I just came across a fascinating photo-essay about the WW2 Anderson bomb
shelters Londoners built in their backyards during The Blitz:
http://mashable.com/2015/07/24/wwii-backyard-bunkers/#6Ko7Xr4WukqP
Not sure if the J-shaped corrugated iron/steel sections are available anywhere these days but you could build a very livable house out of several of these grouped around a central courtyard, either open to the air or enclosed as a solarium for
solar gain.
The covering for the shelter could be dirt or
hugelkultur - check out the first photo in the essay of the woman watering her plants!
