I got 175 kWh/m2 per year. I had to do a bunch of approximating (I don't yet have data for the full heating season, and can't find bills from previous years), so it could be off by 20%.
EDIT (25-06-2010): I finally completed a full year of logging the gas usage, so here is better data:
- During the last year we used 1932 m3 of natural gas,
- The non-heating average is 0.5 m3 per day,
- So we used 1747 m3 for heating.
- Nat. gas has an energy value of around 32 MJ/m3 = 8.9 kWh/m3.
- The house's floor area (without unheated basement and garage) is 110 m2, so
- Our house is at 141 kWh/m2/year.
I'll have to look for old gas bills to get a multi-year average, which would be much closer to being objective, since this past winter was the coldest in 30 years.