Space

The first, and I daresay the most important, rule of creating a happy multi cat household is personal space. Even though they're domesticated, house cats tend to claim their own territories, only sharing that spot under special circumstances. For instance, female cats in the same family tend to have an easier time sharing.

Now, just because your cats need plenty of room doesn't mean that your home has to be palatial. Even apartment dwellers can create a multi cat-friendly space by strategically placing tall, slender cat trees throughout the room. If that is too much, there are also plenty of wall-mountable perches and window boxes on the market. In many cases, though, your cats will really only need a few cushiony chairs, a simple hideaway post, or a prime spot on your bed, shelves, or bookcases.

Separation

In line with the theme of personal space, maintaining a peaceful household also means ensuring that certain things are kept separate. Very territorial or stressed out cats are less inclined to share litter boxes, no matter how clean you keep them. This can lead to them urinating outside of the box. Often, this is a marking instinct. At other times, more aggressive cats will block the entrance. Some kitties, though, are just more particular, and like people, prefer not to use a dirty bathroom. As a rule of thumb, it is a good idea to have at least one litter box per cat. If you're dealing with a bully, you may want to nix the litter box cover, as more submissive cats may feel insecure or cornered if they can't see the entire room.

Playtime

Aside from sleeping and eating, there are few things more important to a cat than playtime. Cats are solitary hunters. Domesticated cats, however, are surprisingly adaptable. Assuming all of their other needs are met, many cats will become more social, and even "hunt" with their fellow house mates.

Toys are a great way to encourage mutual play. In many instances, this type of activity can become a bonding experience, encouraging your cats to interact as a sort of team. It is important to remember that play fighting, while often harmless, can escalate very quickly. So in order to maintain a harmonious home, it is a good idea to provide plenty of escape routes.

One-On-One Time

Believe it or not, cats can feel jealousy. Despite what some people may think, many cats actually love their people. In order to have a happy multiple feline home, try to carve out a little bit of time for each cat. This may mean a bit of extra playtime, or simply a calm moment with your buddy in your lap - it all depends on what that particular cat enjoys the most. That being said, it is important to allow your cats to dictate the time and amount of interaction they receive, as "sharing" affection can sometimes lead to more of that undesirable jealousy.

Conclusion

Now that you know a little bit more about how to have a happy multi cat household, go ahead and fill out your feline family. Just don't get crazy, now!