A bath or shower solves several problems at once from a cat's point of view. The surface stays cool under her paws even in a warm house, it is smooth and easy to turn on, and it is rinsed clean so often that it carries almost none of the lingering smell a used tray holds. The high sides give her privacy without a hood trapping odours around her head, and a walk-in shower tray in particular has the size, the low entry and the clean corners of an ideal toileting spot. She is not being defiant or dirty. She has compared the options on offer and chosen the one that feels cleanest, so the fix is to make a proper tray beat the bath on those same terms.
Read the Bathroom Like She Does
Start by auditing the tray she is rejecting, because its faults are usually specific. A covered tray traps smell inside the very space she must enter. Scented litter that smells pleasant to you can be intense at cat nose height, and a gritty or dusty texture can put her off digging. A tray placed beside a humming washing machine, in a busy hallway or where a dog can ambush her mid-visit asks her to toilet somewhere she cannot relax. Size matters too: many standard trays are too short for an adult cat to turn, dig and cover comfortably, and a high rim can be a real obstacle for an older cat with stiff joints. Note which of these apply before you change anything, because guessing wrong just gives her another tray to reject.
Set Up the Easier Option
Place one large, open, low-sided tray as close to her chosen spot as you can manage. Beside the bath works well, and inside the shower tray itself works even better if that cubicle can be spared, because you are no longer asking her to change location, only surface. Fill it with a good depth of the unscented litter she has disliked least in the past, and scoop it after every use at first, since cleanliness is the whole reason she picked porcelain. Keep the bathroom door closed whenever nobody is in there to supervise, so the bare bath stops being available while the tray habit forms. Praise her calmly when you catch her using the tray, then leave her in peace. Once she is reliable, you can inch the tray a little further from the shower each week if you want it elsewhere, but many households simply accept a well-kept tray in the bathroom as a permanent, workable answer.
When It Is Not a Preference at All
A long-standing habit in a cat who has always disliked trays is one thing; a sudden switch in a cat who used her tray happily for years is another, and that change deserves a vet check before any retraining. Pain when crouching, discomfort passing urine, or early joint stiffness can all make a smooth, step-in shower feel safer than climbing into a tray, and no rearrangement of litter will fix a medical cause. Book the vet promptly if the change comes with straining, crying, small frequent wees, blood in the urine, licking underneath, or trips that produce nothing. In a multi-cat home, also consider whether another cat has started guarding the tray's location, which can quietly push a peaceable cat toward the bathroom instead. A shared rota inside KinsLe keeps the bathroom routine honest, because Care Teams records who cleaned which tray and when, so the fresh option she prefers never quietly lapses.