• To build your own apartment building, you must have, obviously, a building. Ideally an apartment will be around the size of a small house, with objects to satisfy all of your Sims' needs without the rent price being too high, although you can make more expensive ones for your richer Sims.
• Once you are done building all of the rooms, make sure that the only entrance/exit to each individual apartment is through the special apartment door and that the unit is otherwise entirely enclosed so that no other Sims may enter (The game will notify you if a unit is is not properly enclosed).
• Once you are FULLY and COMPLETELY done with your apartment, enter the code at the top of the code list to turn the lot into an apartment building. It is important that you are totally finished building because once you have entered the cheat and exited the lot, you will not be able to modify anything in Build Mode except for flooring, wallpaper, and objects from the Gardening tab. If you really must edit your apartment, you must re-enter the lot and change the zoning back to residential (Second cheat code).

Note: Furniture that the game does not consider "necessary" may not appear to Sims moving into the apartment. To rent apartments fully furnished, enter Cheat Code #3 and when you move in a family, hold Shift and click on the apartment door, then click on the option to rent furnished.