Adding a bedroom

you'll get all kinds of answers for this from very knowledgeable people, but this is first issue you you can resolve right away through your broker before you even settle on a building. i believe co ops, though maybe condos as well, have a set number of legal rooms allowed for the entire building. your broker should know this information and be able to answer this question for you after a fairly short amount of legwork. if they can't answer this question for you, you are working with the wrong broker. another way to possibly ease your approval process with a co op, besides buying a smaller place, is finding the apartment yourself and hiring the broker who specializes in that building, maybe even lives there or in the neighborhood.

The fire code mainly determines if and how you can add an extra bedroom more than the building's board. There has to be egress in the bedroom so that one can escape in the event of a fire.

It all depends on the zoning, as Lucille says. Condo vs co-op is not the issue here. You will need to have any such plan approved by the DOB--you may find it won't approve a second bedroom because of zoning regs.

It has to do with the size and layout of the apartment. You will need to meet the minimum dimension of a bedroom based on building code. You will also need to have a window to provide natural light and ventilation. If the existing condition allows, this is nothing difficult.