What is a conclusive evidence clause?

It is a clause that makes a final certificate or final assessment binding, meaning it can no longer be challenged, unless you formally start a dispute within a set period. A final certificate is the document the contract administrator or architect issues at the very end to state the last agreed figures for the job; a conclusive evidence clause then treats it as proof of those figures. These clauses deliberately end arguments by deadline rather than by who is right: once the period passes, the certificate's numbers, and under some contracts its statements about the quality of the work, become fixed however wrong they may be, so a genuine underpayment can simply stop being recoverable. The trap is that the period is often only weeks, and it keeps running while settlement talks still feel friendly, so people miss it without realising. So when a final certificate arrives the first two questions are what exactly it makes conclusive and the precise date that takes effect, and the diary entry to protect the deadline usually matters more than the reply letter you send.

Say a main contractor receives the architect's Final Certificate on a Tuesday, and the contract makes it conclusive evidence of the account unless proceedings are started within 28 days. The certificate values the account £55,000 below what the contractor believes it is owed. The contractor writes a strong letter of objection the following week and waits for a reply, feeling it has made its position clear. But a letter is not proceedings: the 28 day clock keeps ticking while the parties trade emails, and when it runs out the certificate becomes binding at that lower figure, so the £55,000 is lost however good the underlying claim was. Had the contractor instead started an adjudication or court claim before the deadline, even a short one just to stop the clock, the figure would have stayed open to challenge. With a conclusive evidence clause, the diary date to protect your rights matters more than the objection letter.