Can I layer a coat and a raincoat on a dog?

A warm fleece or knit coat provides insulation but is not waterproof, so in cold rain or wet snow it can become soaked and lose much of its warming effect, while a purely waterproof shell without any insulating layer keeps a dog dry but does little to add warmth in genuinely cold conditions. Combining the two addresses both problems at once, similar to how a person layers a warm base with a waterproof shell in bad winter weather.

The outer waterproof layer needs to be sized to fit comfortably over the inner warm layer without pulling tight across the chest or restricting the shoulders, which sometimes means buying the waterproof shell a size larger than you might otherwise choose for the dog alone. Checking that leg openings and any belly straps still allow full, natural movement with both layers on avoids a dog walking stiffly or refusing to move properly.

For a well-insulated, double-coated breed or in merely damp, mild conditions rather than genuinely cold wet weather, a single waterproof layer alone, or no layer at all, is often sufficient, and adding unnecessary warmth on top can cause overheating during an active walk even in winter. Judging the actual conditions and your specific dog's needs matters more than defaulting to maximum protection every time.

A dog unfamiliar with wearing any clothing at all should be introduced to a single layer first and given time to adjust before adding a second layer on top, since two unfamiliar garments at once can feel considerably more overwhelming than either one alone would have been.