All dog owners expect a new puppy and even a new adult dog to mess inside the house initially, but it can become worrying when a dog who was previously clean starts messing in the house again. This can be due to a variety of reasons.
It is normal for a puppy to mess in the house in the first few weeks and months of their life, as they have small bladders and are not yet able to hold themselves for very long, and don't originally know that it's wrong to do so.
It takes a few weeks to train them to go outside and to ask to go outside. If the puppy follow his mother outdoors, he will see her do her business outdoors and is more likely to copy.
Always be patient and praise your puppy and reward with a treat when he does something outdoors and as soon as he does it, so that he understands what you are praising and rewarding him for.
If he does an accident indoors, it is believed that one should avoid telling him off, and you should never clean the mess in front of him.
Adult dog in a new home
If you have adopted a new adult dog, even if he was clean before, he may mess in the house due to stress. This may be due to the new environment, other pets or children in the house, or simply because he is easily stressed by new surroundings. A dog needs to feel secure, and the smell of his urine helps reassure him. Dogs can also mess indoors to seek attention from their owner.
Spend a lot of time with your new companion to help him feel secure and relaxed in your home. Take him our regularly and praise him. If he was clean before coming to your home, this habit should come back fairly quickly once he feels settled.
Adult female dog in heat
Female dogs go in heat two to three times a year, and often urinate a lot more frequently during that time due to the changes in hormones, which can lead to uncharacteristically urinating in the house during the night or if left indoors too long during the day, and even pooping indoors, particularly if your female dog's stools tend to be softer when she is in heat. Sterilised female dogs no longer go in heat but can still have hormonal variations at the times they would have gone in heat, and can also mess indoors during those times.
Adult male dogs
Owners of male dogs will be used to their dog urinating on virtually anything outdoors to mark their territory. This becomes problematic when he does the same indoors, as it can ruin furniture, carpets etc. You will need to train your male puppy well from a young age, maybe even restricting the access to some parts of the house until he is trained not to mark inside. If the problem can't be easily resolved, castration may be an option, if you do not plan for your dog to father puppies.
With age, dogs sometimes tend to relieve themselves indoors, when they did not do so until now. This can be a sign of old age: your dog no longer has the same vigour as before and walking to the garden represents too great an effort.
Older dogs also often suffer from loss of bladder and bowel control. They therefore become incontinent and cannot help but urinate or poop in the wrong place. There are nappies for dogs to alleviate this problem.
Stress, Fear & Separation anxiety
Fear and stress can be a cause of your dog messing inside, for example when there is a thunderstorm or fireworks outdoors, the noise itself causing stress and fear, and the refusal to go outdoors to do his business as it would bring him closer to the source of fear and stress. Separation anxiety and changes such as moving house, the owner starting to be away for work, etc can also cause a source of stress that affects the dog's digestive system and causes him to start messing indoors. A dog who misses the company of his owner or who wants more attention can start messing in the house to get that attention, even negative attention such as being told off is better to a dog than no attention at all.
There are other reasons that could make your dog start messing indoors rather than outdoors.
Walk was too exciting
If he comes indoors after an exciting walk where he picked up the scent of a rabbit or another animal, he may have been too busy sniffing to think about doing his business, so it is good, if you can, to leave your dog out in the garden after a walk to give him time to do his business before coming in.
The weather can also be a reason. Some dogs dislike going outdoors in the rain or in the snow, and will favour doing their business indoors rather than going outside. It is good to train your dog to like water/rain and to go out in all weathers.
Smell of urine/poo indoors
Dogs are used to doing their business in spots they have used before. Sometimes when cleaning where they have urinated or pooped in the house, some of the smell remains, and will incite the dog to go in the same spot in the house again. Definitely don't use bleach to clean, as despite the strong smell, it does leave some of the scent of the dog's mess behind. It is best to use a 50/50 mix of white vinegar and water.
If none of the above seem to be the reason for your dog's change of behaviour, it is advisable to take him to the vet to investigate possible medical reasons. It could be due to something easily treatable such as intestinal parasites, bacterial infections, or much more serious illnesses such as diabetes, or even some types of cancer. It is better to be safe than sorry.