Is a 12-in diameter tree stable enough to support a tree house?
As requested, The reddish color makes me want to ask if it is madrone, does it shed it’s bark? ,Are the leaves mid size thick or leathery and almost feel waxy? If this is what it is I would not use it for support. Madrone is very hard and cracks and splits easily, it is a large one but I would verify the type before trying to use as it might work for now but the next storm in the PNW and the tree may break apart.