Houses/Lots Disappear

This happened to me recently. I was on the blank lot in San Sequoia across from the library. I built a house and played a while. My sims came into some money, so I built a small studio apartment on the same lot for a butler. I traveled for my Sim's job (interior decorator) and when she got home from work, my addition was gone. The addition exists in world view, but not once the lot is loaded.