If you only have one bathroom or only have a bathroom downstairs, you may decide that you need to build and extension to allow the room to add another one.
If you want to add a new bathroom downstairs you may consider installing it under the stairs but if this is not possible they you may need to have a single story extension built. When deciding on the details of the extension, you need to think carefully about if at one time you may wish to build on top of it. If you do want to then the foundations of the extension will need to be dug deep enough to allow this. It is very important that you decide this early on as once it has been built, you would need to take it down and start again if they foundations were not built to the correct depth to withstand a second story.
Plumbing is often another complication when it comes to creating an additional bathroom. You will need to find out how and where you can get water from and where the drainage will need to go. The toilet will need to drain in to a soil pipe that connects to the sewers and if this is not on the same side as the extension then you may need to run pipes under the ground.