Tile/Slice a 3D Terrain Map

Hello!

I'd like to make a 3D Map/Terrain map for a game I'm start working on soon. The game itself is 3D with a fixed orbital camera and the user should be able to place buildings there etc. but the terrain should be tiled.

I think auto snapping shouldn't be such a problem, As I could "correct" the position to the closest coordinates where the tile would fit, however, I have no idea how to make the tiles visible visually (dotted or dashed lines) and how to to determine if the stuff can be build on that tile etc.

I don't like to split the terrain into real blocks/gameobjects, as this would have a great impact on performance and limiting the level/map design pretty much. Can't use unity terrain system neither, as I intend to port it for android/iPhone too which doesn't support terrain yet. I doubt drawing the lines on the textures would be a good idea. I'd need to draw the tile lines directly on the terrain somehow, but I don't know where to start