How to select a random point within a Navmesh or Layer?

I want to drop items at random position within a layer. For example, I have a street grid composed of a road layer, a sidewalk layer and a buildings layer. If I were to drop vehicles, for instance, they would have to be spawn within the road layer.

Having the area of my plane, selecting a random transform is trivial but then objects spawned would end distributed in top of the sidewalks and buildings. But if I could select an specific layer -or Navmesh for that matter- and get its point set, the random generated tranform will be bound to those limits.

Thank you.