Revit considers them seperate because they've been modelled seperate, remove the room seperation lines, or wall-bounding of the wall instances deviding them and use one room?

Or is that you do want the washbasin and toilet cubicles to be considered as rooms for another purpose?

You'd do well to consider the implication of both - and consider using area-plans for sub-division of "rooms" if it's an area-compliance/study you're doing.

PS : There is always giving the rooms an additional parameter (say "room-group-name") like many do with apartments (for apartment plans using rooms for the color scheme to avoid re-drafted, orphaned, area plan linework) - but that gets fugly when you've other rooms that don't need the differentiation/information.

Snowyweston you biscuit!!!
I removed the wall-bounding and it does exactly as I require! I'm more interested in the scheduling of mechanical equipment (diffusers and such) in rooms and the client has this area (the three rooms together) as one location.
To better communicate needs and requirements in these rooms it's of course easier if I can draw schedules etc with the clients description and list of rooms.

Thank you for the help!