Rather than bore everyone with details of wood amounts, nails etc I've just given step by step photos of this project so that it can be seen how simple these playhouses can be built with little fuss or mess. I did use tantalised wood however for durability(4x4, 4x2 and 2x2). I had no real plan to go from, i just designed it as I went along.
Step 1: The Building
I did use concrete for bedding in the posts because this is top heavy. My size eventually was just to suit the 8x4 sheets for the roof too minimise wastage. 8x2 sheets of chipboard for internal floor, and deck boards for outside. These little kids know how to test the stability of playhouses so I built it fit for an adult in case the wife kicked me out. I roofed over the ply with green felt. Sealing the clout holes and joints with all weather silicone.