The view from above.

When Darren and Lynne Wallis moved into their house in St. Louis, Missouri, they decided their backyard was the perfect place for a tree house. Two sweetgums on the property seemed made for the task, and, at 6 and 4, their two oldest sons were getting big enough to appreciate such a thing.

The finished product, which sits about five and a half feet off the ground, sports a slide, monkey bars, a deck, a sandbox, and two swings. "It's got pretty much everything any little kid would love to have." Darren says. "They absolutely adore it."

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And who wouldn't? A tree house brings out the kid in everyone, from a 4-year-old boy to an 80-year-old woman. "It elevates you to a different perspective," Darren says. "You can see the world from a whole new point of view. Literally."

Harmful to Trees?

Unfortunately, tree houses can be a deal breaker for some arborists and tree lovers, who argue that attaching any kind of structure to or around a tree disrupts its environment and growth cycle. The purest of the purists will tell you there's no way to build a tree house without affecting the tree, even if nothing is attached to the tree itself.

"Foundations affect the root zone, and increased foot traffic can compact the soil," says Scott Baker, a registered consulting arborist in Seattle who regularly speaks at the World Treehouse Association Conference in Takilma, Oregon.

"But there's more than one way to skin a cat," Baker explains. "You can design a tree house in a way that will keep the tree going for a long time." For Baker, that means a structure that takes advantage of both attachment to the tree and a ground foundation, but only when it's done the right way.

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"The most common mistake people make when they attach to a tree," he says, "is that that the framing members of the tree house are immediately adjacent to the trunk of the tree, and of course that's untenable because the tree has to grow."

Baker also points out that any attachment to a tree, if it creates a hole, makes the tree vulnerable to decay at the puncture point. He suggests building in a long-lived tree, because they have evolved and become very good at resisting decay. Even so, he says, pick your tree wisely.

"If you own a beautiful three-acre plot up in the Berkshires and you have a truly magnificent oak tree, that's not the tree I would choose to build a tree house in," he says. "That's your magnificent tree. Leave it alone."

Baker recommends reading up on simple tree biology. "Once you understand that," he says, "it makes the whole process of trying to build a tree house with minimal impact much more feasible. If you don't know much about trees, you're going to blow it." Two good places to start: Tree Basics by Alex Shigo or Stupsi Explains the Tree by Claus Mattheck.

Baker also suggests consulting a qualified arborist. You can find one at the website of the American Society of Consulting Arborists, asca-consultants.org.

"Any arborist that's been around for a while has seen bicycles enveloped by a tree with no obvious problem," he says. "They're very adaptable. So we know that you can put hardware in a tree with minimal long-term effect, but you have to do it correctly."

In the spirit of tree houses that only minimally affect trees, Minnesota Landscape Arboretum in Chanhassen, Minnesota, held a tree house design competition last year. It sent out more than 500 postcards to architecture and design professionals and chose 12 to construct their designs for the exhibit. The one overriding rule: the structures shouldn't harm the trees.

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"They couldn't attach anything to the tree with nails," says Peter Olin, a University of Minnesota horticulture professor and director of the Arboretum. "And they couldn't put foundations down. They had to put down wood chips and blocks." The resulting exhibit boasted whimsical shapes that snaked around the trees' trunks, delighting children and adults alike.

One popular design was that of Tree Man, designed by landscape architect Marjorie Pitz. Made of sandbar willows woven like a basket, Tree Man resembled the upper trunk of a giant hugging a tree. "One arm wrapped around the tree, creating a tunnel that kids could enter," Pitz says. "And there was a raised platform inside his head, which was just a dome."

Another design, titled the Ochocasa (or "Eight House," in Spanish), belonged to Randy Larson, a designer and builder in Duluth, Minnesota. Larson had already designed and built the eight-sided tree house in his own backyard two years prior to the exhibit. When he read about the competition in a builder's journal, he consulted with an arborist to make sure his structure would meet the exhibit's standards. It did.

Larson suspended Ochocasa 10 feet off the ground by hanging it from a compression ring with stainless steel cabling, and the platform was surrounded by stainless steel mesh.

The exhibit lasted just over four months, and the Arboretum counted 20,000 more visitors than the year before. The tree houses have been taken down, but the Arboretum might reconstruct one, a bird's nest, for this year's exhibit on birds.

Many consumers have also taken an interest in trying to build "tree-friendly" tree houses. For Darren Wallis, respecting his family's tree meant building the tree house around the tree, rather than attaching it. "We only have two trees, and they're great big old trees," he says, "so I didn't want to attach anything if I didn't need to."

When Art Casolari of Florissant, Missouri, built the two-story structure in his backyard for his 5- and 9-year-old sons, he was very conscious of the tree's needs. "We attempted to penetrate the tree as few times as possible, and we left large openings for the branches to go through," he says. "We wanted the tree to last. What good is a tree house if the tree is dead?"

The Casolaris' tree house has now been around for more than 10 years, and the youngest son recently revamped the flooring, which had begun to rot. Art also added a deck, closer to the ground, that isn't attached to the tree at all.

Though his boys are older now--16 and 20--the Casolaris fondly remember the days when the family would all sleep out in the house. "It's like camping out," Art says. "It's just neat being up there."

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Accessible to All

Bill Allen shares that sense of wonder, so much so that he decided to build structures accessible to everyone--even the handicapped. His nonprofit, Forever Young Treehouses, has built universally accessible tree houses in Vermont, Connecticut, New Hampshire, and Texas. Allen hopes to have one in every state by 2008.

"We're the club that anybody can join," he says. "Our philosophy is that if you can breathe, you can get into our tree houses."

Forever Young's tree houses have 400 to 600 square feet of space, large enough to hold eight to 10 kids in wheelchairs. Most are 12 to 14 feet off the ground, with ramps of more than 140 feet to ease entry.

To Allen, a tree house is a magical thing, with the ability to lift spirits, heart, and perspective above the everyday and ordinary. "It's pretty hard to be depressed or sad or angry if you're just sitting on a tree limb," he says. When writing a recent speech, he began to make a list of all the advantages a tree house provides: "Reversal of aging, hilarity, stress reduction, insomnia relief, your neighbors will be envious, and you'll eat cookies and drink milk."

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He also jotted down some uses for a tree house: library, nap room, writer's garret, weekend getaway, study hall, and nature retreat.

The best use Allen has found so far, though, is the smile it brings to the faces of children in wheelchairs when they find themselves up among the rustling leaves--a place many never dreamed they'd be.

"There was one little boy." Allen remembers, "who'd been in a wheelchair since birth. He had a fairly rare disorder where he couldn't speak, but he understood everything. He was pretty uncommunicative, and he didn't make a lot of noise. We built a tree house in his backyard, and when we first brought him up there he just squealed with delight."

Allen found his tree houses were also popular with the senior set. Construction of a community tree house in Burlington, Vermont, drew the attention of local senior women, who were anxious to use it for card games and picnics.

At Crotched Mountain in Greenfield, New Hampshire, the tree house built by Forever Young Treehouses and Crotched Mountain Foundation has seen weddings, board meetings, and mediation groups.

"It's a very cool place to have a meeting," Allen says. "It's a good way to get above the small, nitpicking issues."

Kate Ashford writes from her home in New York City.