✽ Updated: Jan.2021I recently looked into creating apartments, as I wanted to add one or two to my current save.
While I was building I took notes on my experiences and whatnot and wanted to share my notes.

Build the house or apartment building as you would normally. This includes furnishing, decorating and recoloring the objects and furnishings. As once room markers are placed you need to use the {
restrictbuildbuyinbuildings off} cheat to edit anything on that lot.
✽ To Turn Editing off. Press Ctrl-Shift-C together to bring up the cheat window and type in
testingcheatsenabled on and then type in
restrictbuildbuyinbuildings on to edit. This turns off the build cheat.
You don't need to furnish/decorate the rooms or spaces that you'll be putting the hidden markers in. You can even take out walls so you don't have to place as many hidden markers. This may help lower the cost of the lot or rent paid by your active family living on the lot.
💡 A Tip: Build the apartment house or building first, save a copy and then edit that when needed.
This means the markers will need to be replaced, but I have found it easier to edit the build without the markers then with them.
Want the apartment house used to make this how-to?
Click Here ✽ To help with Sim routing, it is a good idea to set front of lot and front door to help with routing. In Build/Buy mode:
Set Front of Lot - with
testingcheatsenabled on, press Ctrl-Shift-Click on the space you want to be set as front of the lot. A blue line and animated arrow will appear to show you where front is.

Set Front Door - Press Ctrl-Shift-Click to bring up the pie menu and select Set as Front Door.

✽ In order to have neighbors in your apartment, you need to set at least one NPC door. This will also help the callbox to work.
Add a wall mailbox and a callbox. These are Late Night objects.
Without a call box, you can't call out the the NPC Sims that live in the other apartments to visit with them.
Callbox can be found in Buy Mode > Sort by Function > Miscellaneous Electronics

Wall Mailbox can be found in Buydebug > Misc. Objects

Hidden Room Markers and Public Room Markers are base game items, you can get to the via the buydebug menu with
testingcheatsenabled on.
Buy Mode > Sort by Function > Debug [?].
Hidden Room Marker

Public Room Marker

Go to the last tab called Misc. Objects, scroll down until you see the orb called Hidden Room Marker.
✽ Public Room Markers -This marker is for all the spaces you want your Sim to use, but not own any of the objects. For example, living spaces, such as kitchen, living room, family room, etc. Any space you want to consider shared public space.
✽ Hidden Room Markers -This marker is for rooms your Sim will not have access to. This is considered rooms where NPC Sims will be living.
** Do not put markers in your Sims' bedrooms/bathroom. This is considered their own private space.
~ More Tips and FAQs: ✽ Editing Spaces -You see a grid in the spaces where you edit with cheats off. If you don't see a grid, you cannot edit that space.

✽ Assigning NPC Doors -NPC Sims cannot be chosen by player.
To assign doors to NPC Sims you need to have testingcheatsenabled on and be in Build/Buy mode. Press Ctrl-Shift together and then click on a door to get the pie menu that gives you the option Make NPC Door.
This can be done on all doors leading to hidden "rooms" that are facing public spaces.
✽ Make NPC Door -
Once NPC Sims start moving in (in about a Sim day), you will be able to Left-click on any NPC door and Ring Bell or use a call box if placed on the lot when in live mode.
✽ Odd Routing With House That Has a Foundation and Basement -If a house is built on a foundation and has a basement, Sims may oddly route to the landing when returning home from another lot. The lot type (Residential or Apartment) doesn't matter.
The following conditions may cause this:
House has a basement and built on a foundation
The staircase (straight or spiral) is in an open public area before the Sims personal space.
There is at least one NPC door set in the apartment
To fix, either re-do the stairs leading to the basement landing or wall off the staircase area and add a locked door. Sims will now route properly into the apartment.

✽ Making Changes? -If you want to make any changes to the lot after the room markers have been placed, turn on testingcheatsenabled on, and type in restrictbuildbuyinbuildings off into the cheat window. This turns on the build cheat. To turn off the build cheat type in restrictbuildbuyinbuildings on when finished.
✽ Too Many Sims in the Callbox? Or Duplicate Sims in the Callbox? -This may be by design. To simulate roommates in a rented apartment. They do not come out all at once. When using the callbox, usually only one Sim is called at a time.
If your town has only one apartment building, all the homeless/NPC Sim will be added to your only apartment building. If you place more than 1 apartment building the game may distribute the homeless/NPC Sims between all of the apartment buildings.
If you find that you have duplicate Sims in the Callbox. Make sure you don't have a copy of them in your Library. As the game can sometimes use families that are saved in your library or CAS if town population is low.
✽ Does Lot Type Matter? -It might, depending on your playstyle. In my experience lot type doesn't matter (Apartment lot or Residential) in the way that it plays.
Both lot types you still get Sims roaming around the house, but it looks like you get more Sims roaming around if lot is set as an apartment.
It's less expensive to move into the lot, when set as an apartment lot.
- Example:
* Price On Lot: ~ §50,000+ for regular residential lot
* Price On Lot: ~ §535 for an apartment lot lot
You may find random NPC Sims (roommates) added to your household as NPC Sims. This is normal. To get rid of them you'll have to dismiss them each individually by clicking on the Sim and selecting Roommates > Dismiss Roommate.
The option to turn off roommates isn't available on lots set as an apartment lot. The lot needs to be set as normal residential lot in order to have the option of Roommate Services. To change that dismiss all roommates, evict household to the clipboard and set lot as a regular residential lot.
Turn off Roommate Services and Troubleshooting Tips
https://www.carls-sims-4-guide.com/forum/index.php?topic=26400.msg440850#msg440850 ✽ Q: My Sim is routing all different ways to the new lot? -This is normal.
When placed via library, Sims can get placed by any door or inside the building.
When moving via phone or computer, Sims route from sidewalk to front door.
✽ Q: Does choosing the 'unfurnished' option take away all the furnishings, including all of the furnishings in the communal areas of the apartment? -It shouldn't take away the furniture or decor of the communal areas of the apartment. Just your own private areas with exceptions to the bathrooms. Those stay mostly furnished - no mirrors.
1st Floor

2nd Floor

~ Reference Links:http://simswiki.info/wiki.php%3Ftitle%3DTutorials:TS3_Sims_3_ApartmentsBuilding Apartments
https://www.carls-sims-4-guide.com/forum/index.php?topic=2455.msg59615.html#msg59615Too Many Sims in Callbox
https://modthesims.info/t/450829https://gamelover86.wordpress.com/tag/how-to-build-a-apartment-in-sims-3/https://hubpages.com/games-hobbies/Building-appartments-in-The-Sims-3https://forums.thesims.com/en_us/discussion/310981/how-can-i-build-apartmentshttps://forums.thesims.com/en_us/discussion/820000/no-option-for-npc-door-when-building-an-apartmenthttps://forums.thesims.com/en_us/discussion/893698/creating-apartmentshttps://www.reddit.com/r/thesims/comments/6xyyuc/sims_3_an_apartment_building_without_an_elevator/https://sims.fandom.com/wiki/Apartmenthttps://sims.fandom.com/wiki/Room_markers