When there is a need of another toilet and no room, you may consider about making one under the stairs. You’ll be surprised how functional it will be for you, especially if the main toilet was upstairs. Climbing up can be difficult especially for older persons and the tiny dimensions of the under stairs toilet will be excellent replacement. A project showed in the photos bellow is actual toilet problem solved in a very smart way.
The drawings and dimensions will show you the little toilet area and the images will guide you trough the process of the construction.