I am wanting to add a 400 square foot addition above my garage. I did some research about the size of the furnace and the existing one will handle the new rooms. I am having a problem with figuring out how to run the duct work. The furnace is in the basement. I am adding 2 bedrooms and a bath. I spoke with a hvac worker who had told me that I do not need a vent in the bathroom, as long as there is no window and that I would have to run two 6" pipes one for each room. Do I need a return air vent as well? Is this correct and code compliant?

Put the 6 or 8 inch ducts to the out side of the rooms... if you can run equel or a little less returns to the inside of the rooms