Regarding planning, unfortunately it violates pretty much everything, it's right on 3 boundaries, spanning 2 different ones behind, hence why I want to keep disruption to a minimum during construction, and keep it fairly well concealed. It will be much higher than permitted, but it would be a complete waste of time to try and keep to those, so I am going for the hope no-one gets upset route, fingers crossed. Our immediate neighbours to the side are completely happy with the idea, and as soon as lock-down is over I'm sure their boy will be keen to make some use of it too. I'm only going to have a window on the side which looks back towards our house.
That treehouse looks ace Rich, due to the height a ladder wasn't really a safe option, despite being way simpler to construct than a proper staircase. I ummed and ahhed about having 3 or 4 steps to go around the 90 degree sections, and after building a full-scale cardboard mock-up I think 3 is the way to go

I've only ordered the wood for the platform and the stairs at the moment, its difficult to design much beyond that at the moment given the complexity of the branches - there will certainly be plenty of tree inside the house with branches going in and out of walls. I see this as a good thing as it should break up the shape and make it less obtrusive.