WHEN DON (Francis D.) Lethbridge, dean of Washington architects, built a house for his family almost three years ago every architect in town could hardly wait to see it.

The long awaited house is best described as being very much like Don and Mary Lethbridge: contemporary, cofortable congenial and conversational, but not casual nor careless nor capricious. It's a long way from the prevailing architectural fashion of building assertive, dramatic houses. Instead of screaming a "statement", the house carries on a conversation. The house doesn't raise its voice. It doesn't have to.

"We didn't want the house to be contrived," said Lethbridge, as we were confortably ensconsed in the lightfilled living room. "We thought it would be good to do a house that would be difficult to date. We wanted to avoid fashionable cliches, and build a house that doesn't jar the neighborhood.

"I like the older houses in our area. Kalorama has a great many very individual houses but houses that fit into the neighborhood. One of my favorites is Alice Pike Barney's Studio House (now owned by the Smithsonian).

"Even so, I aimed for a house with a character of its own. As you look at our house, you can see it isn't traditional in any way, but it has the same kind of solidity and air of conservatism as the other houses. Yet it has strong design qualities and it doesn't sacrifice the things we want. There's no other house that's like it.

"It's a house built specifically for a particular lot. I would have built quite a different house in another place." The Lethbridges chose the Kalorama site, at 50 by 150 feet, one of the larger city lots in the area, because h e has always had his office at Dupont Circle and he wanted to walk to work. The site also suited Mary Lethbridge, head of public information at the Library of Congress. They had lived in Chevy Chase while their children were in school, but now all are grown.

With all the attention to aesthetics, the house is also energy efficient -- a true passive solar house. It was also relatively economical to build because it's compact, costing approximately $50 a square foot three years ago. Obviously it would cost more now.

What you see from the outside is a polite building, with an air of reserved formality that carries through the house. It is not a bare feet and beer house.

"I chose the brick because it was closest to that used by other houses in the neighborhood," said Lethbridge. "It's sand-cast, that's important, because it gives it more texture. I thought the roof should be tile, not only because that's common to the neighborhood but also because otherwise it's rather a plain facade."

The 4,500 square foot house is designed, as all city houses should be, with the principal level, the piano nobile, on the second floor. The garage is neatly tucked into the first floor. Visitors come in through a recessed porch, cut out of the mass of the house. "I didn't want to leave it hanging out," said Lethbridge. The house comes right up to the building line. "It seemed obvious as well as practical," he said, "to leave as much private garden area behind.'

We came in through the large front hall with its Jotul wood stove and quarry tile floor, both practical choices for entry halls especially in the winter. The windows go across the wall but start hgh enough to give a feeling of protection.

At this point you have your choice of going down to the study or up to the main floor. We went up the handsome oak stairway, carefully put together as an important object of design.

"The staircase goes up through the whole house," said Lethbridge. "The fact that it is so open makes the house seem less formal. We particularly enjoy looking out and seeing the changing light through the big areas of glass as we go up and down it." Lank Woodworking built the staircase and did much of the other woodwork. Columbia Woodworking did the dining room cabinets. E. A. Baker was the general contractor.

The living room is 12 1/2 feet tall, 22 feet wide and 16 feet deep, so accustomed as we are to 8 foot ceilings, it seems higher. A wall of glass stretches across the living room. Below openable transoms are sliding glass doors opening onto a small balcony. The glass is shaded in the summer by the overhanging third floor balcony. A tall ceremonial arch opens between the living room and a small, cosier library on the west side.

The entertaining rooms are furnished with a careful collection of important traditional furniture and classic modern pieces. The living room is rather formal, with a magnificent inlaid early 18th century chest, though the white sofa and the Stephens chairs are comfortable and the oriental rugs add color.

The much smaller library with its bookshelves (made by Lamar and Wallace) going all the way up to the ceiling, is quite warm and intimate, though not casual. The library walls are covered with the Lethbridge's fine collection of architectural prints and engravings, mostly about Washington and Nantucket, where they have a vacation home. A splendid 18th century Philadelphia bombe chest stands on one side.

The dining room and kitchen are at the terrace level, a half flight above the first floor and below the living room. The dining room has a fine early 19th century New York table with mid 19th century English chairs. The sideboard is a real prize -- 18th century American.The desk is Philadelphia 1800, made for a friend's family.The glass front china cabinet is blond, a piece the Lethbridge's commissioned for the house.

The kitchen, which flourishes with the help of Prescilla Newsome, has a handsome Vermont maple butcher block table from Jarvis kitchen shop at 20th at I Street. The picture tiles on the wall come from Lisbon, a recent Lethbridge trip. Besides two big ovens, the kitchen has an immense Sub-Zero commercial sized refrigerator/freezer with a stainless steel door.

The Lethbridges are often hosts to peasant buffet suppers for visiting authors and architects. They are as interested and adept at gastronomic taste as they are at visual taste. Lethbridge is a great buyer of kitchen equipment and Mary Lethbridge is a great cook.

Sliding glass doors lead to the terrace. "I raised the windows up a bit," said Lethbridge, "because I like some separation from the terrace when I sit in the dining room."

The terrace is a joy, broad flagstone ending with an expansive fish and lily pond on one side and a planting area on the other. Steps lead down to a square of lawn, and then, on the other side, a summer house or pergola. "You look at the pergola from the house and the house from the pergola," Lethbridge said.

Lethbridge and his architect son Christopher, who helped his father with the plans for the house, built part of the pergola themselves. But then Chris had to go back to the University of Virginia (he's graduated now and working in his father's office), so his father had a carpenter finish it. A new clay sculpture, a totem by Roxford Brown, a Cranbrook professor, stands in the garden.

New redwood benches and a table are currently under construction by Lethbridge in the good-sized workshop on the lower floor.

The view of the garden from the bedroom is an extra dividend. "I was surprised," said Mary Lethbridge, "at how much I enjoy sitting and reading in the bedroom and looking out on the garden." On their bedroom level is a dressing room, a marvel of orderly storage, and a bath.

One flight up is an informal living room, with a wall of glass opening out into a balcony. Two bedrooms and a laundry room with facilities for making breakfast complete the floor, so it can be used as an apartment by visiting children. On the top is what Mary Lethbridge calls "the archives," storage space a plenty. It could also accomodate the visiting grandchild. Another balcony opens on the garden side.

The Lethbridges have four children: Mimi (Mary), an artist whose anniversary present to her family, a soft sculpture angel, hangs in the place of honor on the staircase; Cathy Daniel, who has presented the first grandchild; Peggy, who's working in anthropology at the Smithsonian, and Chris, the architect.

"Someone asked if we were building a smaller house, when we moved from our big old 1903 house in Chevy Chase. And I said we couldn't because we were expecting children to not only come home to visit, but bring grandchildren." Currently Peggy is the only one in residence, Chris having just acquired an apartment.

On the lower level Mary and Do Lethbirdge have a joint study, with a system of metal bases supporting stock solid core doors. They provide a place for his drafting board and her typewriter as well as neatly stacked magazines.

The study is saved from feeling dark by high windows topped with a slanting glass roof that goes across the west wall. A planting bench below holds potted flowers.

The eqiupment lives in a room off the study which also serves as workshop. The house is cooled and heated with six tons of Carrier heat pumps: remarkably energy efficient thanks partly to eight inches of insulation in the ceiling and 3 1/2 in the walls.

And, the house's orientation saves bills. The house is a true passive solar house. The overhangs over the principal glass walls, most of which face south, keep out the low summer sun and let in the high winter sun. Three solar panels heat hot water.

Only a few windows are on the east and west where the sun is harder to control and where privacy would be infringed. All the glass is double insulated. As a consequence, fuel bills, including lights, heat, cooking and laundry averaged $135 a month.

To understand the great interest in the Lethbridge house, you have to know who he is. In the 30-odd years Lethbridge has practiced architecture in Washington he has designed a great many buildings including houses, both custom and development. Tiber Island, Carrollsburg Square in the District, Pine Spring, Holmes Run acres in Virginia, and Carderock Springs and Potomac Overlook in Maryland, among others are all projects he worked on with his former firm.

The old firm, then Keyes Lethbridge and Condon, has been called "the Washington post-graduate school of architecture."

More than 14 firms in town have partners (including such luminaries as Hugh Newell Jacobsen, Wilkes and Faulkner, Hartman and Cox) who started their careers working for Lethbridge and his former partners, Keyes, Condon and Florance, and their earlier partners, Chloethiel Smith and the late Nicholas Satterlee. Altogether the group, with Charles Goodman in Alexandria, are the people who brought modern architecture to Washington. The firm has won more than 25 local honor and merit awards from the American Institute of Architects.

If Lethbridge helped remove the heavy hand of cutesy colonial reproduction architecture from the city, he was a pioneer in awakening Washington to the necessity of preserving the good old buildings we have. He often quoted the code: "preservation is better than restoration, restoration is better than reconstruction."

He organized the Joint Committee on Landmarks to identify the buildings we should be saving, and he served as chairman and member for almost 20 years. He's on the design review board for the Federal Reserve System, and has just finished a term on the review board of the State Department's Foreign Buildings Operation.

Lethbridge has held many offices in both the local and national AIA, of which he is a fellow. For many years he was president of the AIA Foundation, owner of the Octagon.

Mary Lethbridge is as well known in her circles as he is in his.Many say she's one of the two best government public information directors in town. She heads public information at the Library of Congress. She is credited, along with other accomplishments, with saving the new James Monroe library from the greedy hands of Congress, who wanted it for more office space.

And what would the Lethbridges change about the house?

"Nothing," they both say.

Now in their landmark 1840 Nantucket house, there's the matter of whether the bay window, added many years after the original house was built, should or should not be removed.