It was minus 3 at Tiny House Ontario for the two days that I was there. A small propane heater brought the temperature up to the place where you could not see your breath, not exactly winter camping but mighty close. Still, I had known this before I went and packed a lot of bedding. Up in the loft I used 2 blankets, 2 comforters, and a fun fur covered (non-animal) duvet. I also slept in fleece with long underwear. Sexy eh?
This was cozy and though I could see my breath in the morning again when I woke up, I slept well and the bedroom is really cozy. Here is what it looks like there.
Have you planned any solar gain, thermal heat retention, solar heating technology? Why not a really small wood fire? Also check out rocket mass heaters.
I have a small propane heater I use in my VW camper van that works awesome. Im sure you could get something just perfect for the size of your home that could run on propane as well.