Is a builder's quote legally binding?

A quote is an offer. If it states the work and the price with enough certainty and you accept it, a contract is formed on those terms and the price is binding, subject to whatever the quote says about variations, provisional sums and rates. An estimate is an indication of likely cost and is not binding, and a document headed "quote" that is full of allowances and exclusions behaves more like an estimate than its title suggests. Two things matter more than the label: what the quote actually includes and excludes, because the price only binds for the scope it describes, and whether the parties then signed a written contract that replaces the quote with its own terms, which the standard homeowner forms do. The commercial protection is to have the quote checked before acceptance for scope, provisional sums, the basis for extras and the payment terms, and to put the agreed position into a written contract rather than relying on the quote alone. Whether a particular quote created a binding contract is a legal question; ask a solicitor if the answer would change what you do.

Assume three quotes for an extension: £96,000 "fixed price" with a two-page scope, £88,000 with £14,000 of provisional sums and "kitchen by others", and £91,000 "estimate only". Once the scopes are put on one basis the second is the highest, not the lowest, and only the first has a price that binds for the work described. The decision is made on the levelled figures and then written into a contract.