The Minister's House in Crossville, Tenn., is 10 STORIES HIGH, over 97 feet tall, and supported by six full-grown oak trees. If you're a total purist about your treehouses and believe they need to be entirely off the ground and supported only by limbs, then this doesn't qualify, but screw you because it's awesome.
You can see more photos here and here, and you know that you want to.
