What is the apartment near badtfl regional office?

[Stubb]

...and how do I get in?

[TooltipTinkerer]

Did you ever figure out what this was?

[Ihateeverybody]

Guessing its part of a quest in a mod. Let me guess in order of likely hood (or alphabetical, I am llazy).

1. Commonwealth Underground and Forgotten Spaces2.0
2. Inside Jobs
3. morexplore
4. Tales from the Commonwealth.

if you have more console you can open the console and click on the door to find the mod, than ask there for better results. If its 1 then likely you have to complete another space before you can open that one.

[Zotliatlicor]

If you have the CC appartment installed, that is close to BATDF and The USS constitution. You likely find the entrance first, but the key is located on a corpse somewhere in the City. (hint Hubrics)

[Bored Peon]

Of course it would have been MUCH easier if the OP provided a screenshot showing the exact location....

[Stubb]

I'll add some screenshots soon