Final accounts go wrong long before the final account.
Under JBCC, the final account is the consolidated record of every variation, every adjustment to the Contract Sum, every loss-and-expense claim, every penalty waiver. If it's not in the final account, it's hard to recover.
Two failure modes we see repeatedly.
The first. Contractors submit on time but don't include items that were verbally agreed on site. The Principal Agent's letter back is short — "no documentary record." The variation is dead.
The second. Contractors miss the submission deadline entirely, then try to revive items in the next valuation. By that point the Principal Agent has issued the Final Payment Certificate and is functus officio on items not raised.
We've seen single contracts where contractors walked away from R2M+ in genuinely entitled work because the final account wasn't run as a contract instrument from day one.
The fix isn't a final account checklist. It's running the daily site instructions, contract instructions and variation orders as if you'll need to defend them in three years.
More on JBCC payment, the final account and how it compares to NEC3 in the JBCC Contract Assistant.
General guidance on how the contract works — not legal advice, and no substitute for reading the contract and its amendments on your project.