A service dog is defined as any dog that is trained to perform at least one task that mitigates it's owner's disability. If the dog is trained to help himit is a true service dog and legally had to be allowed to accompany him in his apartment even if they had a no pets policy.
I live in a place that has a no pets policy and we have a service dog..and recently obtained two more. (I have three kids with disabilities).
I have a good friend in the UK who is deaf. Her dog is not a registered see/hear dog, he is a normal mutt who lets her know if the phone rings, someone comes to the door or, as has happened, if a fire starts. He was never trained. He is not a pedigree dog but a mutt. But he is adorable and I owe him BIG time for saving her life. He may not be in the same class as some "service" dog, but I regard him as such! : ) He is priceless!