Engineered suspended garage slab

[marty]

I have this situation quite often and usually set the floor depth at the maximum likely to be needed which should 12".
If the engineered design comes in less - say 10" then I just raise the basement floor 2" rather than change all the upper levels.

Another consideration is acoustic inulation. If 12" is allowed for it gives space for the ceiling to be isolated from the structure so you could end up with an 8" precast slab with 4" for thermal and acoustic insulation.