The synopsis of the movie -- as quoted by FilmoFilia -- makes it clear:
THE KINGS OF SUMMER is a unique coming-of-age comedy about three
teenage friends [...] who, in the ultimate act of independence,
decide to spend their summer building a house in the woods and living
off the land. Free from their parents’ rules, their idyllic summer
quickly becomes a test of friendship as each boy learns to appreciate
the fact that family – whether it is the one you’re born into or the
one you create – is something you can’t run away from.
This dialogue excerpt refers to the same theme:
This is the site of our new house, man.
What, like a tree house?
No, like a real house. I'm moving out. We're moving out.
What the hell are you talking about? My mom won't let me walk around
the house without socks on. She's definitely not going to let us live
out here in the woods.
No one will find us out here, Patrick. This house, this land, it's
ours! We make the rules, you know? Like men!
Instead of "just living" and having to follow their parents' rules, they leave home and are "kings of their own domain".