16TH-CENTURY chateau in lower Belgium with a moat, towers, a dungeon, a secret tunnel and a secret room--but unfortunately no baths--started Frank and Mimi Hodsoll on their building career.

Today they have a tower, if not a chateau, on their spectacular McLean palisades house. Don Hawkins was the architect, the Hodsolls (with their Belgium experience) the contractors on the house overlooking the Potomac.

Now that the McLean house is finished, Frank Hodsoll has gone on to head the National Endowment for the Arts. When he gave a party for his first arts council meeting in the McLean house, I.M. Pei, the architect of the National Gallery's East Wing, expressed his approval.

"We were glad we had the house," said Mimi Hodsoll. "It was like our credentials in the arts." (Frank Hodsoll's arts background had been questioned when he was appointed to the council.)

Recently, the Hodsolls talked about their experiences while he was a U.S. Foreign Service officer, serving at Casteau, Belgium. They talked with visitors in the white, sky-lit living room of their new house, overlooking the Potomac. Hawkins, along to talk about the new house, settled on the curving sofa in the tower-like end of the living room, awaiting his turn.

The chateau was not your ordinary remodeling job, Frank Hodsoll said. "We found the secret room when the rotted floorboards gave way and I was almost deposited in the floor below."

"The secret room had 1940 newspapers and a bed in it. We found out later that our landlady, a Nazi sympathizer, had run sort of an underground railroad to hide them. Anyway, we fixed up the chateau in great style with two bathrooms--all for $2,000, deductible from our rent. We could have bought our chateau, the landlady's 13th-century manor house and some land for $60,000."

When the Hodsolls returned to this country and bought a Georgetown house with 27 working gas lamps, they found that neither $2,000 nor $60,000 would buy very much fixing-up, so they sold the house.

Then they bought a house in Arlington, where landscape architect Lester Collins designed for them an above-ground swimming pool shaped like a whale with an adjacent oak tree as the spout. But that, too, cost more than the chateau, the manor house and its land. So they did a simpler remodeling, opening up the rear of the house to a less grandiose pool.

But the Hodsolls still had their dreams of glory. They looked around and found an older house, designed by architect Don Hawkins.

"I knew they would be thoughtful clients," Hawkins said "when Frank asked me why I'd designed the dormer and the chimney juxtaposed like that. It was one of those designs you regret later. But no one else but Frank ever caught the error."

Instead of buying the house, Frank Hodsoll took a sabbatical from the State Department, he and his wife bought three acres of land on a steep hillside off Route 123 in Virginia, they hired Hawkins, and proceeded to build two wood-shingled contemporary houses, in the hopes that the sale of the first would help finance the second.

The two houses are designed not to be startling, but to nestle into the heavily wooded site. The first house, sold before it was finished, is situated at the beginning of the Hodsolls' driveway. The Hodsolls' own house is at the jumping-off place, at the edge of the steep palisades.

The Hodsoll house is 4,400 square feet. A contractor bid somewhere around $400,000 to build it, Hawkins said, not counting additions that grew during construction, the pool and its deck and the long driveway. The Hodsolls turned down the bid, but after it was all finished, Frank Hodsoll admitted they should have taken the contractor up on it.

"The first advice Don gave us," Hodsoll said, "was not to be our own contractor. We know now why he said it. Even so, I enjoyed it. If Jim Baker hadn't called me to campaign with Ronald Reagan, I might have gone into the building business." He paused. "Just in time for the building depression. I was lucky."

Most of the problems when building the house were caused by the steepness of the hillside. The land slopes off toward a ravine with a pool on one side and the river on the other. Another problem came, Hodsoll admits, because he did his own surveying, with an eight-inch error. His calculations, as well, unlike Omar Khayya'm's, were not always correct. One time, a surveying error caused them to hit a gas line. They had to call the fire department to stand by.

"One of the worst days," Hodsoll said, "was when we lost the bulldozer in the creek. And the owner wanted it back."

The Hodsolls had set up a trailer that teetered on a hillside, to Hawkins horror. "I was sure," said Hawkins, "that we'd lose him over the cliff eventually. One day when we were talking on the phone, I heard a crash. I thought, 'It's happened.' I didn't know whether to call an ambulance, the police, or a crane. So I just hung on. In a few minutes, Frank came back on the phone--it was only the chair that gave way."

And, of course, the gravel trucks and the crane got stuck in the mud.

The Hodsolls ordered their lumber from Louisiana. But when the big tractor-trailer arrived at the top of the hill, it was obvious it couldn't make it down to the building site. They had to hire a forklift and ferry it down. "At $50 an hour for the tractor-trailer, we ran it down in a hurry."

The Hodsolls found that if "you don't hire laborers, you have to be laborers." Daughter Lisa and son Frank helped their parents dig the endless ditches and post holes, clean up every night and move materials from here to there.

"The best day," said Mimi Hodsoll, "was when we found the local high school had young fellows who would hire to help."

The Hodsolls were by no means happy with all their craftsmen--one error meant the windows in the prow of the living room were eight inches too high, cutting off some of the river view.

They had their favorite craftsman. "Mike Elmore, the dry wall finisher, saved us," said Mimi Hodsoll.

The house would have been complicated even for an experienced contractor. Because of the steepness of the site, Hawkins wrapped the house around the cliff.

You come down the paved roadway past a shelf built out from the hillside to hold a large sculpture by artist William Pye to an entry court, designed by landscape architect Lester Collins. Large boulders are sunk to form steppingstones. The white pool, well below the road level, was built by Moeller Pools, with Bill Moeller's trademark pool cover. A pleasant arbor is on one side and a wooden deck on the other, hiding the fact that the pool is partly above ground. The effect is quite nice, like a woodland pond.

You enter where the main house joins the wing. A long hall goes the length of the house, providing wonderful storage space. In the living room you first notice the high cathedral ceiling, the most impressive feature of the house. The ceiling throughout is made of unfinished pine boards. A long skylight runs the length of the main wing, bringing in a glorious, tree-dappled light. Hawkins turned an ordering error to a great advantage, by interspersing the long skylight with a series of lights only a board wide. "We think it adds to the dappled effect," Hawkins said. The light makes the house always seem bright and cheerful, even on gloomy days.

The living room is almost square, but you step down into a semi-circular seating area with a tower-in-the-trees feeling. Opposite is a fireplace with a display cabinet on one side. A wall of sliding glass faces the stream and the woods. Only the screens prevent you from stepping out into nothingness.

The room is filled with art, both painting and sculpture, including another by Pye. When the Hodsolls lived in Chicago, Mimi Hodsoll, now a saleswoman with Better Homes Realty, ran an art gallery--"and spent my salary on the art."

The dining room is next down the hall. A whole wall of storage in the hall holds the Hodsoll's splendid collection of china. Another display cabinet holds jade objects. The room has another sliding glass wall. The dining table once stood in Skipwith, where Thomas Jefferson was born--"But somewhat after his tenure," said Mimi Hodsoll. The table, as did many of the other fine antiques--an Italian inlaid chest, a handsome sideboard, Chinese Export tableware and the jade collection--come from her family.

The kitchen is larger than the dining room, with not only a wall of glass, but also a small deck. An island in the middle of the room holds a Jenn-Air grill and two gas burners and a long table that serves six. The refrigerator, twice as big as Hawkins had expected, is accommodated in a cabinet with another set of cooking units. An oven and more storage is on another wall. The washer, dryer, laundry sink and more storage are across the hall. The garage has a door which opens into the hall.

A three-story tier of three bedrooms is stacked in the wing, each with a glass-roofed bay toward the view, each with its own bath. On the entry level, the guest bath backs up to a powder room. The guestroom is up a half flight of steps.

The middle level has its own door to a huge flight of steps into the woods, matched by a similar flight on the the pool side. Hawkins likes to think of wedding parties descending to a woodland ceremony. Others might think of a Busby Berkeley dance.

At the foot of the steps is the disco, with its own grand piano and semi-circular floor for dancing, with flashing lights and mirrors. If the dancers are too fatigued, they can go down the hall to the hot tub, with its adjacent changing rooms, and then go out to cool off in the pool.

Down the main hall is a study with two desks for Mimi and Frank Hodsoll, their commodious bedroom, dressing area and bath.

It isn't a Belgium chateau, though there is sort of a tower and you could think of the stream as a moat. But the Hodsolls certainly have settled themselves well in the McLean palisades.