An Indoor Childs Playhouse makes use of unused space in a room.
More details available at http://www.creatrope.com/blog/make/diy-indoor-playhouse/
Step 1: Find a Good Spot for the Playhouse.
In my case, it was an elevated section of a room that had a portion of a corridor below it. It was otherwise unused. Since it lies against two walls, only two walls need to be constructed.
Step 2: Cut and Install Materials, Ladders and Slide
Purchase, then cut (measure twice!) Masonite that can run from the floor of the playhouse to the ceiling. Place exterior reinforcing posts at edges and running across both ceiling and floor. Connect these posts to each other and ceiling and floor.
Step 3: Cut and Install Materials, Ladders and Slide
Cut doors and windows (saving the door piece for a hinged door), and make a ladder from 2x4s and dowels.
Note: The safe building, engineering and use of the following project is entirely your responsibility. The description is how I built it and may not be appropriate for your home layout or intended audience. The guidelines for use and parental supervision are identical to what they would be on a backyard or outdoor playground. It is not inherently safer just because it is indoors.