Washington's typical architectural headache -- the "in-fill" office building at mid-block -- has been given an atypical cure in the design for the National Geographic Society's new headquarters complex downtown.

That the design is unusual is in itself somewhat unusual for Washington. More importantly, it is very good. The Washington office of Skidmore, Owings and Merrill has managed to do a number of things with its terraced, low-lying structure, and do them well.

The problem in this case begins with the existing headquarters building at 17th and M streets NW, designed by Edward Durrell Stone and completed in 1964. Stone's 10-story building -- set upon a pedestal and sheathed in glass and vertical fins of white marble under that airy, flat, protruding top -- conceivably is the most popular private office building in Washington. Clearly it is one of the best remembered. Almost as if it were a huge lettered sign it says "National Geographic" to thousands of Washingtonians and visitors.

Stone's secret was simple. By dressing up an ordinary curtain wall structure in white marble and by calling to mind the conventions of the classical orders -- base, column, capital -- in a most straightforward way, he supplied a convenient marriage between modern architecture and the city's image of itself. He used the same association in his design for the Kennedy Center, and it didn't work because the building is way too big. By contrast the scale of the Geographic building is just about right for its site.

The new Geographic building is a different order of achievement. A seven-story L-shaped structure that cuts sharply away from the street to provide an outdoor antechamber for Stone's building, it adroitly manages to honor that free-standing white tower and to state its own emphatic case at the same time.

Like Stone's building, the new one will project an immediate visual signature, but one that has nothing to do with lingering classical associations. The main impact of the new building will come from the series of sloping cantilevered terraces that snugly wraps around the angles of its M Street facade.

Large, horizontal buildings with terraces that recede toward the top are not at all new in the modernist lexicon. They make a splash as resort hotels where catching the sun is necessary and a quick big statement is desired -- something strong and light to play against the lush mountain backdrop. The Geographic terraces make a lot more sense than that. No mere attention-getting device, they clearly define the building's form and its sympathetic relationship to the neighboring structures.

Furthermore, they do it with green. The broad, sloping terraces will be planted with two-foot-high yews that even from street level will provide beautiful contrasting borders for the granite-faced horizontal bands of the structure. SOM's famous skill with exterior detail is everywhere in evidence, nowhere more so than in the striping of these alternating bands, which will form an emphatic progression: a narrow stripe of polished granite to the broad unpolished granite band to another narrow stripe, and then a swath of green and a shadowy band of recessed clear-glass windows . . . and so on, back to the top.

A quick glance at the SOM history book suggests that the inspiration for the design is a huge headquarters building with receding, planted terraces designed by Edward Bassett of the firm's San Francisco office in the early 1970s for the Weyerhauser Co. of Takoma, Wash. The Weyerhauser building sits astride a bucolic valley, however. The real inspiration of the Geographic design was to bring the idea to the city and make it work.

The green terraces carry on a leitmotif established at street level, where a closely packed variety of trees, hedges, cover plants and flowers will, in time, form a dense visual base for the mass of the building. The planting also promises to make the southern side of the 1600 block of M Street an almost park-like enclave in the center of the city. The word "almost" should be stressed, for the "park" clearly is intended to reinforce the Geographic's sense of privacy, screening the new ground-floor cafeteria from the street and vice versa.

This is apt if unfortunate. The theme is carried through in the unsittable two-foot-high granite wall at the sidewalk that slopes back at a 45 degree angle into an elegant little fence. An opportunity was lost here. With a little effort some space could have been made for sidewalk shmoozers at no damage at all to the Geographic's coveted sense of itself. (One could in fact argue that a sittable sidewalk space, however narrow, would improve the place for all concerned.)

But then, one can't have everything. The Geographic was an ideal client in one major respect for which we can all be thankful. The building would have been a far different, and much, much bulkier, kettle of fish had the society opted to build the package permissible under D.C. zoning. In fact, the new structure occupies less than half of the space it legally could have filled.

This is partly because much of the needed space will be packed underground. Two levels of parking and a full floor for the society's photographic and exhibition labs, taking up more space than all of the seven above-ground floors, will be located in the building's huge basement, which David Childs, the local SOM design partner, refers to as "the great iceberg." As a master plan to take care of the Geographic's complex operations, the SOM design ties things together in superbly efficient fashion.

It does very well on the outside, too, for cars and pedestrians alike. Explorers Hall will stay where it is on the ground floor of Stone's building, so most visitors will continue to enter on 17th Street. But the entrance on M Street, cut away from Stone's tower in a sort of curtsey to that "statement building" (as Childs calls it), is cleverly divided by a platform carrying azaleas and Japanese pagoda trees all in a row. A terraced granite fountain, matching pavers and green borders on all sides will make this a welcoming space in all respects.

Considered as urban design, the SOM scheme plays an interesting counterpoint to the Hartman/Cox plan for the site directly across the street (discussed here last week). SOM defined its context mainly in terms of Stone's building, and its form is a result of that decision. Thus it is less successful on the western edge of the site, where it backs up against Hubbard Hall, a terrific little Beaux-Arts building at the corner of 16th and M streets designed by Hornblower and Marshall for the National Geographic in 1903.

Hartman/Cox's very different problem was to design a speculative office building that would do as little visual damage as possible to the Sumner and Magruder schools, two excellent 19th-century brick structures. Their inventive solution was to copy the styles of adjacent structures (the Victorian Sumner on the west and the Beaux Arts Jefferson Hotel on the east) and to mass a curtain wall office tower well behind the two school buildings.

Where the Hartman/Cox scheme is multi-faceted the SOM design presents a dramatic, unified facade. What the mimetic Sumner project does with mirrors, the new Geographic building does with abstraction. The one is relatively reticent, the other relatively bold. The point is that both ideas work, providing a neat lesson. There is more than one way to skin the cat of urban architectural context.

An even better point is that the two schemes will work together. When the buildings get built, the 1600 block of M Street will shine.