What makes a payment application valid?

Four things, applied strictly. A payment application is the payee's request for payment for a round, and to count it must be served by the contract's date, clearly against the correct round, in the required form to the named recipient, state the sum the payee thinks is due as at the Due Date, the date the payment obligation is fixed, with workings shown line by line, and read plainly as an actual application rather than a draft, forecast or for information figure. The courts are strict because of what is at stake: where the contract allows applications, a compliant one stands as the Notified Sum, the amount that legally must be paid, by default if the payer fails to serve its own notices in time. Judges read any ambiguity against the payee, so tie every application explicitly to its round and attach the supporting detail when you send it, not later once a dispute has started.

A roofing subcontractor emails its surveyor on the due day, the 25th, a document headed Application for Payment number 7, addressed to the named recipient, stating £48,000 due as at the Due Date with a measured breakdown attached. That reads unmistakably as an application against a specific round, so if the contractor forgets to serve its notices, the £48,000 stands as the Notified Sum by default. Contrast a subcontractor who forwards an untitled spreadsheet marked draft for discussion with a rounded total: if a dispute follows, the ambiguity is read against it and the figure may not stand. The difference is not the work done but the clarity of the paperwork.