How do I add a structure that is created by a specific character in a mod?

[MaieArson]

Basically, there's already a mod that's completely finished in every sense of character, except for the items corresponding to it. And I have a concept of not just a structure, but literally an entrance to a separate location, kind of like in that recently released basement mod from the steam workshop, but here the creation of the entrance will be available only to the character himself, respectively.

Also, in this "room" will be a couch that works like a tent, but with seats for several players.
I've even made an annotated "map of the area", similar to the ones on the wiki of the game I'm trying to transfer all this from, albeit with some assumptions, of course.

So, how do i make something like this? If anything, it's welcome for me to do the couch separately, without the location, if it's easier that way.