The apartments in Sims 4 are much different than in Sims 3. I am going to assume you don't play Sims 4, as you would be familiar with tray files in the game as opposed to library files which are used for S3. Not only that, but you cannot save an apartment in S4 as a lot, except as a build within a gamesave or as a room. I hope this makes sense - it is kind of weird we cannot upload or save apartment files as a lot, but that is S4 for you.
Yes, I don't play Sims 4, that's why I asked for the files, and yeah, it's really different, compared to Sims 3, from what you say :o
Yeah, I really wish we could save apartments as apartments because you can't even save them to your folder, only as rooms (which is so weird, and I haven't quite figured that out myself.) So, if you start a game from scratch, you have to rebuild all the cool stuff you did unless you have a save as a base. Sad Face.