Tried to estimate our sq footage, trilevel house with attached 2 car garage, I think I was pretty accurate but wanted to ask if this ballpark figure seems right, 1813 sq feet? Also an contractor said he tells people to plan on $10 sq foot, this seem reasonable?

New workshop to be built, 12 x 28. Concrete piers w/ three beams: 2 outside beams are 3 - PT 2x10's, center beam is 2 - PT 2x10's. Floor joists are untreated 2 x 8's. Floor is 3/4" plywood topped with 3/4" T&G. The shop will sit on a slight slope - 19 inch drop over 28 ft - w/ five 16" piers supporting each beam. I am confused about how much ground clearance for the beams. Is 6" on the low side enough? Should i put down plastic sheeting? Drainage gravel? Due to critters, I will install 1/4" galvanized steel mesh fabric from floor joists into ground. (Outside beams are set in about 12" to allow for cantilever on floor joists). Lowest pier @ 6", with highest pier @ 19" for air circulation. Is that enough? One last question: If 6" is not sufficient, should I excavate to level? If so, what is best way to stop water from running into the excavated area? (Have a small tractor with FEL and boxscraper). Any advice would be greatly appreciated. Thanks!!