Our 8 year old cat has taken to pooing and peeing on the hall carpet. She has FIV so is kept inside and has a litter tray in the bathroom that she has always happily used. We have sprayed the carpet, sprayed feliway, and yesterday caved and put another litter box there. Which she is now pooing and peeing BESIDE. Our house is starting to stink! She has no other health issues and no changes in her life that may have caused stress.....Getting desperate!
Try repositioning the tray in the hall they don't like a tray to be by a doorway. It may not need to be exactly where she is peeing. I was amazed when I moved my litter tray 2 feet and it cured the problem
Get her another tray. Probably in the hall. I imagine that the bathroom is sometimes inaccessible for a start so it's never a good idea to make that the only spot for a tray. She might have been unable to get to it when she needed to and so had to go in the hall, and now has got used to the idea.