After you fill your whole bar for the garden in achievements your prize reward is the treehouse seed!
How Do You Grow A Tree House On Buildabearville?
I think you have to plant an (ever green) tree somewhere and when it is fully grown go to the Lu-mbear yard and I think you can get the tree house or maybe you can just click on the tree. I havent got the tree house yet so... I don't know if that works if it does :) and if it does not :( I hope this helped maybe prob not.....