Hi Scout, thanks for the answer. With your help, I have looked up the building code of my federal state Hesse and found the definition:
Accordingly, I could build a treehouse - which is not connected to the ground - right?
"(2)1Structural installations are installations connected to the ground, made of building products. 2A connection to the ground also exists if the installation rests on the ground by its own weight or is limitedly movable on fixed tracks or if the installation is intended according to its purpose to be predominantly used in a fixed location. 3The following are considered structural installations:
10HESSE BUILDING CODE (HBO)1. Embankments and excavations,2. Storage, parking, and exhibition areas,3. Sports and playground areas,4. Camping, tent, and weekend sites,5. Leisure and amusement parks,6. Parking spaces for motor vehicles and storage spaces for bicycles,7. Scaffolding,8. Auxiliary facilities for the static securing of construction states."