The treehouse above, called Free Spirit Spheres, was originally conceived as a spherical houseboat. It is 10.5 feet in diameter and is suspended by nylon ropes attached to three trees on Vancouver Island.
Heidi’s Treehouse Chalet, Poulsbo, Wash.
Nelson describes this 450-square-foot treehouse as a "chalet-style fairy-tale aerie." The owner especially loves that a ramp, rather than the traditional ladder or stairs, lead to the house.
A view from the living room to the bedroom nook. The ladders go to sleeping lofts.
A queen-sized bed in the primary loft.
Looking into the living room from the bedroom.
Trillium Treehouse: Fall City, Wash.
A more modern treehouse hangs off the side of a western red cedar. This treehouse was started during a five-day workshop at the Northwest Treehouse School, which Nelson also runs.
Scranton Treehouse: Scranton, Pa.
Everyone should have access to a treehouse! At least that's what the people behind Forever Young Treehouses believe. Using volunteers, they have built 19 U.S. treehouses, all of which comply with the American Disabilities Act.
Teepee Treehouse: eastern Pennsylvania
Nelson found this treehouse through his partner Jake Jacob. It was designed with a Native American theme (note the teepee-like roof) and is made almost entirely of salvaged or reclaimed materials.
Porecatu Treehouse: Porecatu, Brazil
Nelson isn't the only treehouse specialist in the world. Ricardo Brunellie, Brazil's preeminent treehouse builder, built this house about 300 miles west of Sao Paulo.
A large extended family uses the Porecatu Treehouse as a weekend retreat.
Beach Rock Treehouse: Okinawa, Japan
This treehouse by Japanese builder Kobayahsi Takashi was constructed with the express purpose of communicating with outer space. "A sparkling beacon among treetops, it is easy to imagine the dome succeeding at its mission to make contact with alien life," writes Nelson.
The treehouse can accommodate six people in two rooms and is on the beach. Proceeds from rentals help sustain the green iguana project.
Big Beach in the Sky, Hainan, China
A three-storey structure with two double bedrooms in a tamarind tree overlooking the beach near the city of Sanya on the island of Hainan. It was devised by entrepreneur David Greenberg based in Hawaii - Hainan is twinned with Hawaii - and the name reflects a magic mushroom-induced moment he enjoyed in 1972.
The two houses, which are set a mile apart, have two storeys, wraparound balconies, coir matting and thatched roofs. Bullock dung powers the cookers and water is diverted from mountain streams. Food is hoisted up by pulley.
Cedar Creek Treehouse, Near Ashford, Washington State, US
The octagonal observatory, linked by a bridge to the house, is equipped with telescope and binoculars for star-gazing.