Mod The Sims - The Treehouse Tree

1) Place the tree on a lot - make sure you don't place it too close to the edge of the lot. You need to leave room to add the walls and roof without getting the "location out of bounds" message.
2) Place columns around the tree - I use 4 - so you'll get the gridlines and can place flooring.
3) Place flooring of choice, add walls, windows and a door if you'd like. (In my example pic there's a little porch so I added an arch, although it could've been a door. )
4) Now- since your sims need a way to get up into the treehouse, you'll need to add some sort of staircase or ladder. Since a staircase would look a bit silly, I went with Marvine's animated ladder. I would recommend using her ladder as it works well, takes up little space AND doesn't look out of place!
5) Decorate the interior any way you'd like, just be sure to keep objects along the walls. You don't want your sims stomping and complaining that they can't move around in it!
6) Optional - remove the columns. You don't really need them.

The best size for the treehouse is 3x4 or 3x3. It's tiny, yes- but you can put a lot more than you might think in that little house. It IS meant to be a playhouse, after all.

If you make it a bit bigger, the tree will stick through the walls quite a bit- but that just might be ok, since it's a house in a tree, lol.

Note about placing the ladder (or stairs, if you insist) - you'll need to place it on a tile so the tree itself doesn't cut through the ladder. If it does, your sims might not be able to climb it.