All you need is adequate structural support, a window that can provide ventilation and an alternate exit, and to not hoard toxic flammable substances down there.
You would think, but there's no such thing as a window that provides adequate ventilation. Many toxic gasses are heavier than air, so they sink to the lowest point of a space. If you're in an underground bunker it doesn't matter if a window is at the top of it, all the air can get displaced and just pool with toxic gasses without you even realizing it. It requires ACTIVE air flow to circulate through, passive airflow won't do it.
All you need is adequate structural support, a window that can provide ventilation and an alternate exit, and to not hoard toxic flammable substances down there.
You would think, but there's no such thing as a window that provides adequate ventilation. Many toxic gasses are heavier than air, so they sink to the lowest point of a space. If you're in an underground bunker it doesn't matter if a window is at the top of it, all the air can get displaced and just pool with toxic gasses without you even realizing it. It requires ACTIVE air flow to circulate through, passive airflow won't do it.
https://www.backdoorsurvival.com/all-of-the-ways-underground-bunkers-can-kill-you-2/