Thanks, Mark, for the quick reply.
Here is some feedback based on your example and my thoughts on surface to volume ratios.
Working from your example I tried:
wall 0.091 W/m2K,
floor 0.096 W/m2K,
windows 0.61 W/m2K,
MVHR: 92% efficient HR, elect eff. 0.36Wh/m3
= 14.45 kWh/m2/a
So it works, but that's R-62 in the walls (for those of us who think that way), 16" of cellulose, for Portland, Oregon (HDD=4,400F or 2,444C not unlike East Anglia...with the Latitude of Venice). And I haven't found windows (or an HRV) in the US that can meet those values. Compared to the Darmstadt (HDD=2,616C) example:
wall 0.138 W/m2K,
floor 0.131 W/m2K,
windows 0.70 W/m2K,
MVHR: 83% efficient HR, elect eff. 0.40 Wh/m3
= 13.79 kWh/m2/a
So, Mark, you are correct that higher performing envelope elements can get me there. But that only begs the question of why small buildings need to (should have to?) perform better than larger buildings to qualify.
In looking at the surface area to volume ratios, I am not sure they are that important. The larger the numbers used, the lower the surface to volume ratio (interestingly this makes it look worse in square feet than square meters!). I think the treated floor area (bigger is better) is the main factor, and here's why.
I start with a surface area (229) to volume (128.8) ratio of 1.778. I can double the height of my building (but not add another floor), making it closer to a cube with less surface (323.3) to volume (553.7) and a ratio of .5838, and the kWh/m2/a value gets worse. But it gets much better if the extra height also gives another floor. So the same surface to volume ratio (tall one-story vs two-story) can result in dramatically different kWh/m2/a values (25 and 8 respectively!). It's the floor area not the ratio.
And, I can double the long side (doubling the treated floor area), making it have only a slightly better surface (406.7) to volume (269.7) ratio of 1.507, and get a better kWh/m2/a value (10.41). Here it's because of the increase floor area, not the lower surface to volume ratio. If the added size does not increase treated floor area on only envelope, the kWh/m2/a value gets worse not better (32.8).
I see that the party wall of the Darmstadt example helps its results a lot. (FYI: I think there may be an error on the height of that wall, Areas K45 shoud be 8.8925, but it's not used in calculating the heat loss calcs.) If the party wall is changed to an exterior wall (Areas D45 changes from 18 to 8, and ignoring the above error), the kWh/m2/a goes from 13.7 to 29.59.
With 14 fewer square meters of floor area but the same envelop, this building would not qualify. Similarly if you shorten (east west direction) the building by 1.33 meters, it no longer qualifies.
So I am still open to being shown how this works another way, but as far as I can make sense of it, it seems that the amount of treated floor area is a much bigger factor than surface to volume ratio. And so small buildings are penalized in some way by the way things are calculated or by the energy per area metric.
In determining how comfortable an individual will be in a building all these factors come into play: outdoor temperature, how well insulated and sealed the building is, the amount of solar radiation reaching the interior, the internal heat gains, the amount of wind outside, and individuals' opinions about what constitutes a comfortable indoor temperature, relative humidity indoors. And we are trying to achieve comfort with the least possible energy.
Perhaps the "Balance Point", the temperature at which a building begins to need space heating, may be a good number to watch. Energy per person? Or as Doug mentions total annual energy. Well, I am still letting this settle in. So if someone can see something I'm missing here, I'm all ears.