How to build a treehouse?

[kitfriendesq]

My son keeps asking when we're going to build him a 'proper treehouse' to expand his cubby on stilts in the garden so I figured the xmas break is a good time to start planning. I did stilt based job in the UK so some experience but this one feels like a proper tree-based job is in order, and the kids are noticably bigger now so something chunkier is in order!

Our old UK treehouse: 

The tree next to our cubby here is pretty beefy and there's a handy junction for the branches at about the right height so I'm thinking:

- Platform sitting in the notch of the branches, 1-2 "beams" in the notch, and then 1 either side of the tree

- Some 'knee brace' diagonals 

- Perhaps a horizontal tie to the cubby, not sure it'll really add much stability as the tree is so thick already

Most of the 101s seem to be American but essentially have chunky beams with slimmer joists on top: https://www.thisoldhouse.com/yards/21017559/how-to-build-a-tree-fort 

Questions I've got already and would love some thoughts on!:

- The US tutorials all seem to have crazy amounts of proprietary tree bolts and brackets. Our tree is mature so I'm not anticipating huge growth, for bolting the beams and braces to the trunk what sort of lag bolts might be usable?

- Economic timber options for the beams, braces and joists 

[kitfriendesq]

Awesome tips, thanks @JacobZ - I've got both an engineer and an architect in my peer group so can run it by them also  

[JacobZ]

That's fantastic @kitfriendesq, it's always good when you have friends like that.

They'll certainly be more qualified than I am to make a call on structural matters.

I'd be interested to see what they say.

Jacob