We serve all these areas too.
Yes, adding a bathroom to an existing floor in a Fort Greene brownstone involves rough-in for supply (relatively straightforward), a drain connection (which requires either connecting to the existing DWV stack or running a new drain to the basement), and a vent. The constraints are the drain routing — gravity-drain rules apply — and whether opening the walls will affect structural lath or original plaster. We've done bathroom additions in Fort Greene buildings and can scope the plumbing rough-in after seeing the space.
When basement access is blocked or unavailable, we work from cleanouts in the stack (if present), through floor cutouts above the horizontal runs, or from the attic on vertical sections. In a Fort Greene building where the stack is buried in a chase wall without accessible cleanouts, we sometimes add a cleanout during the repair to prevent the same problem in the future. It adds time to the job but pays for itself on the next service call.
Call or send a message. We're about 16 min away.