Things You'll Need
Outdoor electrical outlet
Insulated doghouse at least 24 inches by 36 inches interior size
Outdoor-use extension cord
The mat must lie flat--do not allow the matting to bend. Check the manufacturer's instructions on how to secure the edges to prevent sliding. Some have grommets through which you can bolt the mat to the floor or wall.
Verify that use of an extension cord is permitted by your local building codes. If you're worried the dog will chew on the cord, hide the cord inside PVC pipe.
During winter, your dog's own body heat may not be enough to warm the interior of his doghouse. If you need to leave him outside for long periods of time, this will affect his health. Of course, you have insulated the doghouse against wind, have elevated it above the ground or cement, and have covered the entrance with a thick curtain. But adding warmth will go that extra distance to keep your dog comfortable in cold weather.
Space heaters are a fire hazard, and doghouses are small enclosed spaces even for large dogs. Radiant heat is the safest solution. But true radiant floor heating can be expensive, and installation requires an electrician and tiling expertise.
Snowmelt mats designed for all-season outdoor use reach temperatures of 50 degrees and are safe for pets. You can purchase entrance mats as small as 24 inches by 36 inches. You should be able to find this product at your local home improvement store, or it can order one for you. You can also find these products online.
Kennel mats, a similar product, are an indoor/outdoor heating pad designed for dogs. Pet stores can locate kennel mats for you, or again you can find them online.
Verify an outlet is within 6 feet of the doghouse, or ensure the connection to an outdoor-safe extension cord is protected from moisture.
Purchase an electric snowmelt mat or electric kennel mat.
Install the mat inside the doghouse. If the mat is smaller than the full floor, place it away from the entrance to keep the heat inside the doghouse. You can alternately install the mat on the wall of the doghouse for the dog to lean against. The wall is a good solution for habitual chewers. Alternately, you can install the mat underneath the floor of the doghouse where the dog cannot directly contact the mat.
Plug in the mat and watch your dog enjoy a nice, toasty place to retreat from the weather.