As we speak, I'm building a house that gave me the same problem. Solved it as follows: CFE false (if you don't know what it is, I'll tell you) and then in build mode go to terrain tools > level terrain, draw a block of basement underneath, then select a nearby working block and drag over the spot that's not working. It will turn green, then release. Your piece of basement will now be available to put your tile on. If you want the tile unsupported, just build the basement block one block away. Then when you delete the basement piece, it won't take the tile along with it.