The Washington Design Center is set to open for business on Monday. This is good news for the city in more ways than one. The big building in Southwest, part old and part new, adds a refreshing visual touch to an area that would make an ideal backdrop for a movie about a city buried in bureaucracy.

The old brick building--formerly the Terminal Refrigerating and Warehousing Corp.--has been crisply refitted and stands as a reminder of the neighborhood's long-gone days as a center of warehouses and light industry. The new addition, sheathed in glass from bottom to top, is like a bright ice cube bobbing in a sea of gray boxes.

Besides its architectural distinction, the center is a plus economically--adding jobs, visitors, new businesses and tax revenues. And by bringing some 200 wholesale furniture and accessory showrooms together in a single location it will contribute to the productivity of the design industry in the Washington region.

Located at 300 D St. SW, smack atop the Federal Center SW Metro station, the center is modeled upon (though much smaller than) Chicago's huge Merchandise Mart. Admission will be limited to design profesionals and their clients, but even so, center officials expect to attract about a half-million customers every year. The center is owned and operated by the Merchandise Mart, which invested about $25 million to build it.

The most surprising feature of the design by Keyes Condon Florance Architects, a Washington firm whose projects often have been characterized by restraint and understatement, is the shimmering curtain wall that sheathes the addition to the massive brick warehouse at Fourth and D streets SW. In the abstract this strategy of sharp contrast might seem objectionable, an unneeded exercise of hi-tech bravado. In reality it works. If anything the new building--a sawed-off box typical of Washington--could have used a bit more panache.

The design team (David Condon, partner in charge; Thomas Eichbaum, project architect; Bryant & Bryant, associated architects) took a close look at the surrounding environment. The architects realized that, in the first place, the existing red brick warehouse already was dramatically different from the neighboring government buildings. Secondly they saw the immense sameness of those nearby office buildings, a one-note tune in gray limestone, weathered tan bricks and nondescript precast panels.

Because of these clear-headed observations and because the function of the Design Center seemed to call for it, they decided to drop a shining new thing into this drab district. As a result, the sleek glass walls of the add-on structure contribute a sharp accent to the prevailing monotony, and the building as a whole stands out like a sign saying that something different, something special, is going on here.

A certain subtlety was required to accomplish the job. It's easy for architects to say, "Let's be different," but very hard to do well. Three key aspects of the design explain its success: the sympathetic handling of the old building, the careful detailing of the places where the old and new structures come together, and the modulations of the glass fac,ades themselves.

The warehouse was built during the 1920s, and built to last--a red brick fortress eight stories high. Everything about it contributes to the impression of solidity and weight, from nearly windowless walls to massive corners to rows of heavy-looking brick pilasters that ring the building. Before its conversion it was lined with truck docks both north and south. A railway, bridging Fourth Street, ran directly into the building from the west, entering between the second and third floors.

The architects didn't do a whole lot to this imposing edifice, but the things they did do give it an improbable, somewhat festive flavor. To replace the loading docks they applied a new, 20-foot-high base made of handsome eight-inch-square bricks. This supplies an element of contrast while contributing to the building's massive look. The base is punctuated with a parade of recessed, squared-off blind arches outlined with green ceramic tiles--a spirited, unifying touch of color and rhythm at street level.

Unquestionably the most difficult part of the operation was joining the two buildings, with their contrasting, well-nigh contradictory materials: the behemoth in brick and the cube dressed in glass as if for a party. It was a situation of win big or lose big, and the designers did a lot of things right to pull out a victory. Basically, what they did was to weave the two buildings together, forcing an odd couple to engage in a civilized and even entertaining conversation.

Placing the main entrance on D Street, just at the point where the buildings meet, was chancy--it focused all attention upon the contrast between the two--but it proved to be an inspired decision. It gave the architects just the occasion they needed to juice up the design: to pull the top back a bit, to push the bottom out in a little bulge, to stamp the glass fac,ade with mirror-image brick pilasters, to transform the row of blind arches into an open entrance arcade, to stretch that touch of color into belt courses of green or white aluminum that wrap the glass box like a Christmas package.

All of this was done with the firm's typical clean-lined restraint, so the results are neither fussy nor loud. The same balancing act was played upon the stage of the principal glass fac,ade, the one that faces a parking lot along Third Street, where three different types of glass were arranged in patterns that echo those of the warehouse. Vertical stripes of mirror glass panels, set in a gridded plane of silvery, textured glass, recall the brick pilasters of the old building. Similarly, at the bottom of this fac,ade, mirror glass panels form after-images of the squared arches.

The top of the new building, marked with a band of larger, gray-tinted windows, is perhaps the weakest point of the design. One wonders whether the architects could not have brought the building a more emphatic, interesting conclusion up there had city regulations concerning height and bulk been more flexible.

On the other hand, there's not much room to complain. With the Design Center, Keyes Condon Florance pulled off a sophisticated feat. By designing a building that honors its context while rising above it--figuratively speaking, of course--the architects made a lively, lasting contribution.