That's awesome. How did you do it without a supporting ground floor?
It's easy to do, just build the bottom floor, a simple box shape. Then the second floor (the floating floor). Then delete the first floor.
The bottom floor is really there just to get you started on the actual height you want for the tree house, so, in theory, if you could find a tree tall enough, you could have a 'third story' treehouse.
In build mode, once you've got a start at whatever height you want, you actually don't need to have support. In fact you can build floating/levitating houses too. I built a spaceship but it looked a bit corny so that's when I thought about a treehouse instead!
@ ManiSims, I bet you could do this! Building in TS4 is really fun. I used to build a lot in TS3 but would often get frustrated because I had placed the house in the wrong spot on the plot, now we can just move the whole house and continue on without having to start all over again. Start by watching some build videos, The Sims Supply has a few, and just start playing. What I do is really basic but it's fun. There are so many talented people here on the forum who will answer any questions you've got too.