Considering that fixtures are virtually permanent, and expensive, it’s important to get the layout right. What you are seeking is a room that performs its task, is easy to keep clean, durable and at the same time comfortable and a pleasure to use.
Number of bathrooms
For many self-build projects, a common solution is a main bathroom upstairs and a small bathroom or cloakroom downstairs, with one or more ensuites (i.e. bathrooms that are typically only accessible from a bedroom). Jack and Jill bathrooms have more than one access point, e.g. from two bedrooms.
In the Celtic Tiger days every bedroom got an ensuite but it’s worth considering the cost (capital and environmental) of this and the fact that too many bathrooms will lead to underuse (you will need to run water down the traps to clear stale water smells), and the inevitable need to clean them – dust accumulates no matter what.