We have a Booda Dome (looks like this: link) litter box for our cat. We've had it for at least a year and a half with zero issues, however, recently she has started peeing not in the box, but on the stairs of the box. It has been going on for about two weeks, at first I just figured she did a slight miscalculation, but now it has become so frequent I can't be a coincidence. She has even managed to pee through the gaps of the two sections so that the carpet underneath it was soiled.
The litter box hasn't been moved, the litter is the same we've always used, we clean the box regularly and she was happy to use it from the moment we brought it home.

I know things like this are very hard to diagnose on-line, but I'm posting here in the vain hope that somebody has had a similar issue with their cat and has found a solution. I will be making an appointment with the vet tomorrow as well to rule out any health issues.

The cat is a little over two years old and female (sterilised). We moved to the current apartment in January and although we spent some time at my mum's house in the summer, she has never shown any obvious signs of stress when taken to a new place (we have travelled quite a bit with her ever since she was young) - although I do admit, being a first-time cat owner, I might be missing something. The only bigger chance in recent times that I can think of is the fact that both of us went back to school in the beginning of September, which means we spend more time away from home. But she didn't have a problem with it in the previous years she was with us. Or could it be, that as she ages, she gets more sensitive to changes in our routine?

I ordered some cat behaviour books and we brought a Feliway diffuser and I will call the vet tomorrow, but is there anything else we could do that might help?

I know the plastic does deteriorate to some extent with all of the cat wee, so it is it possible it has an odour the cat doesn't like or it could be something that you've used to clean it with, a tray with a roof may not help?

Cat's in enclosed boxes cannot see potential enemies and they don't have a means of escape.

But if she was worried about escaping, why would she pee on her only way out? You should see her face when she's forced to jump over the stream running down the steps of the litter box. Same thing with the smell - if I was a cat, I would prefer to keep my head near the opening.

I've been cleaning the steps with some pet odour remover (this one) - maybe she doesn't like the smell and is trying to cover it up?

When you find the answer, let me know. I have Nine months old siamese brother and sister. They have taken to peeing on beds and cushions.
They have two litter trays, same litter which I clean as soon as they poo. They have a lovely life. But this is getting to me.

Can't you just tap into your search box 'why does my cat pee on the furniture?'

My Cat From Hell, episodes of which can be found on Youtube, may also help. The issue of peeing comes up often.

Not all cats are as meticulous as people think. My wife has had enough of the little b*gg*rs, so I know. Move tray to a new area and put plastic mats underneath.

Actually I have done just that, I put a litter tray upstairs so they have one up and one down. It seems to have solved the problem. They both use the upstairs one more.

Trying to work out why the change works though is a futile exercise. To use a quote (again) from Deanna Troi - "sometimes a cat is just a cat"!

I too take my cat to mum's in the summer for some fresh air and a change of scenery. Recently she has started to urinate in the carrier. I think it's more likely that this is the result of an unpleasant vet visit last May but I wouldn't rule out stress.