So in planning and instant build i can get items to overlap slabs but i cant build that with heartlings and thats bad 
the good one is made with instant build since i can place fences and what not inside slabs.
Making my fences looks perfect.
But i really think this should be a feature…i saw there was a mod for a half fence. but its very rarely needed.
This just need to be a feature more than a bug 
ghost items i guess? that will have no physical mass just looks nice. that way hearthling wont try to use them either