Here is the best way to make a heated doghouse:

Build your own doghouse, with double walls and insulation between the walls. Also, build a double floor with insulation so the floor is off the ground and out of drafts. For the door, either install a dog door with a flap, or nail a carpet square over the opening, attached only at the top. This will keep wind and snow out.

I don't know what kind of weather you have, but if you live somewhere where it gets really cold, you can also add a light to the doghouse for heat. You will need a lightbulb on one of those drop extension cords. Make a hole in the roof big enough to pull the cord through and secure it VERY well, so it won't slip and burn the dog or start a fire. It usually doesn't take a very high wattage bulb, maybe about 40-60 watts, to warm the house.

Having said all that .... let me know say that I think the best solution is to bring the dog inside and make it a housedog. Not only will it be warmer, but it will be much, much happier living inside with its family.

I do realize, though, that sometimes this is not always possible. When I was a child, we had two dogs. My parents would not let the dogs in the house. They did not believe in house dogs, no way. I hated it, but I had to live by their rules. This is how I know how to build a warm doghouse - our dogs lived in houses like this. Now that I am grown and have my own house, my dogs live in the house with me!