Roof Rights?

Make your contract contingent on being able to do what you want. If the seller won't agree to such contingency (provided you make a good faith effort to get permits etc.) then walk away.

I agree with zizizi. Also, think very carefully of what you want to build up there and what it entails. We looked at a similar situation a few months back and realised we'd need to rent a crane to attach to the side of the building (we wanted to build a floor on the roof with a deck on top of that.)

One other thing, make sure that there are no violations currently on the building because until those are resolved, you will not be able to get a permit. That was the other thing we found with the place we were looking at so we decided to pass.