A small house on a roof?
I sometimes go to my grandmother's house, and there is an abandoned Karate club or something near it. Recently, I got bored so I went outside. I circled around that building a couple of times, and I noticed that I can climb to it's roof! It's not too high, about only 5 meters or so. The roof is awesome! It's flat, and then I came up with an awesome idea - how about building a small house there?
Just for the fun of it, it could be an awesome place to come with friends. I need the floor to be at least 2x2 meters, height 1.5-2 meters. I need the house to be made of cheap and available materials. Got any ideas?