Why has my dog suddenly begun marking in the house when he didn't do it before?

Usually it is because of feelings of insecurity or a perceived threat. This perceived threat, for example, can be an introduction of a new baby, a new pet, a visitor or even a new piece of furniture. The smell of other animals on your footwear or clothing can also trigger a dog to feel the need to mark his territory.

For example, a new baby in the home brings new sounds, smells, and people, as well as changes in routine. Your dog may not be getting as much attention as previously. Changes cause him to feel anxious, which may cause him to mark.

Some dogs feel the need to lift their leg and pee on all new things that enter your house, shopping bags, visitors belongings, new furniture, children's toys etc. Many of these dogs are lacking in confidence and by marking new objects it makes them feel more secure having deposited their own scent on these objects.

Some dogs will never mark in their own house but will embarrass you by marking if you visit a friend or relative's home. Your dog feels less secure there and feels the need to make it more comfortable to him by laying down a few of his own familiar scents.

Even a previously housetrained neutered male dog will urine mark under certain circumstances. This doesn’t mean it will become a regular problem. He may urine mark one or twice in a new home and then never do it again.