Can I start building the final account before the job finishes?

You should start it on the first day, because a final account is either a compilation or an excavation, and which one is decided long before completion. The compilation is built as the job runs: each section of measured work closed and agreed the month it finishes, every variation priced and evidenced in the register as it happens, the running total reconciled to what was applied for and certified each cycle. By the time the site empties, it is mostly a stack of already-agreed pieces waiting to be totalled. The excavation is the opposite: nothing measured until completion, so the whole job is reconstructed months later from archived drawings and fading memories, both sides arguing about work neither can now see. Assembled as you go, an account agrees in weeks; excavated at the end it drags for months and slides toward dispute, every line decided cold. Starting early does not mean submitting early: the submission still lands when the contract requires it, but it totals a living document rather than begins building one.

Take a groundworks subcontractor on a phased JCT job who used to leave the account to the end. On one job the site closed and the team dispersed before anyone measured; six months on they were arguing with the main contractor's surveyor about muck-away volumes from a phase neither could inspect, reconstructing dayworks from photos, and the account was still open a year after completion. On the next they changed nothing about the work and everything about the timing: each phase measured and agreed as it completed, the fourteen variations priced and signed off in the month each arose, the account reconciled to the certificates monthly. At practical completion it was already ninety per cent agreed on paper, and was submitted, checked and signed within a month.