Indoor playhouse above twin bed. Cost about $190. Took two full days. Built using the plans off this site. The inside play area is 42in by 39in. The plans have the mattress on the ground. I have a dog that sheds like crazy so I have her mattress on a metal frame but plant to build a low to the ground platform bed.
Make sure to get perfect 2x4s!!! No twists bends splits etc!