Swapping a litter box seems like it should be trivial. It frequently is not, and the failure mode is expensive: a cat that decides the new arrangement is unacceptable and picks a rug instead.
The principle is simple. A litter box is where a cat is at its most exposed, so it is the place where change is least welcome. Change one thing at a time and give it two weeks.
The one-variable rule
A litter box setup has five variables: the box, the litter, the depth, the location, and whether it is covered. Change exactly one of them at a time.
Most protests come from someone changing three at once, moving to a new covered box with a different litter in a different room, and concluding their cat is being unreasonable.
Adding a new box
The easy case, and the one to aim for.
- Put the new box next to the old one. Both available, both clean.
- Fill the new one with the same litter, to the same depth.
- Add a scoop of used litter from the old box. The familiar smell is the single most effective thing you can do.
- Wait until you see the new one being used, usually a few days.
- Only then remove the old one, if you are replacing rather than adding.
Keeping both is better anyway. The standard guidance is one box per cat plus a spare.
Changing the litter
The change cats object to most, especially between textures such as clay to pellets or crystals.
Mix rather than swap. Start at about a quarter new and three quarters old, and shift the ratio every few days over two weeks. If the box starts getting avoided, go back a step and move more slowly.
Moving the box
Move it a short distance at a time, a few feet every couple of days, rather than relocating it in one go. Slow, but it works.
Where it ends up matters. Cats want somewhere quiet with a clear line of sight and no chance of being cornered. Avoid next to a washing machine or boiler, dead-end cupboards, and anywhere beside the food bowls.
Adding a lid
Some cats like the privacy, others refuse a space they cannot see out of. Put the lid on without the door flap first and see how it goes for a week. If it is accepted, add the flap. If the box gets avoided, take the lid off and accept that your cat has told you the answer.
If it goes wrong
Go back to what worked and try again more slowly. There is no cost to this.
Clean accidents with an enzymatic cleaner, not a general household one. Anything that leaves a trace of scent marks the spot as a toilet, and ammonia-based cleaners actively make it worse.
Do not punish. It teaches a cat to avoid you rather than to use the box, and it adds stress, which is often part of the original problem.
The thing to rule out first
If a cat that has always been reliable suddenly stops using the box, do not assume the setup is the cause, even if you changed something recently. Urinary tract problems, crystals, arthritis making a high-sided box painful to climb into, and other medical issues all present exactly this way.
Straining, crying while urinating, blood in the urine, or repeated trips producing nothing need a vet the same day.
General guidance, not veterinary advice.