Charlotte and James Lewis first decided to build themselves a commodious but compact, an elegant but modest house when they realized their daughters were growing up.

"We had in mind sort of a condominium," Lewis said, "so Charlotte and I could live in a core of rooms and shut up the rest of the house when the girls were away and we had no guests."

Then the Lewises met architect Arthur Cotton Moore, whose ideas are on the same grandiose scale as his own 6 foot-plus stature. Moore is better known for such projects as the old Post Office building on Pennsylvania Avenue, the controversial Georgetown waterfront and Canal Square. He has designed only four houses.

Three years and more than a million dollars later, the Lewises have a 9,000-square-foot house (not counting the decks and the eight-car garage) on six levels with five bedrooms, an 80-foot living-dining room, a Jacuzzi room and a large swimming pool, all on a lot with 400 feet of Potomac River frontage. Lewis, a lawyer and a developer, has also hired Moore to design Tycon II, a huge office building at Tysons Corner, as well as Tycons IV and V.

"We began to see what a spectacular house this site deserved," said Lewis, settling down comfortably on the built-in sofa in his new house, as he looked out over the Potomac Palisades.

The site, on the Virginia side of the Potomac, is part of the old Merrywood estate. The cliff is the sort that inspired castles on the Rhine and mansions on the Hudson. The site is protected by scenic easements, and the design had to be approved by the Department of the Interior. The lot, zoned for an apartment building, had to be down-zoned.

"We only heard about the property because we had a friend living nearby," said Lewis. "It was hard to walk it because of the rough terrain and the thick trees. There was only one little level spot. We wanted to build further down, to be a part of the trees and the river."

Moore decided to follow Frank Lloyd Wright's dictum that a house should be of the hill, not on the hill.

Though the house in many ways looks as though it has just been ferried here from Krypton, it is like a contemporary castle. Its lines are sharp, with angles and triangles shaped to fit the crag. From below, the house looks rather like a ship, bulging at the middle with a narrow bottom section. The futuristic design resembles another house Moore did on another cliff near Chain Bridge Road. The style is not modern or post-modern but Moore modern.

The meticulous workmanship matches the spectacular design. Lewis used his experience in building to extract the best from contractor John Georgelas' crew. Railings with compound curves, curving cabinets, marble and mirror mantels and overmantels, elaborate tile work, cypress siding bleached to the color of the rocks -- all these took money and time. "It wasn't easy, but we all learned a lot," said Jim Oakley, the project manager.

From the main road, one might not even suspect the house is there. Reaching it requires traveling down a long, steep private drive. Even in the entry court, the house doesn't reveal itself. All that can be seen is the eight-car garage and what the Lewises call a porch: a roofless, circular space frame, providing a link between the inside and out.

You almost need a road map to find your way through the house. Charlotte Lewis says, "Even I have been confused." The elevator, an elaborate security system and an intercom help. But inside, it becomes apparent that the Lewises have their condominium after all. The four-zone gas heating system is so organized that parts of the house can be cut off when they are not in use.

An entry hall with a semi-circular ceiling with clerestory windows leads into the house. There, a half-moon window repeats the curve of the dramatic staircase that leads down to the main level. To the east is a guest suite: a huge walk-in closet, a bath, an almost triangular bedroom with two views of the Potomac and a small kitchen with a pass-through that can serve the adjacent deck.

Down the hall is the master suite, a true apartment of its own. A sitting room and bedroom are divided by a fireplace, which serves both rooms, and folding doors. The Lewises -- with apologies to Moore, who disapproved -- have installed a gas fire. The wall above and around the marble fireplace surround is mirrored in protuding sections.

Off the bedroom is a small kitchen on the other side is a sybaritic bath with a huge black tub with gold faucets surrounded by blue-black tile. On another level are the wet room/showers and a large Jacuzzi-equipped hot tub. Sliding glass doors lead to a big free-forn swimming pool. The brick floor extend outside to become a terrace.

A bedroom on the pool level belong to Linda, the Lewises' 19-year-old daughter. She has a big brass bed and a walk-in closet. Down another five step are two bedrooms and a den, each with its own bath. One belongs to Laura, 21 who works for her father.

The kitchen itself has an expansive sitting area in addition to the informa dining space and a dining deck. Curving wood kitchen cabinets, as carefully made as furniture, are shaped much like the house. Everything is here, from a Jenn-Aire grill to a boiling-water tap and a separate bar sink, all set in a white tile counter top.

The 80-foot living-dining area is a series of curves. A pale wash of pink in the living area and blue in the dining area are the only wall colors. Every where else is white, and the furnishings were selected with the help of Bonita Peed. The dining room is raised, like a stage, off the living room. At the other end of the living room is a raised alcove with a vaulted semi-dome roof and a built-in curving sofa upholstered in red A bookcase divides it from the living room. The wide curving steps served as seats at a recent party for 70.

The living room proper is punctuated by round columns set between almost 80 feet of sliding glass. Its fireplace is surrounded by an elaborate production of pink marble and glass. Carpeting covers the whole floor.

On the lowest level is still another bedroom, with a sitting area, for daughter Louise, 16. Louise has another of the large tubs -- hers is white and green. On this same level is an immense recreation room, equipped with a large pool table and a long curving bar. "We had to do so much excavation for footings, because of the soil, that it didn't cost that much more to make it usable space," said Lewis.

The tour finished, Lewis, settling back against the red couch under the semi-dome, looked again at the river. "I think this house is the best work Moore has ever done. We set out to do something different. We put a lot of thought in it in an effort to do something extraordinary. Sure, we put more money in it than we intended, but we didn't put a limit on Moore or the contractor. As a result, we have a house that's a work of art. Other people have beautiful houses, but ours is spectacular."