Why does my dog poo in the house when he hasn't before?

The first major cause would be a urinary tract infection - this can be confirmed or ruled out with a simple urinalysis at a veterinary clinic. At the same time, the vet can look for evidence of bladder stones and some types of bladder cancer. If there is no obvious medical reason, I would start asking about recent changes in the household - did a relative move in (such as a new baby or an elderly relative) or move out (such as a child moving to college)? Is there a great deal of stress in the house? Has the dog's routine been disrupted?

Sometimes dogs will urinate inappropriately as a way of acting out or relieving stress, although medical reasons are more likely.