Hang on a minute. The dog has not been abandoned as someone is coming to walk it. You say it's "the owner" ... is it or are you just guessing?
I'm speaking as someone who has had dogs all their life and one of mine is sitting here beneath the radiator in a sheepskin lined basket grinning at me!

Now, I wouldn't keep my dog outside but I know plenty of people who do and I know several dogs who actually prefer it! A friend of mine who lives on a farm has a young dog and it will only settle down lying outside the back door whatever the weather.
It honestly doesn't sound to me like the dog is being neglected because someone is checking on it but I suppose the best thing to do would be to contact the council and if its one of their garages, then maybe they'll do something.
I think you'll find if it's being walked, fed and watered the RSPCA won't do anything.
Wouldn't the easiest thing be just to ask the guy if there's a problem? Maybe the dog has fleas, maybe they have a relative staying with an allergy to dogs, maybe he's in the process of moving and can't take the dog just yet, maybe he's looking after it for someone, maybe he's found it and is awaiting the dog warden.
Sometimes there are logical explanations, it's not always about terrible, cruel pet-owners.