The simplest and most cost effective foundation for a detached structure is a slab with an integral perimeter footing i.e. a thickened edge. As long as your soils drain relatively well, you can just let it 'float' on the frost or you can protect it from frost movement using rigid board insulation. Place the insulation under the slab and extend it out a distance equal to your frost depth around the perimeter. The insulation will retain the natural heat from the ground and keep the frost from getting under the structure.
I have a 28 x32 garage built on a monolithic slab with no insulation up here in Maine and it performs just fine. Give my Guide to Floor Slabs in the link below a read for some additional thoughts on slab construction. Good luck with your build!
