How to make "Building buildable: Unbuildable"?

[Pinzu]

I have two unique buildings, wall pieces that I want to make only buildable over a wall building, so if there was any good way to trick with the editor so it can only be built on unbuildable, it would make it somehow easier for the user to catch on and it would feel smoother reducing the amount of misplaced buildings in need of removal.

Or I'll come up with a work around. Tell you more later maybe.