Planners are a bit like British Football referees.
Everyone loves to hate them, questions their decisions to the point of ridicule and derision , but the only way we can trust the 22 people on the pitch to behave themselves is to have a referee.
I would have assumend that if these two people are clever enough to make a living from being a Sculptor and a Painter,they would also have known that planning regulations in National Parks are fairly stringent.
Gedpipes will also know this about such regulations fronm his time living in the N.Yorks National Park.
There is a fine balance with something like this.
I would think the tree house looks fairly ok with many of us - ecological pleasing etc., it certainly looks good to me , but what happens if that same " many of us " discover someone has erected some hideous construction which has a blighting effect on our, and otherpeoples lives ?.
I am always left with a bad taste in my mouth over such cases, and there have been a few locally recently. In each case I believe ( correctly or not ) the "builder" has built,and knowingly stuck his middle finger up at the Planning Authority,instead of consulting with the Authority prior to building to see what is possible and what is not.
There is no doubt that Planners (like referees) make some bad and incorrect decisions from time to time , but in general ,they try to ensure that our neighbourhoods don't turn into eyesores, shanty towns and shit holes, because of others bad taste and "fuck-you" attitude.

Betsy at work