I pulled these plans from another site I frequent, the first one is one of the best Ive seen, especially for cold weather.
http://www.angelfire.com/nc2/uniquekennels/doghouseblueprints.htm
http://www.pitdogs.org/main/html/doghouse.shtml
Here is another one using just 1- 4X8 sheet of plywood.
http://www.pitdogs.org/main/html/doghouse.shtml