There are two main reasons that cats go outside the litter box. The first is stress, which often causes them to mark areas around the house out of a feeling of insecurity. The second is the box itself, which they either have an aversion to or prefer to replace with a more convenient or luxurious location. To address the issue, make the litter box desirable while making out-of-bounds areas inconvenient.
First, clean up accidents with an enzymatic cleaner (such as Anti-Icky-Poo) and make the taboo locations off-limits.
Second, after your veterinarian has examined your kitty to make sure she doesn’t have any illness that’s causing stress, painful urination or difficulty getting into the box, work on making the litter box desirable.
The number of litter boxes in your household should equal the number of cats plus one, and the boxes should be 1 1/2 times the length of the cat. They should be in a convenient location, at least 4 inches deep, and cleaned every time the cat is ready to enter. That means scooping each box twice daily.
Next, do a choice test to determine your cat’s preferences. To do this, you need four boxes lined up side by side in the kitty’s room. The boxes should be nearly identical, but with one thing different about each. For instance, one may have a scoopable litter, the second the same litter but scented, and the third and fourth boxes may mirror the same litters as boxes one and two, but be significantly larger.