LOOKING UP the grand staircase of the Harris house, you expect Douglas Fairbanks Jr., rapier in hand, to come swashbuckling down, Basil Rathbone close behind him.

Not many houses these days are romantic. But the great hall of the Harris house has all the excitement and spectacle found in the great spaces of a castle. Like castles on the Rhine or the Danube, sited to be defended, the Harris house is set on a hill high above the countryside, its location a defense against the traffic below.

Natural light comes from the sun (and the moon and stars) shining through the slanted glass ceiling. Artificial light is cunningly set to make the great sweeping circles of the staircase seem to float above the slate-black floor. The tall, wide, fixed-glass walls bring in light from fixtures set high in the sun-shield ribbons that wind around above the deck.

Even the family members, who walk through it dozens of times a day, still feel the excitement of its space. The tree-top view out the glass walls changes with the season, now with its thick cover of leaves, soon with the bare branches hard against the sky. Always there is the sense of drama that comes from living high on a hill, and perching precariously on the slope.

"It's the light," said Joan Harris, showing visitors the house the other day. "It's always changing. Always different. I love it."

The house was designed by Arthur Cotton Moore, commissioned by the owners, attorney Don Harris Jr. and his wife Joan. They and their two teen-age daughters, Leigh and Meghan, have great parties with tables spread throughout the great hall. The furniture, in contrast to the house, is mostly antiques, many treasured pieces from Mrs. Harris's family. "We showed Arthur our furniture in our Georgetown house before we started the new house and said it all had to fit in."

One of the principal pieces of furniture that had to be accommodated is a large romantic gold-framed mirror, once given by a long-ago governor of Kentucky (Mrs. Harris's native state) to his daughter. It has an important place on the living-room wall, a few steps down, and across a splendid 1850 Turkish rug, from the court. Its antique curlicues are just the right frame for the reflections of the house's exuberant contemporary contours.

No one who'd ever seen the house was surprised when it recently won an Architectural Record House of 1977 award. The award given by the architectural magazine is counted as one of the most prestigious of such honors. (The house is one of two Washington winners. The other, designed by Hugh Newell Jacobsen, will be covered in The Washington Post Magazine, Fall Home Design issue Sept. 26.)

Arthur Cotton Moore has designed only two houses which have been completed so far, his own house and the Harris house. Both have won Record awards. Moore currently is hard at work on his innovative design for the Old Post Office Building remodeling, which also boasts a soaring cortile or inner court. Though the cortile in the Old Post Office is many times bigger and taller, the Harris court has much of the feeling of reaching up the sky.

The first glimpse of the house comes after one makes a precarious turn from a narrow Virginia county road. It has much the look of a medieval hillside fortress with its circular balconies, decks and sun shields.

When you pull up into the driveway, you encounter in the face of the house only a blank expression - an almost solid white stucco wall broken only by a bit of glass at the entry.

"This was deliberately planned for privacy," Moore explained. "The roadside is closed against intrusion. The house opens out on the private side where the glass is protected by the steep slope of the hill and the tall old trees."

One of the clever design features is the garage door. It's actually a secret door - a wall that slides to reveal the garage. Back in place it appears to be a solid wall. The house's white facade is softened by three magnolia trees.

The family enters the house through a greenhouse (the other side of that solid white wall) with its flourishing camellias and into a small family sitting room adjacent to the kitchen. Guests park in the arrival court and enter a solid wooden door to a glass-roofed entry hall and on into the great hall.

"The house is actually two boxes, splayed apart and connected by the hall on this level and the balcony or bridge above," Moore explained.The hall separates the house into quiet and noisy zones. To the north is the living room with its antique suede-covered chairs dyed to match the Turkish rug.

Beyond the living room's slate-covered fireplace is the peaceful study with its three walls of books broken only by clerestory windows. Here is Don Harris mineral and shell collection with the sign "please do not dust." Dusting, Harris says, breaks off the small protrusions on the shells. This room has handsome knoll Vernon Platner chairs and a glass-topped table,

South of the hall is the dinning room, the kitchen and the girls' sitting room. The dinning room has a storage wall, also reachable from the kitchen. A glass wall faces the swimming pool outside. The deck is entered from the hall and the living room through extra-high French doors.

The curving deck leads around to the house's principal outdoor space, the pool and surroundings, shielded on one side by the garage and green-house, on another by the bulk of the house and on the third by 22 acres of federal parkland. On the fourth side is the steep slope - which has given rise to Don Harris' great invention: a pulley that brings up the morning paper from the road below.

To serve as transition between house and deck there is a bench that circles around, sometimes servings as a step, sometimes as a seat, in one place as a telephone table.

Upstairs, the girls and adults have separate worlds, connected by the bridge. The girls share a bath plus dressing room. A guest bedroom and bath are also on this side of the house.

The house has no basement, but instead a wonderful number of closets, ingeniously planned. The wine closet, for instance, opens in the front hall for loading and from the library, near the bar, for dispersing. Extra glasses are also stored in the hall's bank of closets. And the utility room is tucked in behind, where you'd never know it was there.

It isn't that large a house, only a bit over 3,000 square feet. But it concentrates all the bits and pieces of hall space usually thrown away into the one spectacular space, suitable for swashbucklers or even just plain people who like a house with spirit and adventure.

The house was finished about three years ago. A house of similar construction today would cost about $40 to$50 a square foot, exclusive of land.David C. Cox was Moore's associate in charge. Carcaterra & Associates were the engineers.