[zrail]

Would you be willing to quantify what "mild" means to you, maybe in terms of a USDA zone? There are maps for both US and Europe:

https://planthardiness.ars.usda.gov/pages/map-downloads

https://commons.wikimedia.org/wiki/File:USDA_hardiness_zones...

[laurencerowe]

Passive house standards first gained popularity in Germany and Scandinavia but it seems the principles have been adapted to quite a wide range of climate zones now.

https://en.wikipedia.org/wiki/Passive_house