With the introduction of the "Late Night" expansion pack, "The Sims 3" added a new type of building: apartment blocks. Adding an apartment building to an existing town adds a great deal of additional housing for migrating Sims, giving you the chance to import new families or create a more personalized town with your own characters using only one vacant lot.
Converting into an Apartment
The only things separating apartments from any other tall building you design are the callbox and public room markers. The callbox lets Sims call tenants in the building, and public room markers indicate that Sims are allowed to rent the marked room. Outside of these requirements, you must also fully enclose public room markers in either walls or tall fences with a gate. You aren't able to purchase a wall mailbox for your apartment without modding the game, but using the default mailbox lets you assign NPCs to their doors.
Designing the Building
Think of apartments you've seen before: Many are designed to hold similar-looking apartments on two sides of a long hallway, all housed inside unassuming rectangular buildings. Build what you know, and build tall; the more floors you have, the more apartments you can make for more tenants to live inside. If you want to play on the top of a tall building, try using the Level Skip Markers; these don't prevent NPCs from moving in, but they'll ignore the animation as your Sim passes through them to go home. Remember to add a door for each room you want to turn into an apartment.
Designing the Rooms
Each door can be marked to permit specific Sims passage. You can use this knowledge to help design multiple rooms inside of apartments. Place a bed in the room and click it to set ownership, selecting an NPC in the active family when you have one. When you click the door, you can lock the door so that only the Sim who owns the bed and any friends in a group may pass through. As long as you set this, you don't need to worry about restricting doors inside the apartment, letting you create rooms for Sims inside to freely pass through.
Trawling for Tenants
When you're done with the building, save your game and open the options menu, selecting "Edit Town." Here, you'll be able to create households of Sims that you can move to the apartment building. Although you can have multiple families living inside of an apartment block, each of the households can only have up to eight Sims, similar to a standard family and a normal house. This limit can help you better plan out your apartment furnishings, though, as you only need to plan for a maximum of eight residents per unit. When you're done creating a household, select "Move In Household" to move a family into the building.