Nice project! Over / under storage is definitely the most efficient method.
This is probably overkill for most but if you have some $$ to throw at the problem, you could go the route I took. A couple of years back, I was faced with the necessity of putting up another pole barn for implement storage. I got some building prices and found I could buy one of these hydraulic service lifts for less than half the money. It made sense to take advantage of all the empty space from 5' to 12' off the floor.
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I chose a 14,000# 14' X 10' model and installed a deck on top. I now store all my seasonal implements on it which freed up 40% more floor space in the existing building. It doubles as a grease rack to work on tractors.
I use the same principle in my garage for other types of storage:
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