As long as you have little space available, access to sufficient electrical outlets, ventilation and adequate sound insulation, you can install a new bathroom on the sly almost anywhere in your home. It might seem like you don't have any free space, but it's very likely that you don't know where to look. The short answer is yes, you can install a bathroom almost anywhere you can afford it. However, this will largely depend on your electrical and plumbing installation and the style of bathroom you want, which is another common question.
As long as there are enough outlets, enough ventilation, and some insulation, you can find a way to add a bathroom. Think about it and maybe you can turn even an underused closet into an additional bathroom. If property taxes are already a pain in the ass, increasing the value of your home could end up causing your annual tax bill to rise. A popular option would be to expand an existing bathroom instead of installing a new one somewhere in the house.
Because the space was so small, they hung a wall-mounted toilet and sink, which they rescued from the basement bathroom, and installed a sliding door to maximize space. Ideally, the site of your new bathroom should have an outer wall where you can place a window or access to the ceiling for a skylight. Like a bathroom with a closet, another option for placing the bathroom would be under or above a staircase, since many of these areas have shoe cabinets or pantries. The bathroom is an essential aspect of any home, whether it's new or remodeling an old house, and subsequently adding one can do wonders to spruce up your home.
When designers were restoring a 1920s Craftsman in Seattle, they added a vanity on the main floor to increase accessibility for guests who may not be able to use the stairs to reach the master bathroom. You can also look for an attached garage that you can crash into from a shared wall, an attic to which an attic can be added, or other rooms in your house where you can handle losing a few square feet without causing space problems. If your new bathroom is going to have a bath or shower, it's important (and, in most cases, mandatory) that it's well ventilated. In the past, many homeowners were frustrated with the idea of their dream bathroom because the place was too far from the main drain.
If you include a shower in the basement bathroom, make sure it's well ventilated and that the walls have a protective layer, otherwise you'll rot. The frustration of waiting to shower or not wanting to go up or down the stairs every time you need to go to the bathroom is a reasonable concern. Seriously, it's absolutely possible to install a toilet, even a bathroom, anywhere in the house. According to Lindberg and Cederlind, finding a good location for your additional bathroom is half the battle and depends entirely on the layout of your home.