Superiore, supremo, inferiore and infimo have special usage. You can certainly say “il piano superiore, il piano inferiore” (the higer or lower floor) with reference to the floors of a building, but “il piano infimo o il piano supremo” have a definite metaphysical implication. If you want to refer to the higher or lower floor of a building, you should stick to “il piano piu’ alto (o l’ultimo piano) e il piano piu’ basso”.