At the very least, I would bring in a urine sample to the vet to be tested. There are a number of medical conditions this would rule out.
How many litter boxes do you have and where are they located? Have you tried (temporarily) putting a box near the couch to see what happens?
Has anything changed in your household lately to cause stress (new job hours, someone new or leaving the household)?
Happiness is being kneaded.
Avatar is my childhood kitty, Berries.