I plan on having lots of kids, at least so if space is limited, I just love the idea of having a loft bed built into the eaves of the house with two single beds below. It leaves a lot of floor space too, so no need for a toy-room.
This is a perfect combo of my two favorite ideas for that space under the stairs leading to the basement. A reading nook AND book storage and shelves . assuming that there is enough space for this there.