How to open the closed metal door on the mansion

[Valase]

Here are all the ways I know off:

@Valase

You can open that steel door in multiple ways. The easiest one would probably be to use an acetylene torch on it to cut it open.
You can also blast it open with explosives.
You can smash it open with enough strength and some tools or a Sonic Resonator CBM.
You can dig through the wall on its side with a pickaxe or a drill.
And you might be able to send an NPC to loot the place, as this opens all doors (last I checked the code), but I’m not sure if that is fully implemented yet or how to do that.

As for blasting through it; it probably destroyes whatever’s behind the door, so apply with care.