FOR 21 YEARS, ever since John Kennedy looked at its frayed condition in disbelief from his inaugural limousine, Pennsylvania Avenue has been Washington's favorite building project . . . to talk about. Whole cities have sprung up in the surrounding suburbs while the Avenue plan that Kennedy inspired has gone through visions and revisions to the tune of talk, talk, more talk.

What Kennedy saw along the northern edge of the great boulevard was a succession of buildings undergoing a sad metamorphosis from a once-vibrant commercial strip to a seedy assortment of second-rate uses, a main street in the process of drying up. Ironically, even after the first of the bold new plans for the Avenue was published in 1964, what succeeding presidents, fresh from the ceremony on the Hill, saw along the parade route was even worse: deterioration accelerated rapidly through the 1960s and early 1970s.

Today actions are at last replacing words along the Avenue. The dramatic increase in construction activity persuasively tells part of the tale. What is not yet so clear on the street itself is that much of the most important work, the planning and design work, is done. Cranes and trucks and barriers will mar the vista for some time to come but the look of most of the Avenue has been set, on paper if not in concrete.

It is possible today to conduct a fairly accurate architectural tour of the Avenue of the future, starting at the Willard Hotel project on the western end of the parade route and continuing eastward with stops at each of the new buildings (either completed or under construction or just about ready to start) along the way--nine new buildings in all, ranging from megablock to modest mid-blockstructure.

It is even possible to get enthusiastic about the prospects. As architect George Hartman observed some time ago, the evolution of the Pennsylvania Avenue plan from its initial incarnation 18 years ago to today's more mixed, more complex endeavor mirrors the many positive changes that have taken place in the field of urban planning.

All too typical of its time that first plan, featuring a behemoth National Square at the western end of the parade axis from the Capitol to the Treasury, called for wholesale destruction and reconstruction--an awesome Roman plan in modern dress. Fortunately, the FBI building was the only major structure actually to be built under the dictates of the 1964 plan. Felled by a combination of things, including bad timing (or good, if you disliked it), its own overextended ambitions and a substantial public outcry, it was replaced in 1974 with a plan prepared by the Pennsylvania Avenue Development Corp. (PADC), an independent agency created by the Congress two years before.

Besides being a blueprint for a massive building enterprise, the new plan called for a significant mix of old and new structures and more sophisticated management techniques aimed at recapturing much of the old sidewalk vitality for the nation's ceremonial main street. This plan, too, changed substantially over the years, and mostly for the good, as we can see in an advance look at the actual designs for the new Avenue buildings.

The architectural results are far better than one would have expected even five years ago. Ironically, this is partially an accident: The recession of the mid-'70s produced unforeseen benefits. Had large-scale new construction begun then, architects and planners alike would have been far less prepared to cope with the sizable challenges of designing buildings along the north side--the "main street" side--of the Avenue. The waiting period provided time in which architectural practice could catch up with the revolution in architectural theory.

Developers responded by hiring better architects--the list of firms is studded with award-winning names--and architects responded with a vigorous debate, argued in buildings instead of words, about the ends and means of designing structures for this unique boulevard. Architect-author Robert Venturi has pointed out that "not all buildings are equally important, not all buildings should be high art, in any landscape there should be plain and fancy." Nowhere is this truer than along this crucial stretch of Pennsylvania Avenue, and happily most of the new designs for it reflect this kind of perception.

It should be pointed out that the money paying for the buildings along the north side of the Avenue is private, and also that very little of what we'll see would have happened so soon or been nearly so good, without the guidance and substantial public investment (in parks, plazas, sidewalks, benches, trees, plants and so on) provided by the federal government through PADC.

The corporation was given extraordinary powers when created by Congress a decade ago, and by and large its performance has been an exemplary demonstration of the benefits of sophisticated public intervention in the generally all-too-private domain of city design. This contradicts favored notions of the Reagan administration, which, predictably, has been chipping away at the PADC staff budget at precisely the time when demands for its skilled oversight personnel are increasing.

If the PADC has made a major mistake in terms of architecture, it is a common one: to lure private development dollars in a competitive market it has given away rather too much in the way of bulk. With a single, small exception, each of the buildings would have benefited by being shorter and smaller. A parallel architectural fault is that, following the massing requirements of the incentive zone used by PADC along the north side of the Avenue, designers understandably focused most of their attention on the Avenue itself. The back sides of the new buildings (along F or E Streets) are almost uniformly big and grim.

If the extent of this rebuilding enterprise is not nearly so awesome as the sweeping reconstruction of Paris under Baron Haussmann more than a hundred years ago, it is still one of the bigger and more significant urban redevelopment projects ever to take place in this country, and without question the most important urban design effort to affect the core of the capital city since the 1901 McMillan Commission plan, which advocated the redesign of the whole area along classic revival lines. It is the biggest thing to happen on the Avenue since the Federal Triangle, following recommendations of the McMillan Commission, was built some 50 years ago.

WILLARD HOTEL project, 1401 Pennsylvania Avenue. The most impressive single result of the changes in the federal plans for the Avenue is, of course, the preservation of the grand old Willard, which has towered over its end of the ceremonial avenue for 81 years. The PADC plan proposed a complete restoration of the Willard as a luxury hotel, coupled with new development carved out of the middle of the block where, alas, the old Occidental Restaurant is no more.

For this key building on a key site, the PADC didn't mess around. To get the best possible design, the corporation invited submissions. Nine teams responded. Only two showed any real architectural class, and their similarities as well as differences are worth examining.

One, by Hartman/Cox of Washington, followed the competition guidelines and proposed a genuinely urbane background building between the Willard and Washington Hotels that would maintain an impressive "wall" on the north side of the Avenue. The other, by Hardy Holzman Pfeiffer Associates of New York, departed dramatically from the rules to propose a series of stepped-back structures set at an angle to the Willard and separated from it by an exterior courtyard.

As a PADC staff architect quipped, "The romantics won this one": The Hardy Holzman Pfeiffer scheme, with its staccato massing of new buildings, each a mimicking echo of the Willard, was awarded first place. The project has been hounded by delays, due in part to financial problems resulting in a change in developers, but preliminary clearing of debris from the old hotel has just begun and construction is scheduled to start next spring.

Each of these schemes emphasizes stylistic context, but in strikingly different ways. The Hartman/Cox design was conceived primarily with the grand Avenue as a whole in mind; the HHP model is intended mainly as a sharply respectful comment upon a single building. The first proposal embodied a continental sort of modesty; the second is a brassy, American sort of fanfare, a one-of-a-kind tribute to architect Henry J. Hardenbergh's Willard, a building whose style is Second Empire French but whose swagger is decidedly that of Gilded Age America.

The winning scheme is a lot riskier, and the kind of thing that architects and the PADC fortunately shied away from elsewhere on the Avenue except in an outbreak of silliness in the proposed Naval Bandstand Arch at Eighth Street and the Avenue. But the genius of the Hardy Holzman Pfeiffer design is to recognize that the Willard building is the only one on the whole Avenue that would support their kind of sophisticated, aggressively whimsical treatment. Despite an unfortunate change in program (office space supplanting about 200 hotel rooms) the design promises to get the Avenue off to a suitably fancy start at its busiest pedestrian intersection.

MARRIOTT HOTEL, 14th Street and Pennsylvania Avenue. Modern architecture takes over in the adjacent block, with mixed results. Although the block actually consists of three separate projects, it will read pretty much as a unit: a strong, sculptural, abstract version of Anywhere, U.S.A., circa 1965.

The biggest and best of these buildings is the huge Quadrangle/Marriott development (Frank Schlesinger of Washington and Mitchell-Giurgola of Philadelphia and New York, joint architects), which begins as a 750-room hotel at 14th and the Avenue and courses diagonally back through the block to end as an overwhelming Mondrianesque office-cum-retail monster at F and 13th streets.

The architectural-development team won the competition for this building mainly by a coincidence of conscience and timing: From the very beginning its proposal envisioned saving the National Theater building in the middle of the block at a time when preservationists were increasing their pressure on the Avenue plan. The architectural merits of the winning scheme, too, result from responding to the challenge of accommodating the National, an architecturally undistinguished if culturally irreplaceable structure that is lower by far than standard PADC densities.

Thus the mass of the building was broken up in an interesting way so that it will almost seem to be two different structures along the Avenue. The part of the building that abuts the National precisely matches its height and limestone finish, and this horizontality is emphasized by sleek ribbon window courses. Behind this structure with its planted roof balcony will rise the background plane of the hotel building, with rectangular windows and a contrasting, flecked brick finish. The hotel structure then angles neatly out to the corner of 14th Street, flush with the edge of the Willard facade.

Nice touches . . . and the views will be sensational. Venturi's Western Plaza shines when seen from above. And it is no small thing that the uses of the building, so complementary to the luxurious Willard, will help to bring the PADC parks alive.

NATIONAL PRESS Club, 14th and F streets. Of all of the architects involved in the Avenue enterprise, Edwin Schnedl of HTB Inc. of Washington faced the most trying set of circumstances. He was asked to save the National Press Club Building and add to it, and to give it a new look while inconveniencing the tenants as little as possible.

Schnedl solved these problems ingeniously, devising a four-stage construction schedule, improving pedestrian circulation for the office floors and managing to carve an interior atrium out of an existing light well and the auditorium of the old Fox Theater (the giant steel trusses of which had been left in place when it was filled in for office use). And like most of the new PADC spaces, this one pays attention to ground-level retail. It will connect on the inside with a shopping mall that runs along the enormous F Street facade of the Quadrangle/Marriott building.

Ingenuity has its distinct limits, however. The building will get a completely new face, in part because of the client's desires and in part because Schnedl figured out a way to lessen the impact of interior construction by running new plumbing pipes along the outside of the building, between the existing terra cotta facade and the new applique of a light shade of brick. The unfortunate effect is to deepen the facade by creating a recess of nearly 2 1/2 feet from outer edge to window pane.

Schnedl's idea was to cloak an old notion in a modern suit--to give the modernized Press Club Building a base, a middle and a top. But the result is an unhappy compromise, from top-heavy balconied roof to articulated limestone base. The existing building is undistinguished, but as Venturi said there are merits in modesty. The HTB design converts a trim background building into one of quite aggressive pretension.

1301 Pennsylvania. This building, with its brooding Mediterranean pattern of precast concrete sun-screen window bays, was the first big private office building to get under way under the revised Avenue plan, and already it seems decidedly out of sync. Architect Frank Schlesinger broke up the repetitive mass of the Avenue facade with the exclamation point of an off-center vertical band of more deeply recessed windows, and he added drama to the 13th Street facade with a bravura system of cantilevered, upside-down setbacks. If the building is a bit above average when considered as a type--the forceful, geometric, concrete modernist office building--it is a type we've had our fill of, the same esthetic mold, come to think of it, from which the FBI building was cast. Fortunately the mold was broken along the remaining parts of the Avenue.

1201 Pennsylvania. The one truly distinguished modern building on the Avenue, this sleek, towering edifice sheathed in pink granite houses the offices of Covington and Burling as well as those of its designers, Skidmore, Owings and Merrill (David Childs, partner in charge). It is a building of some pretension, too, but its pretenses are nicely clothed in an abstracted sort of classicism quite in keeping with the setting.

Its distinctive features are at once subtle and immediately apparent, such as the ceremonial four-story entrance bays, the change in fenestration at the top (providing a distant echo of neoclassical precedents), and details such as the recessed vertical panels separating horizontal window bands cut into the granite at a sharp, neat angle. The big entrance is no disappointment. It leads to a clean, dizzying atrium that extends from below ground all the way to a space-frame skylight, 160 feet above ground.

Best of all is the exterior space created by the angle of the building as it shoots out to meet the facade of the neighboring Pennsylvania Building, a vintage 1950s modern office structure badly in need of a sympathetic facelift. This space is a real joy, a subtly enclosed pocket set back from the long open swath of the boulevard. Typically, the building puts its best face forward. It meets E Street as an oppressive, repetitive wall.

EVENING STAR Building, 1101 Pennsylvania. The firm of Harry Weese & Associates (Robert J. Karn, architect in charge) was handed the most delicate task: to design a building that will somehow sit comfortably between the old Evening Star Building, a rich, Beaux Arts stalk jutting some 50 feet beyond the PADC-approved building line, and the speculative Presidential Building, with its no-nonsense grid of concrete and glass.

The architects acquitted themselves rather well. Recognizing the generously inflected Star building as a magnificent sliver that needs no jazzing up, they decided to fill it out along the Avenue, reflecting its materials and picking up its bands and arched window patterns without trying to duplicate them. The nicest touch of all is the soft curve of their new building as it bends back to meet the Presidential structure, a gentle foil for the angle of the SOM building up the street.

Indeed, with its canopied entrance facing onto the tree-planted Presidential building plaza, the Star annex completes the engaging space inaugurated by the angle of the SOM structure. It is a space that would not have existed under the original Avenue plan, which saw the old Star building simply as a sore thumb with ornamental bandages, and called for its destruction.

LINCOLN SQUARE, 1001 Pennsylvania. The erasing of an entire city block is something one hates to see, at least in part because what comes after usually is so awfully big and god-awful. (Think, for just one second, of the 200 block of Pennsylvania Avenue as it used to be on the other side of the Capitol, and the egregious Madison Library building that took its place.) Lincoln Square, the megablock designed by Hartman/Cox, is big--too big--but it is impressive architecture, too.

What the architects propose is to break the block down and then build it up again, allowing the old buildings to dictate as much as possible about the character and form of the new building. Seven of the best old commercial structures are to be preserved and tucked into the new structure; these, in turn, determine the up-and-down pattern of setbacks by which the architects attempt, with some success, to disguise the true size of the project.

Designed in layers, each with its distinguishing materials, the building rises to the full 160-foot height in the middle of the block, the innermost layer being simply a bland background box. Visual interest is concentrated on the outermost layer, which cuts an irregular profile, particularly on 11th and 10th streets. The outer skin of the building, with its double-hung sash windows and its light-stone-on-dark-brick detailing, is intentionally remindful of commercial Main Street, U.S.A., as it was 30 or 40 years ago.

How refreshing a change is this idea from its all-too-common, chest-thumping modernist precedents or the vulgar ersatz classicism that inflicted Washington in the Rayburn building, among other unspeakable blockbusters. There is something quite inspired about the irony of creating a modern megablock as if it were a happenstance accretion of just plain ordinary buildings--an extraordinary ordinary idea that is remarkably apt for the northern side of Pennsylvania Avenue. We can only hope that in the end the sheer bulk of the building will not significantly compromise good ideas and good intentions.

THE APEX-BRADY block, Pennsylvania Avenue and Seventh Street. The Apex building, with its conical turrets, has been a picturesque respite for romantic eyes ever since the turrets were added to it in the 1880s by Alfred B. Mullett, whose chief claim to local fame, of course, is the wedding-cake classicism of the old Executive Office Building next to the White House. Around the corner from the Apex are two modest pre-Civil War buildings where Mathew Brady set up his photographic studio when he was in the chips.

Happily, these structures will be combined and restored to provide a historical note and a low-scale enclave along the monumental street. There is a missing tooth between the Apex and the Brady Buildings; Hartman/Cox will fill it in with a neat, vaguely 19th-century building that will do double duty as an entrance and service core for the combined project.

PENNSYLVANIA Avenue Triangle, at the Avenue and Sixth Street. Jacquelin Robertson, of Eisenman/Robertson, the New York firm that designed the large complex that will be built in the trapezoidal blocks bounded by Indiana and Pennsylvania avenues and Sixth Street and intersected by C Street, says that in urban terms the design was "quite straightforward."

That may be, but it is the kind of sophisticated straightforwardness that is all too rare in contemporary urban design, for it successfully takes into account an almost baffling number of conflicting contexts: the diagonal pull of the two avenues; the changes in scale and style in adjacent buildings (including the nondescript "concrete cages" across Indiana Avenue); the necessity to form a planar backdrop for the Apex building and its historic neighbor, a Richardsonian Romanesque bank building; the desirability of creating a succession of different spaces along C Street, between the two buildings; and the complexities of the program, which includes rooftop condominiums, a hotel, an office building and ground floor retail spaces all around.

That the architects managed to pull all of this together is remarkable, a sign at once of skill and of a general progress in the complex game of urban design. An adroit use of setbacks is again a key. The office building along Pennsylvania Avenue, for instance, aligns itself with the 72-foot height of the Apex building cornice, and then steps back in three stages, and the materials and fenestration change at the first level from articulated limestone to a flat stone-and-glass International Style facade that will, one hopes, be delicate in character. The northern building, although unfortunately higher, follows a similar pattern.

A sensitivity to ground floor uses and complementary outside and inside spaces prevails throughout. The hotel atrium, for example, is not some showy cavern but a modest walk-through space that will spin activities out to the edges of the building and into the streets. C Street, to be narrowed by 20 feet (from 80 to 60 feet wide), will channel people into a richly planted longitudinal garden and out into a beautiful sitting space between the historic structures (the Apex and the bank buildings), and from there to the wide-open "federal space" of Market Square.

Of course, we can cross our fingers about all of this. Good design needs good execution and there is no way to tell precisely how well even the best of the new designs will turn out. Then, too, there is a lot still left to be done from the Washington Hotel at 15th and the Avenue to the Arthur Erickson design for the new Canadian Embassy at the 4th Street intersection, with the big question mark of Market Square in between.

Basically, though, it is a pleasure, and something of a surprise, to be able to give so encouraging a report about a unique stretch of street, the "Grand avenue connecting both the Palace and the federal House" that Thomas Jefferson, among many others, felt would become "most significant and most convenient."