Contact 3 LICENSED plumbers in your area for written estimates. Have an idea of what type fixtures you want to use, and what grade. Do you want to match existing fixtures in the house, or upgrade the new ones. Do you want to use the existing flooring, or remove and replace? Are you considering a change in the wallcovering and type? Is there any ground fault interrupter circuits for the new electrical outlets? Is there an available exhaust fan duct? Is the lighting adequate? Is the existing building code allowing you to add these fixture units to the existing main line size? A ball park figure without this information can run from 33 dollars per square foot to 98 dollars and won't serve your purpose. It would be a disservice to you to make a guesstimate.
: I am looking for a ball park figure of what adding a half bath will cost. It will back up to the existing bath. I live in the northeast.