The contractual payment mechanism decides when design services are valued and paid. No general rule requires full payment on design completion or requires the cost to be spread across the construction work

The contract decides, and a well-drafted one is precise. Design cost divides into pre-contract design cost, incurred before the contract in working the design up to tender, and post-contract design cost, incurred as the design develops during the works. The parties may allocate tender-stage and post-contract design costs to different payment stages, but entitlement arises only under the agreed mechanism. Where the contract uses stage or milestone payments, the pre-contract cost belongs in the first stage or milestone and the post-contract cost is allocated to later ones. Where an advance or mobilisation payment is available, the pre-contract cost is usually included in it; otherwise it goes into the first payment on account. Where there is no stage mechanism, design is paid through interim payments, front-loaded for the pre-contract element and spread for the rest. Design is neither paid in a lump on completion nor withheld to the end.