When should the final account be submitted?

By the contract's timetable where it sets one, and in practice as early as your records allow, because delay usually only helps the other side. Many standard contracts set a timetable for the final account after practical completion (the point when the works are finished enough to be handed over and used): the JCT forms give the contractor a period to submit its account and the other side a period to assess it, while NEC4 runs the same reckoning through a final assessment the Project Manager must make by a set deadline. Some contracts make that timetable bite by letting an assessment become conclusive, meaning fixed and no longer open to challenge, if you do not respond in time. But the practical clock is harsher: every month after completion, people leave the project, records get scattered across archives and inboxes, and memories fade, so your ability to prove each figure quietly shrinks. So submit as soon as the account is properly evidenced, while everyone who can verify it is still reachable, and do not treat the contract's outer deadline as the target.

Say a fit-out contractor reaches practical completion on a Friday in March, and its JCT contract gives it six months to submit the final account. It relaxes, thinking it has until September. By August, when it starts pulling the account together, the site manager who signed off the extra partitioning has left for another firm, the daywork sheets for the out-of-hours works are in a box nobody can find, and the employer's project lead has moved on too, so nobody on the other side remembers agreeing the changes. A rival that had built its account up week by week could have submitted in April, while every person who signed a variation was still a phone call away and every record still in the live project folder. Same contract, same six month window, but the early submitter can prove its £90,000 of extras and the late one is negotiating from memory. The practical deadline, when your evidence is strongest, usually bites long before the contractual one.