I also recently have had this problem. It's very frustrating. I try to place objects in one room and I get the "Placement not allowed" or something like that. It's the same message you get if you try to put stuff outside of the currently active apt. It's also the kitchen which makes it even worse, cause the refrigerator optios are extremely limited, ex: Make single/many. There's no serve option. Any help with this would be much apprecieated.
Edit: I figured it out. It's the room dividers. For some weird reason when they're used the game sees one part of the divided room as not part of the apartment. I'll attach a screen shot for those who need a reference.
All you have to do is move your sims out, change the lot zoning to residential, save/exit/re-enter, remove the room dividers, change the lot zoning back to apartmentbase, save and move your sims back in. And everything should work fine!