Their application contains changes we never instructed
Work has appeared in the valuation with no instruction behind it, and we have nothing to refute it with.
What's happening?
The application lands and there they are: variations you do not recognise, work described as instructed that nobody in your office instructed, quantities that grew without a conversation. The site team half-remembers some of it. Paying rewards the creep and invites more of it next month. Refusing everything without grounds starts a fight you have not prepared for.
Under the contract's change machinery, the burden of showing an instruction, or the authority behind one, sits with the side claiming the money. But a bare we never instructed this does not answer an application; a valuation does. The work needs going through item by item: what traces back to a real instruction, what is design development inside the original scope, what was genuinely never instructed at all, each valued and the result stated with its basis in the notices the contract requires.
Done properly once, the exercise pays twice. The immediate cycle gets a defensible answer instead of a blanket refusal, and the register it forces into existence disciplines every application that follows, because the other side learns that creep gets traced.
The solution
Each item traced, valued and answered with its basis.