Apartment Life Cheats:

To make apartment buildings

Build initially on a residential lot.

DO NOT CHANGE THE ZONING OF A LOT WHILE SIMS ARE LIVING THERE

Enter the Lot. Open cheat box. changelotzoning apartmentbase
Save
Exit to Neighbourhood Screen
Re-enter the Lot and it should now be an apartment building.

If it doesn't work - one thing that worked for me was to check that the apartment doors work the right way. The doormats should be on the OUTSIDE of the door...

Also check to see if you have an old version of the SEDD buyable mailbox.

To enable buy/build mode is similar to the University cheat:

Open cheat box boolprop aptSubLotSpecificToolsDisabled false

To Open cheat box. Note that some cheats requires that you also enter the Postprocessing cheat further down.

Apartment Life Expansion Cheats:

To make a Residential or Commercial property into Apartments:

Enter the Lot. Make sure there are no Sims living in the Lot.

Open the cheat box and enter changelotzoning apartmentbase

To reverse the process,

Open the cheat box and enter changelotzoning residential

Or to make a Lot into a Community Lot,

Open the cheat box and enter changelotzoning community

Remember that with all the changelotzoning cheats you have to exit to Neighbourhood Screen and then re-enter the Lot to see the changes.

DANGER WILL ROBINSON
DO NOT rezone a beach lot into an apartment lot. This will severely corrupt the ocean and Lot terrain and is strongly not recommended!

To show the Lot's Class Open the cheat box and enter printLotClass
To change the Lot's classification, Open the cheat box and enter changeLotClassification [low | middle | high]
The lot will remain this classification until the following cheat is used.
To clear the Class value of the (previously cheated) Lot and reset it to the correct calculatated value Open the cheat box and enter clearLotClassValue