According to veterinarian, cats urinate outside of the box for quite a few reasons, including litter box condition, medical conditions, stress, marking, intercat problems.
Let’s begin with litter box condition, How many litter boxes do you have? The rule of thumb is that you need one more litter box than the number of cats in your house. For example, if you have 5 cats , you should have 5+1 litter boxes at your home. These 6 litter boxes should be placed at 6 different location which can be super difficult, especially living in Hong Kong. But this can minimize conflicts between the cats.
Cats prefer clean, unscented litter boxes. Cats also do not like the the noises from self cleaning litter boxes. Moreover, cats like a quiet litter box area, away from washing machines, heaters etc.
Source: the Catington Post