The prefabs I looked at seemed like they were not very sturdy and would not hold up in the weather. Especially being in extreme weather like where I live. I knew a self built doghouse would last much longer.
It is however a good thing I had gotten good clear plans so that I didn't make a mistake on the type of wood or building materials. Some materials can be dangerous to your dog. The chemicals in pressure-treated wood can make your dog sick, if your dog is a chewer.
The doghouse plans were easy to follow and very understandable. I found them here at woodworkingforhome.info This website also has 1000's of other plans for sheds, swings, birdhouses, you name it-it is there.

