We are in deep frost country, and put tops of footings at a minimum of 48" below grade.
For an unheated garage, the typical foundation is a poured concrete wall arrangement, 48" high 8" thick walls atop 16" w x 8" h spread footings, then a 4" slab inside.
To control our cost and schedule, given the extreme unreliability of the concrete foundation contractors hereabouts, we are considering doing garage foundations using thin (meaning smallest cavity they have) ICF systems.
Have you done this? Does it make any sense for you?
Using a rule-of-thumb cost for poured concrete work at $275 per c.y., a 24 x 28 foundation (just the wall part) costs us about $2,800.
The walls would have an area of 416 sf, and the concrete pour inside would be about 7.5 c.y., for a cost of about $700 in crete. I see it as needing the ICFs, steel, and labor to erect coming in at less than $2100 to make sense financially.