However, there are actually two complete units with one shared common wall only.

Should I gut the entire third floor bathroom and see what's behind the walls and completely renovate while at it?

Hello,I'm a real estate agent, my client is selling / renting out an apartment that was occupied by a previous tenant and needs major cleaning, wall painting and fix some lights and a crack on the kitchen backsplash.

They wouldn't have dared color on the walls, flushed Ducky down the hole, or anything else. 

I did pretty much all the work myself and did kitchen cabinets repainting and added finishes, added subway tiles to the front wall and back splash, added laminate through out the house, painted, and fixed up the yard and cleaned the privacy fence and painted the outside trim of the house and so on.

He wants to replace carpet and paint walls.

@Rush Wall Thank you.

Insulate the F out of that one wall for sound issues... 

Thank you for reaching out.Thanks @Moty Wall.

We are contemplating on installing a new electric boxHere is what we are wanting to do.Add a hallway (will require moving a non load bearing wall.)Add a powder room about 3 feet from existing bathroom with medium grade finishingUpdate 2 small bathrooms with medium grade finishingAdd additional toilet in master bath and move plumbing for shower and existing toilet about 3 feet.