Can a new bathroom, toilet, sink, and shower be vented through an existing 1 1/2" vent? The bathroom will be in the basement, and the vent is part of kitchen on the first floor. Would a 2 1/2" better, or can I use what I have if I tie into the vent above the kitchen sink ?
Thank you... Rick

Although the inspectors prefer 2"vents the Standard Plumbing Code allows 8 fixture units to be serviced by a 1 1/2" vent. Your bathroom group,(6 fixture units) plus your kitchen sink, (2 fixture units) falls within this limit.
Run the bathroom vent up and revent back into the kitchen vent at least 6 inches above the sinks flood rim. Good luck, Tom