When thinking about treehouse ideas, choose a healthy, long-lived hardwood for maximum support, with load-bearing branches at least 8 in. in diameter (larger if the species is a softwood). The best trees include maple, oak, fir, beech and hemlock. You don't have to build it very high, just high enough so nobody gets a bump on the head when walking underneath it. Don't know what kind of trees you have in your yard? Here's how to identify trees.