we have a rowan tree with about a 2ft diameter trunk. Any advice?

The bigger the tree, the bigger the tree house. I’ve seen plenty of tree houses in trees way smaller than yours. Should be fine as long as you don’t go too crazy.

Never built one, always wanted to but, they don’t always have to be supported solely by a tree.

I suppose if they’re a true tree house they should be, but you see plenty of magical structures that just incorporate the tree. Other supports are used too.

edit: not diameter, circumference.

So ~8″ diameter? Does not sound big/strong.

Have you tries hanging off a branch to see if it will support your weight?

I can climb it and I’m 13st. Thinking about it, I’d say the trunk is about a foot in diameter. It splits into two trunks at about 4′ and then grows to about 20′. I can climb confidently to about 15′.

My biggest concern wouldn’t be the direct loading from the tree house and occupants, but the increased wind loading that the tree will experience due to a large surface area structure at a height from the ground. But maybe I’ve spent too long listening to the structural engineers in my office grumbling about such things.

I’d be happy to build something fairly lightweight in V between the two trunks, extending a little beyond them if needed, if it were mine. Maybe a small platform and a simple roof.

Rowans can be susceptible to rot in the base of the trunk, and you might not see it until it blows over. Adding additional sail to it might be risky. The unions of the branches are not always the strongest either. I’d be tempted to use upright struts to support most of the weight if you want any thing bigger than a small platform in the canopy.

Also allow for the tree to grow, assuming the house will be a round for a few years.

Build the tree house / platform around the tree have the support primary on other legs. No worries.

sounds ridiculous i know, but be wary of planning permission requirements…

https://www.pinnaclearchitecture.co.uk/2017/02/20/do-i-need-planning-permission-to-build-a-treehouse/