Look for systems, not a silver bullet technology
I realise you need a flippant ending, but you need to be more than just digging up the garden to install these. Mine are about 4m under the surface. Putting one in an existing site is also difficult, running a large diameter pipe into an existing building can be tricky.
Also, note that since I'm in Adelaide and the air temperature is very hot here, that I'm actually using these as "cool pipes", delivering air that is well under the ambient temperature to the input of the evaporative air conditioning and the to back of the fridge, freezer and computer rack. The installation had to be designed when the house was rebuilt, so that it could come up through the foundation slab.
Note that there wasn't any one technology that had a massive payoff. It was the accumulation of the technologies. So the passive design and careful roof, wall and window choices keeps the house cooler to start with, the cool pipes improve the efficiency of the cooling system, allowing the cooling to be evaporative rather than a power-hungry compressor, allowing the evaporative unit (basically fans and a water pump) to be run from solar cells. Water is from underground tank storing rain from roof, thus cool. Result: cool house on 45C days, with no grid or mains water input. Capital cost of the cooling system was about $20K, or roughly twice a ducted airconditioner system, but the running cost is $0 compared to about $1000pa (it's really hot here).
Heating is a hydronic solar system in the slab. For temperature areas like Adelaide (annual minimum -5C) it works well. Cost was about A$15K, running cost is $0.
The near-off-grid design for a 12m x 20m suburbian block cost A$150k, compared to about A$100k for a conventional house. I'd say about $20k of that was construction inefficiencies that a wider use of the technologies would correct. My house is also smaller than a conventional house on the same block, since having eaves and shading implies less interior space. There is a great deal of usable semi-exterior space (the so-called "outdoor room"). I've been told be visitors that my new home is larger than the old one on this block. That's an illusion, it's just that every metre of the new house is a pleasure to be in (good natural lighting, temperature, etc).
Equiping the new house was difficult. About five years before the re-build I started buying appliances with an eye to their efficiency. If I hadn't have done that, then the evening wattage of the house would be about triple what it is. As it is, the house of four near-adults pulls less that 200W of an evening, with higher excursions as the fridge and freezer do their thing (although they do so much less often than a typical installation, since from beyond the doors they are in a sealed, foam-insulated cavity, fed from a cool pipe and exhausting directly outside).
I'd say the additional design -- if it were paid for, rather than done by me -- would be $20k. And that's the real problem. No developer wants to have an 'administrative cost' of 20% of a house's value. They'll always be undercut by Shonky Brothers. This is why regulation is needed. Market forces fail due to a lack of information -- it is only years after the sale when one neighbour pays $2000pa for energy and the other pays $0.
In summary: there is no technology which is a silver bullet. The silver bullet is a *system* -- a cooperating and self-reinforcing set of technologies and techniques, each of which contributes maybe 20% on its own.
I don't know anything about Europe, but there's no excuse for a domestic house in Australia to pull any energy *on average* from the grid. What we need is a regulation saying so, so the building development companies (which build most of the houses here in Australia) construct houses to meet that goal.