NEW YORK (FORTUNE Small Business Magazine) - Anyone can nail a couple of two-by-fours into a tree and call it a fort, but if you seek a two-story treetop home with plumbing and electricity, call Treehouse Workshop of Seattle. Used for everything from vacation homes to backyard retreats, the houses start at $18,000 for a basic box to $335,000 for a 1,600-square-foot two-bedroom with fireplace. The eight-year-old firm averages one new project a month. The downsides? Co-founder Jake Jacobs, 54, has had to repeatedly convince insurance companies that workers dangling in treetops are safe. He also fields calls constantly asking if he's for real. "It's one in 100 who's actually serious about building something," he says.

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