Francisco, the condo association decides whether you have roof rights or not. Normally, they run with the unit when its sold and are recorded in the condo declarations somewhere. If the unit does not have roof rights, you are subject to the whim of the condo association, which would have to agree to give you those rights and file some legal documents recording those rights. One reason roof rights may not currently exist is a rooftop deck is not up to code. So normally the condo board will not want to pay to make those changes, since you need permits, etc. I am sure something creative could be done where you pay all expenses, but as a buyer I would want a sense of whether the board has considered granting roof rights in the past and how they view it now. Otherwise, they could just say no and that is that. Try calling the Board president and see if they are willing to discuss it. Your realtor should be guiding you as well.