Siamese cat eliminating outside litter box fix

So your vet's flagged the litter box size as likely the culprit — that's a really common and very fixable issue with Siamese. They're lean but surprisingly long-bodied, and most standard trays just don't give them enough room to turn around and position properly, so they end up hanging over the edge or avoiding the box entirely. Before I point you to a specific tray, a couple of quick questions: is your Siamese male or female, and roughly how long is their body from nose to tail base? Also, is the current box covered or open?

Right, that covered box with a flap is almost certainly a big part of the problem — possibly the whole problem. For a male Siamese at 45cm body length, you'd want a tray at least 67–68cm long ideally, and the flap door forces him to push through it every time, which many cats find stressful enough to avoid altogether. The combination of too small and too enclosed is a classic trigger for exactly what you're describing. Has he always used this box, or did the avoidance start around a particular event?

That timeline makes a lot of sense. Moving is a significant stress event for Siamese especially — they're territorial and routine-dependent. The covered box probably felt more claustrophobic once he was already anxious about the new environment, and once a cat starts eliminating elsewhere, it can become a habit even after the stress fades. The fix here is straightforward: open tray, much bigger. I'd go with the Savic Senator Open Cat Litter Tray in the large size — it's 56cm long, which is a meaningful step up from standard trays, and the high sides at the back and sides contain scatter without trapping him. It's widely available on Amazon UK, usually under £20, and it's one of the most consistently recommended trays by vets for cats who've started avoiding.

Fair challenge — in an ideal world, longer is better, but the Savic Senator at 56cm is the largest practical open tray that's widely stocked in the UK without going into specialist or custom territory. In reality, the shift from a cramped covered box to an open 56cm tray is significant enough that the vast majority of cats resolve the issue. The removed flap door matters as much as the extra length here. If after two weeks he's still not fully using it, the next step would be a storage box hack — a large plastic storage container from somewhere like Wilko or Amazon, around 70x40cm, with one side cut down as an entrance. But start with the Senator, and I'd be surprised if it doesn't sort it.

Put one where the old box was, and a second one — even a temporary cheap tray — near where he's been going behind the sofa. Cats often choose a spot because it feels safe to them, so placing a tray there temporarily redirects the habit rather than fighting it. Once he's reliably using the Senator for a few weeks, you can gradually shift the second tray a foot or two at a time toward a more convenient location and eventually remove it. Also keep the new litter box in a low-traffic spot where he won't feel watched or interrupted — Siamese are more sensitive to that than most breeds.