A futon bunk bed is a popular way to make good use of a small space.
You can get bunk beds with either a single or full-size futon underneath. During they day the futons fold up to create comfy seating and when an extra bed is needed they quickly pull out.
They are popular for tween and teen rooms where kids like to have friends round for sleepovers but don't want to take up lots of space with an extra bed. They also work well for college students and studio apartment dwellers as they provide seating underneath the bed making very good use of space. A bunk bed with a futon means you can have visitors to stay even in a college student's room or a small studio.