leylandii trunk growth
In our garden we have inherited a cluster of 3 Leylandii with a good 1m+space within where we plan to build a small (low level) tree house for our toddler. The tree house will be supported fully by posts and fill the space created by the tree triangle. My question is how much space do you think we should leave for trunk growth? I don't know the age of the trees but they are atleast 15 feet tall with a trunk diameter of approx a foot each (maybe more).
What would you advise?