[gsdave]

On the slab, I can tell you for sure that the guy quoted you the absolute least expensive, simplest slab possible for a garage. This is almost always going to be less expensive and simpler than building from an engineered drawing. However, it may or may not be something that works for your intended use. You should have given him the drawing when you asked for a price in order to get a good number. But he should have asked as well. From your description of the events, I don't think he was trying to screw you, but I don't know anything about the guy.

However if you don't have the final drawings, you should just wait for them as the soil borings may affect the final slab design and you may be getting another change from the next guy. You cannot expect a contractor to agree to build one thing and then change the specs on him and expect him to make the changes for free. If it were me, I would invite three bidders to look at the site and give them a copy of your drawing and request a bid from each.

FYI, I just paid $9800 (including soil treatment, permits, etc) for a 30x48 slab for a future shop and a 24x30 parking pad off of my garage. I was supposedly 'getting a deal' because they made a mistake during construction on the house.