How can I stop a cocker spaniel from peeing in the house?

If he's still really young he just might need more time. It took my lab 7 months before she stopped having accidents inside. If hes older it's likely a territory/dominance issue. I'd consult a dog trainer. In the meantime you can try this to help with the smell: https://www.craftylittlegnome.com/whole-house-smells-like-dog-urine/

If he was house trained and started peeing inside you might need to take the dog into see the vet. Cocker Spaniels pee when excited and just tend to take longer to potty train then most breeds.

Also, if a dog has heart failure, kidney failure or diabetes, they often will develop bathroom issues at the same time

If there are no health issues causing this, then it's up to you to make sure he is on a schedule and you venture out to the potty area with him. When he goes, act like you won the lottery. Do not allow him loose in the house unless he has urinated. As for the areas that he has used indoors, get a good enzymatic cleaner for those. If it is just one or two spots in your home, you could place his food and water bowls there, they won't mess where they eat. It's only temporary, once he knows that outside is the only acceptable place to pee, you can go back to your old routine.

How old is the dog? Has he or she been properly house trained? Any health issues? How long have you had the dog? New home? Did you move? Letting them out without proper training, the dog then doesn't automatically know he or she is supposed to potty in the yard. Cockers are stubborn, and sometimes take a longer time to train. I know, I had one. Sweetest dog on earth, but mine was dumb as a rock from poor and improper breeding practices. Some couple thought they could make fast cash by breeding them with no clue as to what they were doing and never took any training and lied about certification and got into a heap of trouble. Need I say more? Problem was that now there were many improperly bred animals. Here are some sites that are Cocker Spaniel specific.

https://m.wikihow.com/Train-a-Cocker-Spaniel

https://zimfamilycockers.com/potty.html

https://www.petcarerx.com/article/how-to-potty-train-a-cocker-spaniel/531

https://www.about-cocker-spaniels.com/potty-training-your-puppy.html

Be careful with crate training. If not done gently, it can traumatize them and then it backfires. Some have a tendency to be anxious or nervous. They feed off your energy. If you are anxious and nervous, it transfers to your dog. Remember, you are the Alpha dog. And everyone in the household must be on the same page or all you will be doing is confusing the poor baby and accomplishes nothing. And takes forever and a day to get trained.