Subdividing a large lot?

I thought about it, but haven't got to it yet. I have had a bad experience with a 2 family household in TS3 as I had an upstairs unit and a downstairs. I locked the door and allowed only the appropriate sims. They spent their entire time trying to use objects in the other unit's apartment. Therefore, I had to go back on that decision quickly.

I believe TS4 should implement a subdividing technique, such as placing a special door like TS2, so designate what is owned by who. It can be used for apartment buildings, 2 family houses, and many buildings per lot. Everyone wins.