To keep a doghouse warm, construct the house properly for cold weather, incorporating a door flap and insulation in the floor, ceiling and walls. The house should be large enough for the dog to turn around but small enough that the dog's body heat can help warm the inside. An added internal wall between the sleeping area and door flap serves as a windbreak and adds warmth.
A simple and inexpensive way to heat a doghouse is to place an electrically heated kennel mat on the floor or hang it on an interior wall to keep from hampering the dog's sleeping area. Another way to keep a doghouse warm is with a heater box mounted in a corner of the ceiling out of the dog's way. A heater box works in a similar manner as a traditional space heater, but uses a light bulb to heat the area.
Building the doghouse next to the main house allows for using the dryer vent for heat while keeping the doghouse under the eve and out of the weather. Placing the doghouse in the garage is another way to keep it out of the elements and warm without the need for insulation or added heating products.