What already exists about Requirements? Something must if you know that something is busting the budget while some stakeholders are clamoring for it. Are you going to do more detailed requirements in the hope that the cost of that requirement can be shown to be less expensive? Thart is an unlikely outcome from my experience, but hey, it might happen.

You can't do a plan for anything until this issue is resolved, and your manager should know that. Having a group of stakeholders who can disagree with each other does make this a more "interesting" issue; how have disagreements been resolved in the past? If that doesn't help, are those 2 counties willing to pay extra to get the requirement met? If not, are there other requirements that can be removed from this expansion?

It all comes to setting priorities that are agreed to by the stakeholders, only then will you know what is to be delivered, then you can plan the work, which may mean getting more detailed requirements than you have now... but you can't get to this point with this issue unresolved, that leads to project failure.