What is the difference between a payment application and an invoice?

An invoice is a request for payment of a sum already settled: the price is agreed, here is the bill. A payment application is more muscular. Under a construction contract, and the Construction Act behind it, the application is your periodic assertion of what the works are worth at a given date, set into a machinery of dates and notices the paying side must operate correctly. Once a valid application is in, the payer owes a response in a defined window: a Payment Notice stating what they will pay and its basis, and, if they mean to pay less, a Pay Less Notice by its own deadline. Fail to operate that machinery and the application itself can fix the sum due, payable in full, however generous its numbers. So an invoice at the wrong time earns nothing but a queue position, while a proper application, on the right date in the right form, is a legal position with a clock running on the other side. The rule: know which regime your contract runs, apply on its dates in its form, and treat the application as the formal document it is, because the protections attach to the machinery, not the politeness of the request.

Take a joinery subcontractor under a JCT subcontract who, out of habit from their shopfitting clients, emails a PDF invoice at the end of each month for what feels right. Some get paid, some part-paid, one ignored, and there is nothing obvious to do about it. On advice, they switch: same numbers, now submitted as interim payment applications on the subcontract's application date, in its form, referencing the payment clause, with the measure attached. Two months later the contractor's surveyor goes quiet on an application and no Payment Notice or Pay Less Notice arrives inside the windows. Because the paperwork was an application inside the contract's machinery rather than an invoice outside it, the Notified Sum becomes the sum due, and the account is paid in full without a fight the subcontractor could not have afforded. Identical work, identical values; the difference was which document carried them.