The specification is the agreement. Every choice from the layout to the tap belongs to a document, and a change agreed verbally halfway through is what turns a fixed price into an argument at handover.
Payments follow milestones. Deposit, insulation and boarding complete, furniture in, electrics commissioned, handover: staged against progress on a build that runs for months, so neither side is financing the other.
Parts arrive from everywhere. Windows, roof lights, heaters, fridges and upholstery come from different suppliers with different lead times, and one late item can hold a finished van in a bay for a fortnight.
The bay is the business. A workshop with three bays and builds taking ten weeks has a fixed number of conversions a year, which is what the diary and the waiting list actually have to reflect.