Apartment
From The Sims Wiki, a database for The Sims series that anyone can edit
An apartment is a lot where Sims can live in The Sims 2: Apartment Life. It is similar to a dormitory, however unlike a dormitory, anyone of any age can live in it. You can live in an apartment by selecting a room's door, and selecting "buy apartment(-$). You can edit out the flooring, appliances but you can't remove objects that already exist in the room(e.g. sliding dresser, fireplace) without using cheats. You also need to pay the rent whenever it's scheduled.
You can complain to your neighbors if they make too much noise, using the "Knock on Wall" interaction or you could knock on their door and blurt out your complaint.
You could also decrease your rent by having a high relationship with the Landlord or Landlady or by having a Roommate that will help you pay your rent. However, you will increase your rent when you make too much noise or your neighbors complain about you. It's up to you!
[edit] Building Apartments
A user can use pre-designed apartment buildings, or can design their own buildings with the use of the 'changelotzoning apartmentbase' cheat. A user could, in theory, build many types of rental dwellings, from bachelor-sized apartments, to multiple houses sitting on one subdivided lot, to single houses that contain multiple households, to even a single rental home sitting on a lot; the builder need only meet a certain number of requirements.
First, all units (individual apartments, for instance) must be completely separate from one another. There can be no doors that lead directly from one unit to another one. Units must have only one connection to "public space" (any area intended to be outside the units i.e. hallways), and such connection must be the Unique Separator door. The Unique Separator is the door in the build menu that comes with a doormat - the doormat and door are changeable, but the mat must face outside the unit for the apartment to be considered whole. Apartments can span multiple floors, but an apartment on a foundation may not have a connection to the ground (if, for example, the apartment were a house with a fenced-in backyard and a deck).
Once the unit(s) are completed, the 'changelotzoning apartmentbase' cheat is used to convert the building from single-family residence to apartment. If the apartment is not built correctly, a pop-up will appear telling you this, and will tell you to use the 'changelotzoning residential' cheat to go back and identify your mistake(s). If the apartment is built correctly, the lot will be changed to an apartment, and will appear as an apartment building when seen from the neighborhood. Make sure all architectural and structural changes are made before the conversion to an apartment. Because of the Build Mode restrictions in apartments, making major changes to the structure are impossible after the lot has been rezoned.
There is no maximum number of units in an apartment complex, but the user may only place 4 families in the building, so any remaining units will be available for Townies to move into. Users with less powerful computers should avoid building apartments that are too large, since large buildings with many units and many families will drain system resources.
