Three quotes for the same extension routinely differ by a third, and the reason is almost never that one builder is greedy. It is that they have priced different jobs. Until the exclusions, the provisional sums and the specification are lined up side by side, the numbers are not comparable, and the cheapest is usually the one that has left the most out.
Quote, house extension estimate and calculator: three different things
A quote is an offer to do defined work for a stated price and becomes a term of the contract once accepted. An estimate is a considered guess and is expected to move. An online calculator is a range derived from other people's projects, useful for deciding whether to start and useless for deciding who to hire. Treat the calculator as a sanity check on the quotes rather than as a target the quotes ought to hit.
The exclusions decide the final bill
Read what is not included before what is. Common exclusions are groundwork beyond an assumed depth, drainage diversions, asbestos, structural steel, making good to existing finishes, kitchen and bathroom fittings, decoration, and building control and planning fees. Each is a real cost that will arrive later. A quote with no exclusions list is not generous, it is incomplete.
Provisional sums and prime cost sums
A provisional sum is a placeholder for work that cannot be priced yet, usually foundations, because nobody knows what is under the garden until a hole is dug. A prime cost sum is an allowance for an item you have not chosen, typically the kitchen or the flooring. Both are honest devices and both are where budgets move. Ask what the assumption behind each one is, and what happens to the price if the assumption turns out to be wrong.
Make the three quotes comparable before you choose
Give every builder the same drawings and the same written specification, then put the quotes into one sheet line by line. Where a line is missing from one, ask for it rather than assuming it is covered. Where the totals still differ after that, the difference is real information about programme, risk and quality, and it is now a decision you can actually make.
Questions people ask about house extension quote
Is a quote legally binding?
Once accepted it normally forms part of the contract for the work described. That is precisely why the description and the exclusions matter more than the figure.
How many quotes should I get?
Three is the usual advice and is enough, provided all three price the same drawings and specification. Three quotes for three different jobs tell you nothing.
Why does a home extension calculator disagree with quotes so much?
A calculator works from a rate per square metre. Real prices are driven by ground conditions, access, the structure needed and the specification, which is where most of the variation lives.
Should the VAT position be checked?
Yes. Most domestic extension work is standard rated, but some conversions and certain energy saving measures are treated differently, so ask how the quote treats it.