Cats are normally fastidious creatures. So people are invariably surprised when kitty decides that the litter box is no longer the place to relieve herself. Unfortunately, as any veterinarian will tell you "inappropriate elimination" is not an uncommon problem. In fact, it’s one of the most frequent feline behavior problems and often a reason cats are taken to shelters.
Any time your cat exhibits a new behavior problem, you should rule out medical reasons first. A trip to your veterinarian is in order. When I looked up "increased urination" in a veterinary database, I found 92 possible diagnoses. Urinary tract infections are relatively common, especially in male cats, but the problems could be related to anything from kidney problems to thyroid diseases.
After your vet has ruled out any medical cause for the house soiling, it’s time to look at behavioral reasons. The first is sort of obvious: the cat hates the litter box for some reason. The reason may be that the litter box is gross because you never clean it. Cats have limits, so try cleaning the box more often. Also if you have other pets, sometimes they will terrorize the cat, so kitty doesn’t want to visit the box. In that case, you need to get another box and make sure kitty can use it in peace. Otherwise, the cat may find that another place that she likes better than where you have put the litter box. Once a cat has gone in a place, she’ll do so again unless you clean the area really thoroughly with an enzymatic cleaner. A good rule of thumb is to have as many litter boxes as you do cats, plus one (i.e. if you have two cats, have three litter boxes). Not everyone needs to do this, but if you’re having problems, it’s an inexpensive experiment.
If the cat is urinating on vertical surfaces, he may be spraying, not just relieving himself. When a cat sprays, he doesn’t squat; he lifts his tail and it quivers. If you see this behavior, you have a different problem. Although spraying is most prevalent with unneutered male cats, neutered males and even female cats will spray. Because spraying is a marking behavior, cats are usually doing it when they think there is a threat to their territory. Perceived threats could be a new cat or other pet in the house or other change. Some cats will spray inside, even if the threatening cat is outside.
Cats don’t understand the concept of revenge, so they aren’t spraying to "get back at you" for something. Sometimes they do spray out of frustration though. Think about recent life changes. Has anything happened recently that has changed your cat’s world? Sometimes when the human takes a new job, for example, the cat becomes frustrated because of the lack of contact and play time.
If the problem is spraying, get the cat fixed, if you haven’t already. Although it may not stop the problem, it removes any hormonal element that could be causing the problem. See if you can figure out when and where the cat is spraying and thoroughly clean the areas with an enzymatic odor neutralizer. Also look into products you can spray on surfaces that deter spraying. Cats live in a scent-based world. Just because you can’t smell it anymore, doesn’t mean the cat can’t. Otherwise, your house can end up being one giant kitty toilet.
As with most behavior problems, look at it from your cat’s point of view. If you can understand what is causing your cat to do what she’s doing, you are that much closer to dealing with it.