Watching buildings get built has become a habit born of necessity for walkers in downtown Washington during recent years. Rarely has it been more rewarding than in the 1600 block of M Street NW, where the various parts of the Sumner School office project have been put together like pieces in a giant architectural puzzle.

So subtly was the design conceived, and so skillfully was it executed, that in a few years' time unknowing passers-by likely will assume these interesting buildings and the handsome public open spaces they surround to have been there for many decades -- which is only partially the case.

Unmindful of how shabby were the two 19th-century school buildings on the site just three years ago, or how close they came to destruction, these strollers may think simply, "What nice old buildings." The more observant might notice the new curtain wall structure behind the old schools, and conclude, "Well, that wasn't badly done." But few will give a second thought to the large new blond brick building on the eastern edge of the property. Actually an appendage to the curtain wall building, it looks like a separate piece that blends without much ado into this exceedingly civilized block.

And only architecture aficionados, versed in the history of styles and materials as well as knowledgeable about the city's past, will pay any heed to a new attachment to the back of the Sumner School along 17th Street. One must look quite closely, indeed, for the telling signs of a sympathetic restoration.

The special genius at work here, in spirit and in detail, is summed up in a statement by Warren Cox of Hartman-Cox Architects, the principal designers. "The whole project really is a big collage," he said, "just like the city itself."

This antiheroic attitude, this refusal to make the aggrandizing personal statement, and above all this subtlety, are precisely what the city needs from its architects. They connote a respect for tradition, an awareness of contemporary needs, a recognition of the piece-by-piece ways in which cities are built up over time and an acknowledgment that in a given city, only a few spaces require extraordinary architectural showmanship.

The Sumner site was ideally suited to illustrate the virtues of this kind of architectural thinking. It possessed, most notably, the spectacular Sumner School, designed in 1872 by Adolph Cluss, the city's preeminent architect of public buildings in the post-Civil War years (the Smithsonian's Arts and Industries building and the Franklin School, among many others, are his).

With its peaked Victorian Gothic clock tower, its polychrome tile roof, its hooded windows and other textural elements, the Sumner was the architectural showpiece of the block. It remains so today, thanks to Hartman-Cox and the preservationists who fought over the years for its retention. (It is hard now to remember just how rudely the Sumner was treated. The sharp pyramidal tower, for instance, has been rebuilt after having been shorn off more than a decade ago in a thwarted prelude to complete demolition.)

In addition, the site had the modest Magruder School at its very center, a little red brick schoolhouse of later vintage and lesser distinction than the Sumner, but even so a respectable companion piece. The Jefferson Hotel, a Beaux-Arts structure designed in 1923 by Jean Henri de Sibour, brought the block to a distinguished conclusion at 16th Street.

Two other conditions gave the architects room to operate. One was the size of the site, which, thanks to the diagonal line of Rhode Island Avenue to the north, was expansive enough to accommodate the big increase in density behind the school buildings. The other was a client, Boston Properties with First City Properties, that understood and sought architectural excellence. The skill of the architects took it from there. (Warren Cox was the partner in charge with Mario Boiardi and Stephen Vanze assisting. Navy Marshall and Gordon Architects also participated.)

Saving the Sumner was a given in the development competition that led to the project. Saving most of the Magruder -- the rear of the building was sliced off and the rest was dismantled and moved eight feet to the west -- for use as the centerpiece of the composition was inspired stroke number one. Inspired stroke number two was imitating the Jefferson Hotel -- in height, style, color and cornice and banding lines if not in materials and detail -- in the design of the intervening office wing. (The popular M Street Deli was destroyed in the process, but its devotees will be happy to know that the same shop was allotted space inside the new building.)

By conceiving of the project as a group of related parts rather than as a monolithic whole, the architects were able to piece together an entire city block in a most natural and appealing way. The result, in all of its complementary variety, forms an exemplary sequence of spaces. The flagstone courtyards flanking the Magruder building serve admirably as welcome mats to the office entrances, and they also will be pleasant places for people to meet, eat, talk or just relax and observe the passing scene.

By extraordinary coincidence, across the street are the finely landscaped open spaces of the beautiful new National Geographic building (Washington office of Skidmore, Owings and Merrill, architects). Together, these make a uniquely satisfying urban ensemble.

Not everything pleases, of course. The off-center penthouse is a dull and lumpish thing, though fortunately none too visible from the street. And I'm not sure yet about the blah gray color of the curtain wall, though it's far better than the more common blah brown.

Basically, though, the Sumner project is a series of delightful surprises, from the classical banding of the curtain wall to the way the brick skin of the Magruder seems to have been folded out onto the surface of the background building to mark entrances. Like much in this project, the latter detail is deceiving; the entrance-ways are of new materials and design.

Among delights unseen from the street are the restored Sumner School interiors (incomplete as yet, and worthy of a separate column) and the lovely sky-lit connecting link between the restored and the new buildings, an enchanting space where architectural wit entertains while soft light entices.

It is all almost too good to be true, but fortunately we will be able to enjoy, admire and learn from it for many years.