County regulations forbid traditional outhouses (and I've actually tried to develop this property by the book). So ours is built over a concrete holding tank. You can see it in the top picture. 2x4 construction, fully insulated, proper door and windows (of the type people put on houses), laminate floor, cedar paneling inside, pine on the outside. (The cedar really keeps down the bugs--there are basically none.) I put a little propane camp heater (powered off a 1 lb propane bottle) in there in the winter. Run that for an hour or two, and it's bearable inside. We tend not to go out there if it's below 0�F.
The logs on the main cabin have a lot of mass and it takes forever to warm up the whole cabin when it's cold out, but it's still nice sitting by the stove, which puts out a huge amount of heat.