What is the cheapest way to build a one car garage?

Check the larger home center for those open, metal frame covers, usually under $1000.

Then there is the pole shed, with a gravel floor and poles set without cement, say 10' walls, you should be able to get a good shed for under $2000 if you can build your own trusses. Look for kits too.

What do you know about the footings or foundation poured around the perimeter of the pad? You want to build walls strong enough to support a two-storey building. Is there a foundation and how strong is it? And any posts you install will have to sit on footings also, not just a 4" pad. Your municipality has building codes to enforce. A good place to start this project is with their office. They can give you good advice and the rules.