Hi, Carla. I built a bed by modifying the plans for a bunk bed that were in The Family Handman magazine a couple of years ago. It turned out real nice and it had 2 big drawers in the bottom. As I recall, this could be built with a study area on the bottom also. Don't forget to comply with all the safety instructions in the article.
I'm not real sure what you mean by "built-in" though. Do you mean a Murphy bed?