A bathroom is often the only sensible tray site in a flat, and it is also the room where odour has nowhere to hide. Towels, grout and warm damp air all hold smell, so the routine has to remove waste fast and wash the tray itself, not mask what is sitting in it.
The Tray, the Caddy and the Mat
Start with a high-sided or covered tray if your cat accepts one, because high sides keep litter and spray inside a small room where every scatter lands on a floor you walk on barefoot. Some cats refuse a roof, and a cat who will not enter is a worse problem than scattered litter, so treat the cover as negotiable and the high sides as the part to keep. Set the tray on a washable mat that catches the grains carried out on her paws; shake the mat into the tray daily and wash it when it starts to hold smell itself.
Scooping at least twice daily, drop each deposit into a sealed caddy kept beside the tray, and empty that caddy into the outside waste rather than letting it accumulate through the week. The sealed caddy is what makes twice-daily scooping practical in a flat: each deposit leaves the litter immediately but leaves the room's air alone, and nobody has to carry a bag through the flat after every visit. Keep a lidded bin as the backup for full litter changes and for the caddy liner, so there is never an open bag perfuming the bathroom between trips downstairs.
Washing Rhythm and Ventilation
Top-up-only trays eventually smell of the tray itself, because urine soaks the base and old litter coats the plastic. Give the tray regular full tray washes once a week: empty it completely, wash it with hot water and a plain unscented cleaner, dry it properly, and refill with fresh litter. A spare cheap tray makes this easy to keep, since she is never left without a toilet while hers dries. The litter change is also the moment to check the tray base for scratches and crazing, which hold odour no washing reaches; a scarred tray needs replacing, not scrubbing harder.
Use the bathroom ventilation as part of the routine rather than an afterthought. Run the extractor or open the window after a visit for a short spell, especially in a windowless internal bathroom where moist air lingers. Keep cleaning sprays and air fresheners away from the tray area itself, because strong scents at her nose height can push a cat to choose the bath mat instead, and keep the bathroom door arrangement honest: a tray in a room she cannot always reach is a routine that fails at night.
When Smell Survives a Clean Tray
If the routine above is genuinely being kept and smell still builds, look for where waste is escaping the system: litter tracked into a damp corner, a mat washed too rarely, or misses beside the tray from a cat who dislikes the cover. Fix the step that is failing rather than adding fragrance on top of it. A sudden change in the smell of her urine, straining, or visiting the tray far more often than usual is a different matter: mention it to your vet, because urinary problems announce themselves in the litter tray before they announce themselves anywhere else.
Write the routine down for the whole flat, with the twice-daily scooping, the sealed caddy, the once-a-week full wash, the washable mat, the lidded bin and the ventilation as one shared set, so a flatmate covering for you does every step rather than the visible ones. Written once into Family Sharing through KinsLe, the twice-daily rhythm and the weekly wash stay visible to each household member sharing her care.