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JCT and NEC are the two main families of standard form construction contracts used in the UK. JCT contracts are common on building projects and come in versions like the Standard Building Contract and Minor Works. NEC contracts are used more on infrastructure and engineering work and offer several pricing options, including a priced contract with a bill of quantities.

A GMP contract reimburses the contractor for actual costs plus a fee, like a cost-plus contract, but caps the total at an agreed maximum. The owner carries the cost risk up to that cap, and the contractor carries it above.

Guaranteed maximum price is often treated as a fifth type, since it borrows the reimbursement model of cost-plus but adds a cost cap. Some lists also include design-build, though that describes who holds design responsibility rather than how price is set, so it can combine with any of the pricing types above.

Start with how well defined the scope is. A fixed-price contract works when the design is finished and unlikely to change. Cost-plus or GMP fits better when the scope is still moving. Time and materials suits work that cannot be scoped in detail before it starts, and unit price fits repetitive work billed by measured quantity.

Changes usually go through a formal change order, a written request describing the new scope, the cost impact, and the timeline adjustment, approved by all parties before the work proceeds. Without that process, disputes tend to show up later over who agreed to what.