There are a number of variables involved in adding an additional floor/new deck to an existing 2-family home (zoning approvals, structural implications, budget, timing, etc.). However, assuming your home isn’t located in a landmarked district, the process shouldn’t be as overwhelming as you might anticipate. Once designed and permitted, the construction would entail framing out and enclosing the new addition with the demolition of the existing roof and interior fit-out to follow.