Cats are creatures of habit, and when those habits begin to include inappropriate elimination, whether urinating or defecating in places other than the litterbox, this can cause great stress and frustration for the feline and his human family.
Typically, when a cat starts eliminating inappropriately, it's either due to an underlying medical condition like a urinary tract infection, struvite crystals (tiny, sand-like particles which develop in urine that is not acidic enough), intestinal distress, or the cause is behavioral.
It's necessary to first rule out the medical reasons for this behavior, so if your cat begins to show symptoms of frequent visits to the litterbox and/or straining to eliminate, but nothing or very little, is produced, your furry friend needs to be evaluated by a vet as soon as possible, in order to get a definite diagnosis and treatment. Antibiotics will be prescribed for a urinary infection and dietary changes and/or nutritional supplements to acidify the urine, will be suggested, if crystals are found. An immediate vet visit is even more important for male cats, because their urethra (the tube leading from the bladder to outside the body) is so narrow, a urethral blockage from struvite crystals can occur very quickly, and then this would become a life-threatening emergency for your kitty.
If a cat experiences pain or discomfort upon urination or defecation, he will come to associate the PLACE of eliminating-the litterbox itself-with that pain, and then choose to eliminate ANYWHERE else. Cats in this situation usually choose 'soft' materials to go on, thinking this will avoid the pain. Carpet, throw rugs, bathmats, clothing, backpacks, sofas and beds are usual, favorite places.
In order to prevent your cat from returning to the places he's already soiled (if he smells it, he'll keep thinking they are appropriate toileting places), you can use an enzymatic cleaner to completely remove all traces of stain and odor. Nature's Miracle, Zero Odor, and the additional products found in pet supply stores, home improvement stores and online, are good for this purpose.
Cats can also sometimes start eliminating inappropriately if there has been any kind of change in their lives or environment, as this causes stress and anxiety. If the reason for eliminating outside the litterbox is NOT found to be medical, after an examination by the veterinarian, it would have to be treated as behavioral.
Cats are creatures of habit and do not take kindly to change. If you have had any new people added to your household, or if a 'usual' person has left, if you got new furniture, carpet, appliances, another pet, etc., or if your usual daily hours at home and/or away from home have changed or you've gone on vacation recently, any of these situations could cause your kitty stress.
Feliway/Comfort Zone for Cats is a cat-appeasing pheromone which can help to relax stressed cats and curb urine marking and/or inappropriate elimination. It comes in both a spray and plug-in diffuser. The diffuser plugs into an electrical outlet like an air freshener, but has no scent to humans. Major pet supply stores carry Feliway, you can also order it online, and some vet offices carry it.
There's another product which may help get your kitty back to using the litterbox, if he has stopped due to a medical or behavioral cause-it's a clumping kitty litter called 'Cat Attract'-it contains herbal attractants which encourage use of the litterbox. It too, may be found in major pet supply stores or ordered online.
It's best to have more than one litterbox, if you don't, already. Some cats prefer to urinate in one box and defecate in another and some cats don't like sharing with their companion cats. The rule of thumb is usually one box per cat, plus one. If you have a multi-story house, a box on each level is also appropriate and usually prevents accidents. Of course, keep all boxes immaculately clean at all times.
Please keep in mind that if your cat starts eliminating ourside of the litterbox, your first step should be to bring him to the vet to determine if the cause is medical or behavioral.