Given the floor and the wall use the same quantity of materials (one of the peculiarities of Dorfspace) that'd be one step away from saying "pile that floor up into a wall!" for no additional material costs. (It can be done like that already, except for the additional designation needed, a wait that may or may not be worth factoring into the execution of the "combo" command and the discontinuity of job participation that likely splits the labour over two dorfs.)
Right now, the 'unit block' does not have room for a specific ceiling. There's an (optional, natural-only until dug away by channelling or down-stairwelling) floor, then a height of space within which may be a built floor (of negligible height for most purposes) or any floor-to-ceiling construction that reaches up to the 'optional floor above' and maybe beyond (no 'optional natural floor'-sized leaking gap!). Built furniture can sit on the top of a wall below, atop the ONF or atop any built floor. Including leak-proof floodgates and doors that both reach down to whatever they are built upon and up to whatever upper limit that cell has (again, leakproof against whatevrr the Z+1 barrier might be).
There's some sort of non-Eulideanness to the verticality, maybe. ⅛th + ⅞ths = 1, but so is 0/8ths + ⅞ths. Or it's 8/8ths added in both cases, but still leading to 1 as a total.