I see lots of references to a 4" slab being the standard thickness, but I have a couple things going for my house plan that I thought might allow a thinner slab: 1) The slab isn't load bearing, the footers / concrete load bearing walls are isolated from the slab with expansion joints. Lightweight nonload bearing walls will be on the slab, however 2) I plan on putting flooring over the slab, probably vinyl sheet. There might be an underlay mat too. Hairlines shouldn't be visible 3) The house is smallish and divided in half by a load bearing wall, so the largest slab will be 32'x16' 4) I'm in a frost free environment. So what do you think? I will still do 6" for the 1 car garage, but was thinking 3" or 3.5" for the rest. On the other hand, saving materials on the base of the house is very intimidating.

Every time I've tried to thin out a 4" slab, I've been sorry. On your largest slab, you are talking about a difference of 1.5 yards of concrete, or less than $150. Is that worth it?

Very good point, thank you.

3.5" thick slabs (thickness of a standard 2x4 often used to form the slab) are pretty common and generally code allowed.