I WANT YOU to design us a house that soars and flies," Gunther Koening told architect Jerry Mumma about a year ago.

THe house, in a rural section of Montgomery County, is finished now. And in a sense, it does soar. "At night the roof seems to float above the lighted ribbon of glass between roof and wall," Koening explained to guests the other day.

The house's light-headed look comes from its Uni-strut system, in which the steel-roof structure actually balances on four interior concrete block piers, a method not uncommon in commercial construction but rarely used in housing.

The one-story six (or so)-room house is a modernist but mannerly cube of glass and white-stuccoed block in a sea of green grass.

Almost half the walls are glass, and there is a huge central skylighted roof. Yet the worst winter heating bill was about $150, lower than might be expected because of some energy saving measures. The house is heavily insulated. Inside, storage cabinets stretch across most of the north wall. The north wall is sunk into the hillside and most of the glass faces southwest.

Aside from all the esthetic considerations, the Koenings, both economists, laid down a strict budget for Mumma. "We didn't want any surprises," Koening said. The structure cost $50 a square foot (only the house - not counting land cost, drive, septic tank or well). The, total square footage is about 2,500.

Mumma admits that the use of the steel-framed roof supported on piers did not save money - "it might have even cost a bit more," he said. "But what it did do was give us a great deal more freedom. Since none of the walls were load-bearing, we could open it up with the skylights and the sliding walls to the outside."

The clerestory windows encircle the house completely to make the point that all the walls are free-standing, none are load-bearing. The glass rides in pockets in the framing so that if a heavy show adds weight to the roof, the glass has some place to go.

Above the Uni-strut support, the rather flat surface is covered around the perimeter with built-up roofing. But the entire center section of the roof is opened up with 16 or so 4-foot-square skylights. The skylights make the kitchen a great atrium.

The house also fulfills Linda Koenig's request of Mumma. "I want a big porch," she said. Mumma gave her a living room and a dining room which open up to become porches. On both the east and south sides, two 7 1/2-foot-wide glass doors slide into pockets, completely opening up 15 feet on either side. Screens also slide across the opening. A 4-foot overhang shelters the glass.

The house is the classic pavilion in the woods. "We were really scared when we had that big windstorm the other day," said Koenig. "Some of those tall trees really bent and clashed overhead. It was frighting to watch."

The house, for all its bouyance, also has its feet planted firmly on, or rather in, the ground. To provide privacy and to help cut the energy bills, the north and west sides of the house are sunk halfway into a gently sloping hillside - bermed, as it is called. Space for a sunken terrace and a garage was carved out of the hillside. On the open east and south sides, the land barely slopes away from the house. It is as if the house had ridden a wave and now was on the ebbing side.

A portion of the front of the house is surfaced with wood slats on plywood, an interesting textured approach, continuing inside. TO the right of the front door are a coat closet and entrances to the guest room and bath. The entry is a few steps above the 15-foot by 25-foot living room. The ceiling of the dining room and living room is a lofty 10 feet. The bedroom ceilings are 9 feet high, still a good foot over the conventional 8 feet. The kitchen, thanks to the skylights, goes up to 13 feet. The height of the ceilings adds to the sense of space and light one gets in the house.

The sweep of glass (insulated, shatterproof) goes the length of the living room, ending with a rectangular fire-place. Though you'd never know it until you went outside, the other side of the fireplace is a barbecue. "We've used it constantly," Linda Koenig said. A wooden terrace, a boardwalk so to speak, extends along two sides of the house to expand the entertaining area - the suggestion of landscape architect Lester Collins.

The dining room is around the corner from the living room. Wooden doors slide into pockets to open the dining room into the kitchen for daily use. The Koenigs enjoy formal dinners, and then the doors close to make what seems to be a wood-paneled wall. All the doors inside and outside the house slide away into pockets, completely out of the way.

The kitchen, with its teak cabinets, is really the center of the house. The basic kitchen is a broken U-shape. A long, double-faced counter moves around on heavy rollers to make any configuration you'd like. It can even join the built-up buffet in the dining room. The kitchen cabinets are kept to a square in the center of the atrium. A corridor extends around the cabinets on all four sides, separated from the kitchen only by the cabinets. A long wall of cabinets was planned as pantry, but the Koenigs found they had more books than groceries, so it now works as an extension to the library.

Most of the visible wood in the house is teak - "I saw their furniture was primarily teak," Mumma said, "so I thought the house should be compatible." Linda Koenig said the couple had to buy hardly anything for the new house. All of their old furniture was already contemporary, and fitted in neatly.

The master bedroom has a long double-basin counter made of Corian, a hard, white processed material. Gunther Koenig's closet covers the whole north wall, to help insulate the bedroom. Linda Koenig has a big walk-in closet.

On the other side of the house are the library with two desks. Gunther Koenig is chief of the economic division of the World Bank. Linda Koenig is assistant director for the Western Hemisphere of the International Monetary Fund, and they have lots of work to bring home.

A bedroom and bath adjacent to the library allows it to work as a guest suite when the two Koenig daughters are home from college.

It wasn't at all easy, the Koenigs said, to find a builder who was interested in taking on not only a contemporary house, but one with an unusual construction method - far from the conventional balloon framing.

The Koenigs were very fortunate to find a builder, William Morrison (who since has died), who was not only willing to take the job on, but was truly interested in the novel method and anxious to make the whole thing work. "He was wonderful," Linda Koenig said. "We spent many Sundays out here talking about the house," Gunther Koenig added. "And his crew was very accommodating as well."

To the Koenigs, the construction method permitting the house to open up to the land means that they can watch the deer, the birds and the squirrels close at hand. Because the house and the Koenigs have come to be a part of the countryscape, the wild creatures accept their presence.