Are there any other signs of major settling or movement? Do the doors/wiindows work. Major cracks or anything inside? Obviously, frost footings are ideal, but holy smokes is that a lot of back breaking work. So if you don't see a lot of evidence of settling/heaving, I'd question whether it would be worth it in the end. There are plenty of little cabins that aren't on real foundations. Now if you were going to add on to it, or add a story, or sink a pile of money into it, that would be a different thing.
Frost protected footings do work, but the rub is that you have to bury the foam, typically 4' out from each post. Doesn't work to just lay it on the ground.
If you really wanted to add a foundation, you could jack it up high enough so you could fit some equipment under there. Or do like a friend of mine did and build a foundation next to his little house on piers, and built ramps and used coffin hoists to winch the house onto the foundation.
Or you can do what I did with my little cabin. I poured 16" x 16" x 6" pads dug just below the organic layer and put blocks on them. This has worked well, though it a very forgiving site--a well drained gravel knoll. All me doors work, no cracks in the sheetrock, and it helps out my property taxes:smile:
