I've been anti coats for dogs on the basis they have good coats without the need for additions. I did buy a coat for my old cross breed, whippet like, fine coat and as he got older, he did feel the cold. He actually enjoyed it being put on and it was good to see hi enjoying walks without shivering.
That was 15 years ago and my next two dogs were water retriever types so they had undercoats and didn't need any help
I've just bought a lightweight water proof called Lovelonglong- amazon (sorry folks) £12.99 for my spaniel. It has long legs and a zip along the back so easy to put on and off. I Never thought I'd put a coat on my spaniel but whilst I don't mind the time needed to care for her long skirt and feathers, its not much fun for her. She's a good girl, been trained to be showered, brushed and combed, good with the hair drier but, it's the time needed and the effort to get knot and matts out. We used the coat a couple of days ago. She was very unimpressed, walked in an odd way initially, obviously felt uncomfortable. But -despite being in long very wet grass and mud, only her feet and the tips of her ears needed cleaning. I hope she'll adjust to the coat. I wish I could explain 'wear this girl and no need for a long grooming session'