How does the JCT final account work?

The JCT final account under SBC/Q 2016 is the final adjustment of the Contract Sum, done through the final adjustment and Final Certificate provisions in section 4, and it ends in a Final Certificate that under clause 1.9 becomes conclusive on the matters it lists unless proceedings are started in time. The sequence is fixed. Not later than six months after practical completion the Contractor sends the Architect/Contract Administrator, or the Quantity Surveyor if so instructed, all the documents necessary for adjusting the Contract Sum: the Variation account, daywork sheets, loss and expense records and the rest. The Quantity Surveyor then prepares, and the Architect/Contract Administrator sends the Contractor, the statement of all the adjustments to the Contract Sum, together with the ascertainment of any loss and expense, within the period the clause states. The Final Certificate is issued within two months of the latest of three events: the end of the Rectification Period, the date of the Certificate of Making Good, and the date the statement of adjustments and ascertainment were sent to the Contractor. It states the adjusted Contract Sum, the total already certified, and the balance due either way. Clause 1.9 then makes the certificate conclusive evidence of the matters it names, including that all necessary adjustments to the Contract Sum have been made and that the loss and expense paid is in final settlement, save where proceedings have been started within the period the clause gives. Under the Design and Build Contract 2016 the same job is done by the Contractor's Final Statement, or the Employer's Final Statement where the Contractor does not produce one, with a similar conclusive effect. What goes wrong is sending the documents late and losing the Contractor's grip on the timetable, a statement of adjustments that was never checked line by line against the Contractor's account, and a Final Certificate that lands while the parties are still arguing.