If you build a doghouse yourself, it should be just the right size and insulated. Most of the commercial dog houses are easier to keep clean and last forever. They are lighter weight and well insulated. Be sure the door doesn't face the wind.

I wouldn't use a light bulb to heat a doghouse. I've heard a couple of horror stories.

They make a plate-shaped warmer that you can heat in the microwave. It's called a Snuggle Safe Microdisc. It costs $19.99 at Drsfostersmith.com. After you heat them for about 5 minutes, they will stay warm for up to 12 hours.

I used one in a heavy box on my porch for my stray cats last winter. Worked well and safely.

Mary