Extension Cost Calculator
Use your measured total extension floor area and an agreed visitor-supplied £/m² rate or explicit range. Record the agreed scope, VAT, optional outside-scope quotes and only your chosen contingency.
How the estimate works
Main entered estimate = explicitly supplied total net floor area × your positive £/m² rate. An agreed single rate supplies both endpoints; range mode requires explicit low and high rates. Each VAT-excluded amount is multiplied by (1 + its supplied VAT percentage ÷ 100); VAT-included amounts are unchanged. Add only explicitly confirmed outside-main-scope quotes. Complete VAT-resolved subtotal × your supplied contingency percentage ÷ 100 is a separate contingency; total = subtotal + that contingency. No monetary total is shown without a named agreed scope, complete prices and resolved VAT. No rounding is applied to intermediate amounts.
Visitor-input budget arithmetic, not a current-market benchmark, quotation, guarantee, tax treatment decision, building/structural design or legal submission. Quotes can materially vary with region, date, access, ground conditions, specification, services, finish and inclusions. No stock, live pricing, Materials List, automatic capture or saved output is produced. Floor area is supplied once; footprint and storeys are not used to infer it.
Worked example
Hypothetical arithmetic only, not a current-market price or a quotation: a supplied total floor area of 20 m² and an agreed rate of £1,000/m² including VAT for a named agreed scope give £20,000. A separately quoted £500 including VAT, explicitly outside that scope, gives a £20,500 subtotal. If you choose a 10% contingency, it is £2,050 and the arithmetic total is £22,550. None of these numbers are defaults, benchmarks or promises about actual extension costs.
What to measure and check
Start with the explicitly measured total extension floor area on the same area basis as your rate. Include each non-overlapping floor section once. The shared area is already net: this tool makes no further opening deductions. Obtain a positive rate, its agreed scope and VAT treatment from your provider or adviser. Choose one agreed rate or supply both endpoints of a range. These are your figures, not Calculator.Shop prices.
Is this a current UK extension price or a quotation?
No. All rates and extra amounts come from you. No current benchmark preset is supplied. Quotes materially vary with scope, region, date, access, ground conditions, specification, services and finish; obtain comparable detailed provider quotes. The result is arithmetic, not a quotation guarantee.
Which area should I enter?
Supply the measured total extension floor area once, using the same area basis as the rate. Add each non-overlapping floor section once. This tool accepts floor area only and does not multiply footprint by storeys or deduct openings again from the already-net shared area.
Why does missing scope stop monetary results?
A rate needs a named scope and your explicit agreement with the provider. Unknown inclusions make an apparent total misleading. Area is still shown while you supply the rate, scope and VAT facts. The calculator does not judge whether the agreed scope is adequate.
Can I use one rate rather than a range?
Yes. In one-rate mode your positive agreed rate is used for both budget endpoints. Range mode requires both positive low and high rates for the same scope and VAT basis; the high rate cannot be lower than the low rate.
What happens when VAT is unknown?
There is no complete VAT-resolved total or contingency amount. If scope and rates are known, entered nominal amounts may be shown clearly marked VAT unresolved, not payable totals. For VAT-excluded inputs supply the provider-confirmed applicable percentage. This tool does not decide tax treatment.
Can I add fees, kitchen costs or other quotes?
Only when they are explicitly confirmed outside the main rate scope, with their own amounts and VAT basis. Do not add included components twice. Completely untouched optional rows are ignored; a named or part-completed row with no price remains missing, not zero, and prevents a complete total.
Is a contingency automatically added?
No. A separate contingency is calculated only from a percentage you explicitly supply, after all active prices and VAT are complete. Blank creates no contingency row and is not advice to choose zero.
Does this create a Materials List or submit anything?
No. It is a pure calculation of supplied facts, with no automatic capture, storage, Materials List generation, building design, legal submission or provider booking.
Where the numbers come from
Formula version v1.0. Implemented and tested 10 November 2026. Product-dependent figures remain your supplied facts; a cited example is not a selected product or a universal yield.