There was a lot of discussion about this in the UK a couple of years ago when the RSPCA made a statement that in some cases dressing up animals could be seen as breaching their right to express normal animal behaviour, and might be uncomfortable or restrictive for them. The main arguments appear in this article. It seems to have ended there though, and certainly people are still dressing up their pets without being prosecuted.

But even though it's not illegal, that doesn't mean it's the best thing for the animal. Of course small dogs needing a coat in the winter, or dogs that have been injured and can't keep themselves warm, are another matter; but you could ask yourself what would you want if you were a dog? The fact is, animals aren't evolved to need or want coverings other than their own hair, so it's more likely to be uncomfortable than not. I would be more concerned about the behaviour that goes with it; if you've bought an expensive coat for your dog, you won't want the animal to run around and get dirty - and that, you could argue, is both unnatural and unkind, for dogs need freedom.

The final question: who are you doing this for? Certainly not for the dog.