This idea of living underground was a passion of mine for many years. But, all the books I could find about (so-called) "underground houses" described nothing except conventional, stick-framed houses with a bit of dirt over the top. This approach is NOT "underground housing".
The advantage of living "underground" is heat-sinking into the earth. A foot of wet mud over the roof of a house is NOT a heat sink! (For that matter, a tunnel a foot under the surface isn't a heat sink either.) If you don't want to modify a natural cave, then mine one. (Wild caves were formed by water and water may be a recurrent problem, as well as the danger of radon gas and collapse.)
I decided on a South-facing marble or granite vein. I could sell what was in the "hole" to pay for construction, and what a lovely home it would be. Just think of the "storm shelter" aspect, as well as the "bomb shelter" advantage. As I pondered it further, a zig-zagged, alcove entrance with overhead defense ports became mandatory to fend off the frenzied horde in the event of a nuclear event. (The zig-zag keeps an RPG from ending up in your living room! The blast door is beyond the stone zig-zag tunnel.)
One of my ideas was to dig a well from above the bluff, right through the future mine. When the mine caught up, this yields fresh water inside the cave (without venturing outside) and a ventilation chimney (to be disguised above).
A waste settling pond just below the cave would have a tiny, drilled tunnel leading into the cave. Catfish have a 90% food to meat conversion ratio. Food scraps and other waste dumped into the pond via the tunnel would yield an emergency protein source inside the cave. Scraps and waste are flushed out to the pond by water from the inside well and fish can be lured right into the cave.
I was (obviously) designing for the long haul in very bad circumstances.