If you don't know exactly what the slab is, then you'll be making a big gamble building on it, IF your local jurisdiction/HOA will even allow for it.
If the slab is in exceptionally good condition, flat, level, etc then you could excavate around the perimeter and cast a new footing to support the building.
If you are careful with the excavation then you could use the slab as the inside form for the stemwall.
This is very doable, assuming the slab is level and in good shape.
I did something exactly like this a few years ago. We had an existing slab off the back of the house. It was supposed to be a patio, but was not sloped away from the house. Turned out to be a good thing. I excavated right around it, over did the footings and the rebar, tied into the existing footings (which the op won't need to do) and we extended the tiny back atrium type room to make a game room. Came out great.
The virtual tour is still up even though we sold it 2 + years ago.
http://tours.tourfactory.com/tours/tour.asp?t=1216335
Here is a pic of the footing in progress. The slab was a little thin on the corners, but had never cracked. I just pulled some of the fill out from under and pushed concrete up in there when we did the stem wall for extra strength.
The room addition is the back room by the pool with a hip roof. From inside, it is the one with the pool table.
We were in an HOA and a municipality with very strict standards. We had no issues with either.
