My daughter, a 'tween' at the time, really wanted a treehouse. We didn't have any trees that we wanted to burden that way so she got a house 'in the trees'. The basic wood was all free from construction friends. As were the windows, cabinets, sink, stove, door, etc. The furnishings were all free from friends also. We paid for nails, nuts and bolts and roofing and siding, insulation, wiring, plumbing, etc. Still need to add on a bathroom and closet.
She will never live in it as originally planned. There are prerequisites to earning the privilege and she's not close. So we have a nifty little guest house.
I don't remember the dimensions. It's close to 15' by 18' with a four foot front porch.
(cross my fingers that I did this right)