Room without foundation (playground for children)


I want to build, without a foundation, a room of 8mx2.5m, attached to the house, being the playground for children.

The only condition is that I can install plasterboard on the inside.
What options do I have?
Can I use some wooden posts fixed with concrete in the ground?
Can I use the wall of the house to exempt the construction of a wall?
What thickness should be the wood used for the walls (80x80), but for the roof?