But do you have, say, an understairs cupboard area which you could turn into a tiny cloakroom and install a second loo - a macerator one? OK, macerator loos are a bit tricky, but at least if you have two loos then you could knock the bathroom and original loo into one room.
Another vote for keeping them separate and adding a small sink to the toilet. Our second toilet is a converted airing cupboard so probably only about 2' x 3', it has the smallest washbasin I've ever seen, but it does the job.
Could you put in an ensuite - even if just toilet and sink? Then you and dc always have somewhere each to go and visitors always just have to fight over main family bathroom. It is tricky in a flat though if you have limited space.
Thanks for your thoughts. I don’t think another loo is an option, as the rooms aren’t really big enough to accommodate an en suite. Plus ventilation would be an issue. Bathroom isn’t ventilated.
Although there are cupboards elsewhere in the flat they don’t have the ceiling height to accommodate an adult, plus are on the wrong side of the building for pipes. So I think we are stuck with current location.
The bathroom is really grotty at the moment, so I am having difficulties envisaging it looking decent. It is a small awkward space, there is a lot (well a bit) of space round the loo. Adding them together would make a bigger room. But due to constraints with pipes the loo would have to face into a wall where the door used to be.