We had a bomb shelter at our last two houses, the last one we had was built in WW2 and designed to provide shelter for three or four families, it was in the back yard of our little terraced house, basically they removed the outhouse and coal shed and replaced it with a big prefab concrete box made from rebar and heavy concrete all dimensions were one foot thick including the roof, it had a steel set of doors with rubber seals and a hand cranked air filter. I turned it into a single family shelter and workshop.
The house we had before that the previous owner had spent thousands re-enforcing a cellar that had been there since it was built, extra walling, doubled up on the amount of floor joists to one every foot and increase the size from 8 x 4 to 10 x5 inch and added a second layer of floorboards over the original floor giving a floor thickness of 2 1/2 inches. he also put extra vertical screw up braces to support the floor etc. The coal door to outside got clad in painted mild steel about 4mm thick. That was my man shed
I've got nothing in this house we live in now