You got it, I want to know if triangle-triangle intersection happened. Intersection of OBB and AABB won’t be precise enough to determine if the plane and ground with height intersects i.e. if there are hills. As far as I can tell it does always intersect because bounding box of ground uses max height.
I think mesh-mesh intersection on triangle level would be “nice to have” for the whole community, not only in case you want to place a building site.
Edit: Thanks to this PG of @Raggar, I got it:
Adapted PG for Plane-Ground: