RW, there are almost more things to be aware of than I can list. You need a lot better foundation for that big a structure. You probably should use 6x6's or bigger for the base. Rough cut lumber may not meet your code since it is not grade stamped. You want 60 percent retension pressure treated for ground contact. 40 percent or treated to rejection is not sufficient. For a building that big, you may have to get a building permit. If so, you will have to draw up plans. If you have a tornado, hurricane or other high wind and it slams your garage into your house, insurance may not cover the damage unless the garage is built and ancohored correctly. If it blows away and hits your neighbors house, you could be sued. Is that enough for starters? Try searching the web for garage plans. That could give you a good start.