Best thing to do, initially, is to contact your local council and ask to speak to the building inspector. In my experience they are very helpful. You don't, at this stage, need drawings or anything, simply explain what you want to do.
As each council organises things differently, I don't know whether they would offer an appointment at the offices, or do a home visit (I have had both). It is fine to do this at the initial stages, and indeed, because they are so experienced, may make helpful suggestions.
Without knowing the general layout of your house, drainage & plumbing systems, it is impossible to say what may, or may not, be allowed.
However, I would add that it may be worth your while thinking even more broadly about making your home fit to live in as you get less able. That is when talking to a local builder may be helpful.