A few years ago, when the boys were four and eight, we graduated them from the queen-sized mattress they all shared to these bunk beds, with M on the top and E and Z sharing the double bottom. This set-up worked fine until sometime last spring, when Z decided he didn't want to share a bed anymore and started sleeping on the futon in our sunroom.
Our house is kind of small, with only two bedrooms (we didn't plan on having three kids when we built it), and while someday we hope to finish off the daylight basement, we needed a faster and cheaper solution before it got too cold for Z to sleep in a room with three walls of windows.
C and I finally agreed that a loft bed offered the best solution, and he finally found time to work on it in November (after I got the room cleaned up enough to bring the bed in, and bought a short dresser to fit under the bed).
Z has been so happily excited to be sleeping up high in his new space, and every morning (when they should be getting ready for school), I find him and E up there, conspiring together over something (so thrilled to finally be allowed up high after being kept off M's top bunk for so many years).
With M moving into his teenage years (gulp), he's probably going to be the next one looking for his own space, at which point we will really have to finish off the basement, but for now, the boys' dormitory (pronounced with an English accent, like Harry Potter) will have to do.