Not all homes are built the same, so some houses will have the main bathroom upstairs whilst others may have it downstairs. Overtime many houses have been adapted to allow for two or more bathrooms but homes with only one bathroom still remain.
When it comes to buying or selling a home, bathrooms are taken in to account. Some people do not like the idea of having their only bathroom downstairs and others may prefer to have two. You need to think about the layout of the house to see if there is the possibility of adding in another bathroom if you wish to do so. It may be that you have a bathroom upstairs but have room under the stairs to create a small toilet or perhaps you are considering have an extension and can build a bathroom downstairs along with an en-suite upstairs. Some people turn small bedrooms in to bathrooms or en-suits but this may end up knocking value off your home, so if you are thinking of selling at some point, you need to take this in to consideration as you may decide to leave things as they are in order to get more money for your home.