The tree house at Alnwick Gardens is one of the biggest I know of (not that I know of many, except that mad one in Tenessee) and is made from sustainably sourced Canadian cedar, Scandinavian redwood and English and Scots pine. It has a well-reviewed restaurant doing local fare, and has wobbly rope bridges to cross. There’s also a lunch place called The Potting Shed in the top part of the tree house.
There’s also a lake with a pair of swans, the Lady Swan was nesting
The Alnwick Garden website is https://www.alnwickgarden.com if anyone is looking to go there.
All photo’s can be clicked so you can go large and see more 🙂