No walls on floors.
Is this ever set to change? It seems like a fairly critical flaw in construction.
Probably not, as both are constructions. If you want a wall there, don't bother building the floor. If the dwarves need someplace to stand to construct, use a bridge.
IMHO, when building a wall on a floored location, the floor should count as a partial wall. There COULD be generic mechanics for it in game. It would just need to be linked or changed to use the levels of the fluid system. The dwarfs should be able to automatically change the construction to whatever you wanted by removing partial wall sections. Just designate a wall as a fortification and a dwarf will come in and cut 2/7 of the wall away without requiring the dismantling of the wall. They all take 1 stone right now. If that changes in the future (it should I hope) then they can adjust the amount of returned stone during the fortification transformation.
In actuality it could be changed so that each tile could have volume of some material (water, marble, magma, gold, etc.) 0-7
An empty tile is 0/7
A natural floor tile is 1/7
A constructed floor: 2/7 (requires 1 stone?)
A support: 4/7 (requires +3 stones unless floor exists, then +2. If wall exists, return 3 stones from extraction.)
A fortification: 5/7 (requires +4 stones unless floor exists, then +3. If wall exists, return 2 stones from extraction.)
A wall: 7/7 (requires +6 stones unless fortification exists, then +2. If floor exists, +5.)
It would be an intriguing way to get rid of the excess stone (besides dumping or hiding it) as well.
Of course, this could lead into the idea of having partial walls block the flow of liquids as mentioned in an older thread... but that would require some liquid handling modifications.