Just put some posts under it and you should be OK, bro.
Seriously, building one on the second floor is not much diff than a first floor, just longer posts. The real diff is the posts will be much longer and you will have to account for horizontal deflection. You overcome deflection by installing intermediate support between posts. Tell your lumber yard sales guy what you plan on doing, and how far off the ground you will be. They should be able to let you know how much post-to-post support you need and how often. It is a very easy/quick equation.
Once you account for the horizontal deflection, other than working that high off the ground, it is pretty much the same as a first floor job.
Also, if you are noticed, the city will require a permit, which will require stamped drawings for permit review. Find a good lumber yard to buy your wood from and they should be able to draw it up for you with calculations and all for a very small amt. Sometimes for nothing, if you are buying enough wood.