Dr. Horwitz, a Veterinarian says:
A dog might urinate indoors for several reasons. If the dog was house trained and suddenly has begun to eliminate indoors, a medical problem could be the reason for the behavioral change. A veterinary visit to check a urine sample would be a first step. If the dog is lifting their leg and urinating, they may be marking and neutering the dog may help. If the dog has always urinated in the home, then perhaps they were not house trained in the first place. Close supervision both indoors and outside can help retrain the dog where they should eliminate. If the dog eliminates only when left alone they may have separation anxiety. A visit to your veterinarian can help sort through the possibilities and then help design a treatment plan.