Regionnus Rattis
Expert
It's all about trade offs.....
With the below garage type, there is not a second form of egress, and the building might be on top of you.
With any below ground unit water may be a problem.
With any above ground unit, it won't offer as much protection.
That was my thought. I think I would much rather have an above ground shelter that can be seen, than a below ground shelter that could get covered and overlooked.
Build one into the ground that is two-thirds below grade and just outside the house/garage. If the crawlspace of the house isn't damp, then the chances are that you are not going to have a water problem.