I've been thinking more about basement apartments.
Nearly every older home in the region I'm looking at has a basement. These generally have 6-7' ceilings and extend ~5 ft underground. Many of them are unfinished and used for storage and utilities (as previously mentioned), while others are finished and used as an additional living room. Some buildings are extensively rehabbed and modify the front/rear windows for egress so that they can meet the building code for bedroom occupancy.
If I think just from a pure energy perspective, a basement has a lot of things going for it. The fact that it is mostly underground means you have stable temperatures year round (I'd estimate in Zone 7 that most summer basement temps never exceed 65 and winter basement temps never go below 55 or 50). You have a naturally quiet environment for sleeping as well. I think with adequate design for passive ventilation, one could live in a basement year-round in the region without the need to spend any money on heating/cooling utilities.
There are challenges of course. You will need adequate ventilation and a way to keep humidity in check. There's also the issue of limited natural light and the fact that culturally most people don't want to live in a basement. Then of course you have the zoning challenges mentioned above. You'd have to figure out where to put cisterns. Basements carry some flood risk, although a lot of that can be mitigated by not living in a flood zone and ensuring you have countermeasures for any sewage backups.
If I go back to my model in my last post, an alternative on a house that has a suitable basement height could be to raise a basement up to code to be able to fit 2 bedrooms and a bathroom. This would involve finishing the basement and modifying the windows to provide egress. Bathroom could be added in the middle space of the building so that each room has 2 windows (ie one room on north side of building, one on south side). I'd keep the plan the same on the ground floor with kitchen and living space. Unsure about what would happen with the 2nd floor. Those would still be very habitable rooms during shoulder seasons, but would likely be very cold in winter without any heating and very hot in summer without any cooling. Then there is the issue of freezing pipes if there is no heating element during winter.
So does this make sense from a practical perspective? Probably not because energy is still so cheap and reliable in most parts of the country. Nevertheless, in a power outage during a heat wave or cold snap, having a basement area for sleeping could be the difference between life and death (especially if we're talking a 100 year heat wave when electricity grid is down). The issue still remains what to do with everything on the 2nd floor and where to put cisterns. I think it would be possible to bury a cistern in the backyard, but that is costly because it requires removing of concrete and is a permanent solution unlike putting plastic cisterns in the basement.
Maybe a compromise is to just outfit the basement as mentioned in my earlier post, but ensure there are appropriate measures to turn the basement into a sleeping shelter during an energy crisis event? So basically, ensure there are no major health risks and that proper ventilation is possible without the need for electricity. Then one just needs a few cots on hand to turn the area into a temporary sleeping shelter during a cold snap or heat wave. Daily activities would still take place on ground floor and roof deck. Pipes freezing are the biggest risk, but with the cisterns still in the basement then this would be survivable. Paired with a PV array on the roof that powers chest freezer and maybe charges electronic devices when sun is shining, one would be pretty resilient at least up to a few weeks in a grid outage scenario. Bigger issue might be security to keep masses away from your stockpile, PV array, water, shelter, but at that point things are turning into a post-apocalyptic movie anyway so likely low probability compared to other outcomes.