This is the same with condos. The coop or condo board want to make sure that the structure of the building itself can withstand the alterations that you might want to make. Buildings are concerned about load-bearing walls, extensive plumbing needs like washer/dryers, "wet over dry" or a bathroom over a bedroom for example. You can ask the agent who represents the seller or the building super what might be allowed or has been allowed in the past, although that is never a promise of what will be permitted in the future.