Building my own "apartment" style houses

If you mean functional apartments like the ones that came with City Living 'hood San Myshuno, its impossible on an ordinary residental lot. That being said, I do have several apartment buildings in my games and I just make sure that have the doors to the apartments locked for ALL residents except the residents that are supposed to live there. I lock each door manually by clicking on each of the names of the ones that are not supposed to be in said apartment. That way, any visitors and services can enter the living areas regardless. The only ones not able to enter are the ones that are not supposed to be living in the apartments that have not been designated as their own. It has worked quite well for me but there is no rent, only bills so you better make sure there is a mailbox. I have had to use the pause button quite a lot though. So keep that in mind, but having a multi residence on one resident lot is possible.

For instance. I replaced the BFF's household little house for a big 4 level apartment building with a basement that is a communal gym and a big backyard with a communal area for plants and a swimming pool and other backyard stuff.

In the beginning, each of the BFFs along with one other Sim occupied their own apartments and they did not/do not enter each others apartments except when I unlock it specifically for them. For instance, Travis and Liberty can enter each other apartments because they are good friends and have a 'key' to the apartment of their friend. Liberty Lee has a restricted access (I unlock the doors for her when needed and then lock her out when she is finished) to the other two of the apartments when things break down because she is the owner/super of the building and makes all the repairs/upgrades to her renters/neighbours apartments.

The residents numbers have increased to the limit 8 Sims and despite that, everything is going quite well. Summer Holiday has moved out into the Partihaus household and a single mom with her kid has moved into her old apartment. Same system as before and everything is going well. The other Sim that is not part of the original BFFs now has a kid and a girlfriend, along with the latest addition, a little kitten, that will become the building's roaming cat when he grows up (he is living in Liberty's apartment for now).