By far the best of the shelters in the Family Shelter Designs booklet is this underground masonry shelter.
Of course it would be pretty expensive to build but at least there is room to exist in the thing.
It's called a "new construction" shelter but it could be an addition to an existing structure too.
This model is very neatly detailed with the wall studs, floor, air blower, shelves, vent pipes and even bunk beds.
Click Photos To See Larger and PDF File of shelter plan can be downloaded by clicking link at bottom of page.
Belowground New Construction Clay Masonry Shelter PDF 2.1Mb