IT AND KAREN SICKELS want a treehouse. They are not grade-school siblings begging their father to build them a backyard fort, nor environmental advocates plotting a yearlong stay at the top of a redwood to protest logging practices. They are just a pleasant couple from Southern California — he a 64-year-old semiretired real estate developer and she a 50-something former teacher — who want to live part time in a tree.

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Two years ago, they bought 80 rocky acres several hours from their house in La Jolla. Now that they have transformed it with apple orchards, olive trees, grapevines, wild turkeys, pheasants, even 13-foot sunflowers, they need a place to stay and enjoy it. So they are planning a treehouse. A $150,000 treehouse, to be precise.

Mr. and Mrs. Sickels had long talked about the possibility. "It's always intrigued me," Mr. Sickels said. "When you enter a treehouse, you really are kind of separating yourself from the earth, isolating yourself. We really saw it as a kind of private retreat for the two of us."

The search for the treehouse of their dreams brought Mr. and Mrs. Sickels to the TreeHouse Workshop Inc., a Seattle design and construction company. Pete Nelson and Jake Jacob, the owners, are among a handful of builders specializing in treehouse construction for adults, and they are perhaps the best known of this small group, since Mr. Nelson has published three books on the subject. (Each also has a second business: Mr. Nelson builds traditional homes, and Mr. Jacob reclaims and remills wood from demolitions.)

Since the TreeHouse Workshop was founded in 1997, the two men have built about 40 treehouses for clients, most in the $40,000 to $60,000 range. "It startles me how often people say: 'Treehouses? Cool! But you can't be serious?' " Mr. Jacob said. "Well, we are serious."

One of the charms of treehouses, he said, is that like cabooses or tugboats or spaceships, they fascinate the child in almost every adult. "A treehouse is kind of like space travel in that it's something that captures your imagination when you are a kid," he said. "Today, the average middle-aged American can't go out and become an astronaut. But you can have a treehouse."

If the TreeHouse Workshop builds the house for Mr. and Mrs. Sickels — the two sides are still in negotiations — it will be its biggest creation to date. Most of the structures are built as guesthouses, retreats or offices near a main house. Electricity and heat are commonplace, and many have a sleeping loft or futon. A bathroom, equipped with an incinerating toilet, is optional. The houses are built mostly from recycled wood from Mr. Jacob's business.

A tree is a living, growing thing, Mr. Nelson and Mr. Jacob remind clients, adding a level of complexity to construction that is absent in building, say, a fishing cabin. Before starting any project, they "interview" the trees. That means climbing into them and getting a sense of how strong they are.

"I don't want to sound New Age or something," Mr. Jacob said, "but when I go and interview a tree, part of it is just checking out if the tree is cool about this idea."

This, he explained, is because, over time, the house will literally become part of the tree it is attached to. A healthy, vital tree will treat a properly installed treehouse as it would additional branches, growing wood to support it. That is why, they have learned, heavier, multilevel treehouses are often best built in stages. The first platform is added, perhaps for a porch and ground floor, then over the next two years, the tree adapts to the load. If done properly, by the time the second level is added, the tree is ready for still more weight.

For treehouse fans, a highlight of the year is the annual World Treehouse Conference, held each Columbus Day weekend by Michael Garnier, the owner of the Out 'N' About Treehouse Treesort, a bed and breakfast in the branches near Cave Junction, Ore.

Mr. Garnier is something of a legend in treehouse circles for his almost uncanny ability to envision and then install sturdy, level foundations in trees. The conference typically attracts 40 to 60 people, ranging from builders like Mr. Nelson and Mr. Jacob to novices who are itching to build their own treehouses.