Hi there, I'm Dr. Sara. I'm a licensed veterinarian in the US who works exclusively with cats and dogs. I'm sorry to hear that you're having this very frustrating issue with Bella. I've been there - it's not an easy problem to fix. Unfortunately, it's also a very common problem. Because it's so common, though, I have quite a lot of info on the topic :)
Cats are super fastidious, so some are very sensitive to a dirty litter box. Some cats absolutely will not pee or poop in a box that's not immaculate, so watching the boxes for cleanliness is important. Cats also tend not to like hoods on their boxes (especially bigger cats - they get too cramped) so I always recommend removing hoods from boxes. Cats also don't like to eat and potty in the same place, so food and water dishes shouldn't be near the litter box.
The ideal number of cat boxes is the number of cats in the home PLUS ONE. Each box should be in a separate spot - a cat will look at two boxes right next to each other as one box, so spread them out. Make sure there's at least one box on each floor of your home too, in case stairs are an issue, perhaps for an older arthritic cat. Boxes should be far from very noisy or unpredictable areas. A washer or dryer or furnace kicking on at just the wrong time can startle a cat and turn them off from the box entirely.
Confinement to one room or a smaller portion of the house away from the problem areas can also help "retrain" the cat to using the litter box. The fewer options they have, the more likely they are to choose the litter box. The longer the house soiling has been going on, the longer the confinement will need to be to establish that good habit. It's also a good idea to gradually allow access to areas of the home once they're using their box in their confinement rather than just turn them loose. Cats do respond favorably to positive reinforcement such as praise and extra special treats when they use the litter box, so if you find her in the box, patiently wait until she is done and then praise her with a very special treat.
It's also a good idea to try a new type of litter. I usually recommend ADDING a new box rather than changing any of the existing boxes. If you're using clumpable litter, I'd rotate through trying pine litter (Feline Pine), recycled newspaper litter (Yesterday's News), crystal type litters, or corn cob litter. Each type of litter has a different scent and different feel on their feet which some cats may like more or less. Sometimes you'll find that he will like one of these new litters better.
In a case where a cat is eliminating in just one specific place, there are a couple of different ways we can react. One would be to recognize that that the kitty is showing you their preference in location, so give them an acceptable option for pottying in that location by placing a cat litter box there or as close as possible to the offending spot. Obviously this is going to be more reasonable in some places than others in your house. The other way you could handle that would be to do something to make that particular place undesirable to the cat by placing something like packing tape, double sided sticky tape, plastic wrap, tin foil, or strong odored diffusers or sprays in the area. Sometimes this fixes the issue, but other times, unfortunately, this can backfire and cause the cat to just go find a new spot to use.
If these things aren't working and the situation is really desperate, I'll consider allowing them to become an indoor/outdoor cat with time outside. If they want to go outside and like to be out there, it often alleviates or eliminates completely the urinating in the home. Obviously, this approach comes with its own set of risks, but IF the choice is to rehome or euthanize them or to allow them to have some outdoor time, I'd choose the latter. I've actually done this with my own cat and it's solved our issue.
I've had some success with pheromone products like Feliway or Comfort Zone. They are geared more toward behavioral marking but sometimes can have an overall anti-anxiety effect and help with litterbox issues.
Lastly, I would consider a course of anti-anxiety medication like fluoxetine. In many of my patients this alleviates or eliminates the problem. Social stresses in a multicat household or other anxiety disorders in cats will commonly manifest as house soiling.
As I said, I absolutely appreciate how difficult this problem is - I've been there myself and it breaks my heart. I miss having my cat in bed with me every night, but I also cannot have him peeing in my house. My dad is retired but he practiced as a vet for 50+ years. As you can imagine, his view on "house soiling" cats was pretty black and white - he used to say "they live in my house, not the other way around". But times have changed - they are such a part of our family that they are not disposable and we aren't going to just give up on them. I'm sorry that you're having to deal with this. Please let me know if I can elaborate on any of those individual things that I mentioned - as I'd be happy to do so :)
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