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.
Tiny House Ontario 
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.
Yes, I thought about this and I did what I knew how to do. I build the house facing the right direction for sun and wind. I put in lots of windows to catch the winter sun and big roof overhang to keep it out in the summer. The cloth porch over the big door all work for shadow (summer and unfortunately winter too. Still, so far, I am not there in the winter. With no running water (well is expensive about 10 K with the pump) and no furnace until next year! Dickinson Marine furnaces, http://dickinsonmarine.com, are typical for Tiny House structures about 1.5K for the furnace and installation.
I stayed in the winter a few times last year and it was too cold with no furnace to live there; however, it won’t take much to heat it! I used a small tent heater (not very cost effective or safe) to do the job and it worked OK… I really look forward to the furnace getting in, but the budget is the budget and other things seemed more pressing this year (the bathroom).
xo L