If you have an upstairs bathroom and would like to have a downstairs one too you need to think about where you are going to build it. Some people decide to have an extension built on to their house or convert their garage but this can cost a lot of money will require planning permission.
If you are looking for a cheaper and easier alternative then you may be able to install one under the stairs. Many staircases in homes create a lot of wasted space. With not a lot of work you can often covert these small spaces into a downstairs toilet.
You need to think carefully about where the waste will go and how you will get water to the new bathroom. If you cannot get hot water then you may decide to install an emersion heater instead.
There used to be a rule that said you must have at least two doors between a kitchen and a toilet but this has now been relaxed as long as there is a sink in the toilet to allow for hand washing.
You may be able to do much of the building work yourself if you are quite good at DIY but if not it should cost too much to get the professionals in to build it for you.