If the Mayor is not with you - forget it
I didn't think France was outright corrupt in the way Spain (and most of the rest of Southern Europe) is.
Certainly, from what I gather from people down there, is that they tend to adopt the 3rd world practice of letting you do all kinds of stuff, until you p1ss off somebody important, and then the sky caves in on you. This is how they run aviation regulation, for sure.
Local planning, I don't know.
Here, it is done by a planning consultant knowing the right people and - bribes are a very 1970s thing; a colour TV doesn't go anywhere anymore - striking a deal with a councillor who is known to object, to withdraw/modify another planning application perhaps belonging to another applicant.
So it is a case of you paying the planning consultant to know the right people.
But if you don't want to pay £200/hr or whatever,
you have to know the right people, and this is going to be tricky in a foreign country... but actually it is damn tricky in the UK, too.