whether a budget holds: VAT, design fees, the planning application, Building Control, and an allowance for what nobody can see until the ground is open. Left out, they make a figure look lower and more complete than it is.
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’ single-storey extension cost guide (4 March 2026) and Checkatrade’s house extension cost guide (table updated June 2026), both of which publish £1,800–£3,000 per m² for a single-storey extension. Checkatrade marks its figures as excluding VAT; the FMB does not state a VAT position, and we treat its band the same way. It is a national benchmark, not a Cambridge quotation, and it is a single-storey figure: a two-storey extension is priced from total area and a developed design, not by scaling this band.
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 (half the householder fee), £249 for prior approval on a larger home extension, all from the Planning Portal fee schedule and the government’s indexation notice. 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. For the fees and allowances you type in, tell the calculator whether your figures already include VAT, so it is never added twice; the statutory planning fee carries no VAT.
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% of the total project cost, and 15% or more on older or structurally involved work. Here it is calculated on the construction allowance and its VAT, not on fees or allowances you already have a figure for.
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.
Filling those four boxes with national averages would make the total look more complete than it is. We would rather show you the gap and help you close it with written figures.
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 quote, 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 single-storey extension cost guide (4 March 2026) and Checkatrade house extension cost guide (June 2026, excluding VAT) for the £1,800–£3,000 per m² band; Planning Portal application fees for England from 1 April 2026; FMB contingency guidance (February 2026); HMRC VAT Notice 708. Prepared and reviewed by Daryl Edwards, Company Director, Prestige Building and Brickwork Ltd. Last reviewed 5 October 2026.
Ready to price your project?
Send us the address, what you have in mind and any plans or photos you already have. A description and a postcode is enough to start; we will tell you whether we can give an early figure or whether a site visit would be more use.