Placing a building over undug tiles results in "not permitted" for the digger, even though they have the skill to dig any hardness but are unconstructive. This prevents anything from happening (cant build because tiles need dig, but dig is not permitted). This occurs for any building type like floor tiles, ladders etc.
If the building is removed, Turner happily dig the tiles. Turner is unconstructive.
Even though Turner is unconstructive, he should be able to dig but this is being blocked whenever the dig command is issued by placing a building,
Place building on hard or very hard tiles Nothing gets done. Digger (with the skill) is prevented from digging (not allowed)