Ever so gently, the new corner building at 19th and K streets NW prods a passerby to muse about the history of western downtown--about the dramatic (some would say catastrophic) changes of the '50s, '60s and '70s, the architectural battles of the '80s, the reemergence of modernism as the century draws to a close.

With its curved cornerpiece--very much a shape of the '90s--the building is sleekly up to date. But it is not entirely new. It is, in fact, an addition to and transformation of a smaller building on the site, and parts of that building--the triangular, white marble verticals of the middle floors--are literally embedded in the new facades.

Folding the old into the new has become a time-honored tactic in Washington architecture. What is most unusual about the Millennium Building--the self-explanatory (and self-congratulatory) name of the new occupant of 1909 K St.--is that the old parts are not all that old. Nor are they "historic," in the conventional, approbatory sense of that word.

No, indeed. The existing building dates back only to 1973--a time that many would think of as the bad old days despite their relative recency. This was when office buildings were still marching westward on K Street and to its north and south, gobbling up row houses, apartment buildings and older commercial structures block by block.

The 1973 building was typical of its time. That is to say it was a serviceable office box designed in a hand-me-down modernist style. Practically everything of '60s and '70s vintage along this stretch of K Street is an imitation of something. There is a boxy knock-off of Mies van der Rohe at 1825 K (one of a half dozen or so in the city) and, my favorite in the imitative vein, the Mercury Building at 1925 K, designed with aluminum window modules in horizontal homage to the 1953 Alcoa skyscraper in Pittsburgh.

Like several other downtown buildings, the '73 version of 1909 K St. was stylistically indebted to Edward Durell Stone's headquarters for the National Geographic Society, completed in 1964 at 17th and M streets NW. However, where the marble fins, vertical proportions and high, perforated eaves give Stone's white building a certain lightness, the boxy frame and triangular marble columns brought the eight-story K Street incarnation soddenly down to earth.

Most of the post-World War II office buildings in the western section of downtown already have been renovated or are in serious need of it because of technological, rather than architectural, obsolescence--their heating, ventilating and air-conditioning systems are seriously out of date. But it is altogether rare for anyone to insist that even parts of the original facades be saved.

To the contrary, the customary procedure has been to gut a building, reuse the structure and apply an entirely new skin. One can trace swings of architectural fashion in the style of the new facades. In the '80s, when the West End renovation process began, second-rate historicism prevailed--arches and turrets and belt courses were tacked on like so many Christmas tree ornaments. In the early '90s, crisply modern, unornamented new facades prevailed--in the main, also second rate.

As for 1909 K St., the surprising decision to keep the original marble slabs was more sentimental than anything else. An ardent admirer of Stone's National Geographic, developer Albert Abramson, of the Tower Cos. of Bethesda, was the moving force behind the emulative '73 design. When the time came to renovate, saving the marble was his strong desire, even though the building was to be a lot bigger, with four stories added to its top.

Architects Joseph Boggs and Michael Patton, of the Annapolis firm of Boggs and Partners, were skeptical but willing; they went through a dozen or so serious alternatives before deeming one to be satisfactory. "It probably cost us more to save the marble than to start from scratch," Boggs says. "But we made lemonade out of lemons, I think."

Boggs is right, basically. For a building that dares to call itself the Millennium, the new design is disappointingly button-down, but its cool formalism is, nonetheless, a rather alluring addition to the cityscape. It is certifiable that those triangular slabs of marble look a lot better as secondary accents in this sophisticated collage than as foreground elements in the '73 design.

The marble slabs are, in fact, the keys to the facade design--as so often happens, elements initially perceived as limitations proved to be catalysts. Duplication and extension of the marble wisely was ruled out in favor of subtle contrast. New elements of the facade were sheathed in different, if complementary, materials--clear and gray-green panels of glass, silvery metals and fine gray granites--and the marble verticals were made to float, ghostlike, in an asymmetrical composition.

Both the palette and the subdued mood are continued in the spacious lobby, where there are angled steel beams, quiet gray granites, warm woods and ebony glass planters. The relaxed formality of the space is amplified by a pretty, horizontal waterfall and, on the walls, Karl Knaths paintings on loan from the Phillips Collection. Four Barcelona chairs from Mies van der Rohe are definitive symbols of status and, as always, icons of architectural allegiance.

The building's new aesthetic is more than skin deep--it signifies a thoroughgoing renovation and reconfiguration of interior systems. The building's Web site--www.1909KStreet.com--brags that the design is "environmentally state-of-the-art," and lists dozens of attributes, from chlorofluorocarbon-free central chiller to energy-efficient glass, to support the claim.

Like that of Cesar Pelli's shiny 1996 effort directly across K Street, the building's enlarged size--from eight to 12 stories--is due to the developer's purchase of space allotments from the old downtown east of 15th Street NW. Called transfers of development rights, such exchanges, in which smaller buildings in effect sell a certain amount of air above their roofs, are a good way to help historic structures resist speculative pressures caused by high-density commercial zoning.

In this case, the purchase helped the old Masonic Temple building at Ninth and F streets NW gain a new lease on life, and enabled the Tower Cos. to construct a bigger office building where it will do good rather than harm. Architecturally, both the Boggs and the Pelli buildings indicate that in western downtown, modern architecture is doing a lot better the second time around.

Then again, this is not to make a mammoth claim. The first time around was pretty much a disaster--the replacement of a vibrant, attractive, mixed-use neighborhood with a nine-to-five office center was a terrible idea, and the architecture altogether lacked inspiration. There remains lots and lots of room for improvement.