Daydreams in sunlight filtering through oak leaves, a rope ladder, secret passwords and BEWARE signs -- there's nothing like a tree house. It may cost over $100,000 to buy and average house these days, but you can build one for under $100 in a tree. It's a perfect home-away-from-home at home, a natural hideaway from bullies, bill-collectors and the rest of the world passing below. And what better place to put the kid who crayons on the walls?

"The Tree House Book," by David Stiles, complete with step-by-step illustrations, was our bible for three weekends of contruction. The only hard part was convincing the kids,

Nine-year-old-Marc:

"I don't have time for a tree house. I have Sunday school and soccer and guitar. When would I use it? Nobody else has a tree house. I never heard of a tree house."

Sixteen-year-old Bonnie:

"If you put a hammock in it, maybe I'll use it."

Eighteen-year-old Darren:

"You're kidding."

But tree houses aren't just for kids. As Stiles points out, an elderly Greenwich Village couple lived in one year-round during the '30s. A whole community of secret tree houses is said to exist on the California coast, compete with electrity and plumbing and unknow to the tax assessor.

But we just wanted one on three trees in our back yard.

"You'll love it," we told the kids. The work began.

Stiles has noted two approaches to building a free-form tree house. The first is to carry a bunch of boards to the site and go at it spontaneously. This is recommended for people who like to deal with problems as they arise.

The second approach is for people who need to know what they are doing ahead of time -- like us. So we followed one of the book's four designs. But choosing the tree is the most important decision. It has to be strong at least one foot thick at the base for one tree, eight inches thick if two or more trees will support the structure.

After much deliberation and one family council, we picked three trees, 50 yards behind our house, within six feet of one another, for the foundation. These were augmented by a 4"x4" creosote-treated post, guaranteed to keep away rot, mildew and termites, at least for one generation. But tree houses can be built on one tree or even four trees; and, if you don't have trees, an airy gazebo could rise atop four creosote-treated posts.

We found that tool requirements are minimal; A hand saw, drill, wrench, hammer, level, square, nails, bolts and wood are all that's necessary. Experience isn't needed, either. My husband became construction boss solely by virtue of having completed a usable tie-rack in eighth-grade shop.

When building a tree house: 1. Gather your tools in advance and mark them with bright tape so they don't blend with the underbrush. ("Where's the hammer? I just had the hammer!") 2. Use a level; measure more than once. Also, mark wood on one side. When it's remeasured it'll be at the same point. ("That's not the figure I got when I measured it.") 3. Wear gloves to prevent splinters and save your hands.

How high should a tree house be? Well, the first falling out usually occurs while the tree house is being built. However, it's just as easy to feel lofty one foot off the ground as six. Stiles recommends building a tree house low enough to be reached when standing on the ground, but it's only necessary to reach the platform or floor -- once that's built, it's possible to stand on the platform and construct sides, roof, windows, doors, porches, railings and even a crow's nest.

Because the platform is the most important part of a tree house, we devoted one entire weekend to building it. A platform must be level. Since trees are the platform's foundation, and nature is not symmetrical, this takes much measuring and mitering. But if this is done properly, measuring and constructing the rest of the tree hose atop the platform is a breeze.

Our platform is about six feet square, and its sturdiness derives from six joists bolted securely to the trees. The joists, 2" x 6" x 8' pieces of builder's-grade pine, were store-bought.

But atop the joists, six boards of one-inch-thick barnsiding were left over from a previous basement renovation. We foraged for old nails, boards and shingles. Barnsiding walls of four to seven feet meet a sloping plywood roof. (This slope allows the rain to run off.) Leftover wood shelving will serve as future built-in benches. And, later, store-bought shingles, together with some found hanging around the garage, will dress up the roof.

And a funny thing happened on the way to building our tree house: Everybody wanted to help. Marc even recruited future club members. Suddenly, there were many small pairs of hands. They held the boards in place, carried materials to the site and applauded as each new part of the tree house took shape.

Esthetically, a tree house can be pleasing; as a family project it's fun. It can be large or small, for tots or grownups. It doesn't require heavy equipment to build and everything can be built on site. There's only one problem -- when it's finished, there usually isn't room for everybody.

Nine-year-old Marc:

"I'm the one who wanted a tree house. This is for me and my friends. You guys are too old."

Sixteen-year-old Bonnie:

"This is a place for me to read in the hammock, by myself."

Eighteen-year-old Darren:

"I thought this was suppose to be for the whole family."

And it has been.

I've lived some of my own best times in that tree house -- alone. I really don't want to talk about it as a solitary refuge after a family feud, a place to go to duck the fallout, but it is.

After one particular Sunday dinner, when the roast was pronounced "gross," and a debate began on the equitable distribution of a section in our house, I retreated to the airy gazebo.

There, sitting cross-legged on the weathered gray barnsiding, I was no longer a cook, parent, policewoman or judge. I was a kid again. A gypsy, perhaps. As I surveyed the tops of beech and oak trees from my penthouse, troubles dropped off. Bits and snatches of Robert Louis Stevenson poems came drifting by. So much for negative vibes, when you take three deep breaths after climbing seven steps into a tree house.

Next, I'm going to get a tetherball, a great direct-action toy for liberated parents. CAPTION: Illustration 1, no caption, By Susan Davis; design by Richard Whiting. Illustration 2 through 5, Building From the Ground up; Copyright (c) 1979, by David Stiles. Reprinted by permission of Avon Books.