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.
The rafters were salvaged from a local 100-year-old barn.
Solace Treehouse: western New Jersey
A classic treehouse designed by Dan Mack. In his book "New Treehouses of the World," Nelson writes, "This is the kind of place I dreamed about as a kid."
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.
Wilderness Treehouse Lodge, Alaska Eric Schmidt's treehouse, in the foothills of the Alakan range, is 90 miles from the nearest road with black and grizzly bears the immediate neighbours. To get there, you have to fly in from Anchorage to a nearby lake, and then hike for 90 minutes. Not for the faint-hearted
Treehouse Hotel, Punta Uva, Costa Rica
This two-level timber dwelling in the Gandoca-Manzanillo Wildlife Refuge, on the southern Caribbean coast, owes its existence to the green iguana. It was built by an amateur botanist who runs a breeding and research station there for the endangered species
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.
Green Magic Treehouse Resort, Kerala, India
An eco resort with two treehouses set in 500 acres of rainforest high in the mountains. Occupying enormous ficus trees - inspiration was taken from local Paniya herdsmen's platforms - access is by water-powered lifts
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.
Hotell Hackspett, Vasteras, Sweden
Built by artist Mikael Genberg, the Hackspett (Woodpecker) is suspended high in a 350-year-old oak tree. Inside, there's a single bed, hammock, kitchenette and toilet, with an outside balcony to enjoy the views. Access is by rope, and food is also winched up
West Bay Treehouse, Roatan, Honduras
High in the thick foliage of a century-old mango tree with views over the Caribbean Ocean, access is by a timber spiral staircase
West Bay Treehouse, Roatan, Honduras
There's a double bed, toilet, running water and electricity and a balcony large enough to swing a hammock. Walls are waist high, allowing 360-degree views.
Cedar Creek Treehouse, Near Ashford, Washington, US
A double-deck treehouse built to make the most of the stunning scenery in Gifford Pinchot National Forest: views of the Sawtooth Range and Mount Rainier, as well as meteor showers. Sleeps five and has a sun room and kitchen
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.