Save yourself some money and pour the footing @ the 30" depth to be below frost, the width and thickness is a local call, if 12" is fine for the width as suggested by the building department, then go with it, now find out the depth or thickness from them. Once complete build a stem wall from the footing to a predetermined point above final grade, this could be six inches, could be eight, it is a local building department call. Stem wall can be CMU, Pour in place, or even ICF if you wish.
The stem wall can be narrower then the footing, saving you concrete which is expensive. Pour your slab afterwards and but up against the stem wall. If it's a garage you are going to need slope from back to front to allow water to run out, don't pour it flat.