Sell the house for a profit and by another house with property big enough to put up a 5000sf shop with 4" slab with rebar and build it. stratospan has some pretty good deals on steel buildings, my shop is stick built with 4" slab and rebar, its 3500 psi concrete, works great for me, i did however figure out where i was installing my lift and dug some footings in that area about 2'x4'x18" deep, kinda overkill, but i figured what the hell whats a little bit more mud in the grand scheme of things. build it bigger than you need because some day you will want it bigger.