I built a treehouse for my kids some 30 years ago. It's still there, and the tree is unharmed.
1: See what each tree has to offer in the way of support. I chose a 400-year-old yew, which had divided into three trunks, and perched the - triangular, 5 metre - platform on the branches of all three, encircling the centre.
2: The three outer bearers were carved to - loosely - fit over each branch. Then galvanised steel housebuilding straps were fed round each branch, leaving plenty of room for the branch to grow, and gangnailed to the bearers; NOT to the branch. NO metal of any sort was put into the tree.
3: Then the floor planks [all pressure-treated] were oak-dowelled to the bearers.
4: To give the kids access up the trunk, I bored 50mm holes angled downwards through the sapwood; made 50mm stepping pegs out of some of the smaller, lower branches of the same tree, and hammered them tight into the holes – hoping they would be 'accepted' by the tree as grafts. They haven't sprouted as I had hoped; but they haven't rotted either.
5: Then the Wendy House was bolted down to the floor planks with galvanised coachbolts and nuts.
LEARNINGS:
1: Even 400-year-old trees GROW much more that you expect. The branch straps were filled after 20 years, and had to be relaxed, to stop strangling the branch.
2: Even 400-year-old trees MOVE much more that you expect. We could feel the platform shifting and swaying gently when the wind blew. The over-rigid joints between bearers, and between bearers and floorboards, pulled apart. But the overall structure remained. The platform tilted slightly, as one branch grew downwards; the others straight or upwards.
3: The patch where the bearers rested on each branch had rubbed the bark off the top over a surprisingly long part of the branch; but it was otherwise unharmed.
4: Because it follows the tree's natural form, the platform [and the Wendy House] are almost invisible, even from underneath the tree itself. And completely so, from outside.
5: The kids used to play in when they were young; as a 'parent-free zone' where they could take boyfriends/girlfriends, and smoke, when they were teens; then abandoned it, in their twenties.
We sold the house recently. So now the treehouse is awaiting its next load of kids - hopefully - from the new buyers . . . . .