You should be fine where your at, as far as not needing a full foundation. Worst case is if they (the building dept) requires a foundation you should be able to build a pole barn style building over the existinget slab. I wouldn't recommend it, the slab will shift with the frost and the building wont -it'll play hell on the doors. Your better off placing the building on the slab and letting it float together.
Check with the local building inspector, let him know what your looking at. After you get his input take the dimensions of the slab (make sure it's square) to someplace like Menard or Home Depot, they'll do the design for you foooooooor free