Our overrun, our cost
If we misjudged the effort on agreed scope, the price does not change. That risk belongs to whoever wrote the estimate.
Scope-driven overrun
If the work grew because the brief grew, that is a change request — quoted and agreed before it happens, never applied retroactively.
Dependency-driven overrun
Where a delay comes from a third party or from awaited inputs, the date moves. Whether cost moves depends on whether idle capacity was reserved, and that is discussed at the time.
Talking about it early
Overruns are raised as soon as they are visible, not at the invoice stage.