Safe distance from belly
How much distance must be left between the bottom of aircraft and the floor of the cabin?
In small airliners, e. g. ERJ 135...145 series, it is not much. Looking from their website, it looks like 37 cm from bottom to floor (which is 153 cm wide)
But what about a widebody belly?
Lockheed Tristar offered optional lower deck lounge - only PSA took that option. But they were required to add extra reinforcements to protect the belly lounge in case of belly landing. How much weight did it have?
A widebody belly still has a floor of some width... so, what exactly are the fuselage modifications required for a lower passenger deck?