Dr. Horwitz, a Veterinarian says:
It is important to distinguish house soiling from urine marking. A dog may eliminate indoors due to separation anxiety or a lack of housetraining or access to the appropriate elimination location so differentiating between house soiling and urine marking is essential. Urine marking is typically small amounts of urine deposited on vertical surfaces. There are many reasons a dog may mark with urine indoors. An intact dog is more likely to mark his territory with urine and neutering decreases urine marking with 80% of dogs showing at least a 50% improvment in the behavior and nearly eliminating the problem in 40% of dogs. Dogs will also mark their territory indoors in novel areas, on new objects and if they are anxious. Changing the environment to prevent access to the areas, making the area aversive and treating underlying anxieties can all help diminish urine marking behaviors.