I do not nowadays have dogs but i used to live in a sunside apartment in eastern Finland in North-East Europe with two 40cm high, around 10kg dogs, one of which was fat.
Take care that the dog does not spend time in sun. On a walk walk shadowy side, in the shadows of trees and breeze, plan walks so that there are no long pieces in sun. If the dog gets exhausted, keep a pause and leisurely pace, maybe take water and a cup for dogs along on walks. Do not leave the dog to a hot car.
If it is hot at home, ventilate a lot when it is cool time of the day and also when there is breeze. Let the dog stay in cool under the ventilation window. Keep curtains open so much that one can see trees. If it is unbearably hot, shower while avoiding arupt cool or cold and while listening to your dog, paws and maybe the back, but not stomach, with cool water, or let the dog walk to a lake ( or to a washing water bowl of cool water ). Take care of foods but only light summer food according to the weather, but not a tough diet but instead for a meal fatless milk or cooked vegetables or vegetable beef.
From my Finnish blog parantamisesta.blogspot.com