How do you manage a multi-cat household?
By understanding that cats are facultatively social — able to live in groups, and not obliged to — and that almost every multi-cat problem is competition for resources rather than a personality clash.
The principle that resolves most of it. Cats in the wild share space with related individuals and avoid conflict by time-sharing and distance, not by confrontation. In a house they cannot leave, so tension is expressed indirectly — through blocking, staring, ambushing at doorways, and through symptoms rather than fighting.
The resource rule. Provide one of each resource per cat, plus one, distributed around the home rather than grouped together. Grouping them creates a single contested area that one cat can control.
Litter trays, which is where the rule matters most. Three cats need four trays, in separate locations, and a covered tray with one entrance is a trap a dominant cat can guard.
Food and water bowls, separated — and water away from food, which cats prefer.
Beds and high resting places. Vertical space is the most under-used solution, because height allows cats to share a room without sharing a level.
Scratching posts.
Entry and exit points, since a single cat flap is a chokepoint.
The signs of tension people miss, because they are not fighting:
One cat consistently not present in shared rooms.
Eating quickly, or waiting.
Over-grooming, which is a stress behaviour.
Urine spraying or toileting outside the tray, which is very frequently a social stress signal rather than a training failure — and is the most common reason cats are relinquished.
Cystitis and recurrent bladder problems, which are strongly associated with stress in cats and are a genuine medical consequence of social tension.
Blocking doorways and staring, which is aggression that leaves no marks.
What helps: more resources, more space, vertical routes, separate feeding, gradual reintroduction where relationships have broken down, and not assuming cats will "sort it out" — they frequently do not, and the cost falls on the least confident cat.
Related cats introduced young cohabit best.
General information, not veterinary advice.