I can't find the link now, but there was an episode of Grand Designs here in the UK (a show detailing private individuals developing interesting or unusual home

I can't find the link now, but there was an episode of Grand Designs here in the UK (a show detailing private individuals developing interesting or unusual homes) where the owner was building a passively heated house based on an idea by his architect father. The ground beneath the footprint of the house was insulated around the sides to a depth of about 2m, effectively extending the thermal mass of the house into the ground. After construction, it took about 2 years (IIRC) to warm to a stable level, but thereafter required little to no energy to stay at a comfortable temperature year round.