You will probably need at least 36 to 40 square feet if you want an Adjoining bathroom floor plan. This Adjoining bathroom floor plan typically translates to a 5' x 8' foot footprint, typical of a small bathroom in older homes. To get an idea of your dimensions, look at the size of your small bathroom. You have two standard options for this size. You can place the door directly across from the sink if it opens on the 8' side. The toilet should be near the sink, and the shower or tub should be against the 5' wall. A towel rack or shelf can be placed directly across from the toilet. This design provides both space and functionality.