Quick answer

Only where the contract wording allows: a guaranteed maximum price has no fixed meaning, and whether the contractor can exceed it depends on which risks the contract transfers, so an increase is available only for employer changes or for risks the wording leaves with the employer, as Mowlem v Newton Street shows where a clear risk-transfer clause defeated a claim for unforeseen work

Only where the wording of the contract allows it. A guaranteed maximum price has no fixed legal meaning; whether the contractor can exceed it depends on which risks the contract transfers. The intention is to place risk on the contractor and allow no increase except for changes instructed by the employer, so an increase is available only for employer changes or for risks the wording leaves with the employer. In Mowlem plc v Newton Street Ltd a clause describing the contract sum as a guaranteed maximum price, with the contractor accepting all risks including responsibility for information given by others, defeated a claim to be paid above the maximum for instructed concrete repairs to an old building. There is no standard guaranteed maximum price form; the FIDIC Silver Book comes nearest. A contract that carries the label is not always what it appears, and matters such as increases in cost from changes in legislation are commonly left outside the maximum and paid as extras.