It took a quarter of a century, from 1902 to 1927, for Congress to adopt the famous McMillan plan devised for it by Daniel ("make no little plans") Burnham and his friends. It won't take that long this time around.

Congress in its finite wisdom can, of course, do what it pleases, or what it thinks it can get away with. But before doing anything the legislators ought to consider carefully the stated and implied lessons, as well as the flaws, of the new Master Plan for the United States Capitol, published last week after six years of deliberations on the part of a distinguished team of architects, planners and scholars.

The axiomatic truth that no plan is perfect applies with special force to a document that attempts to set a 50-year (or longer) course for a client so unpredictable as the Congress of the United States. In the tradition of these things, the new master plan is ambitious, grand and even, in crucial respects, grandiose.

One can almost hear the authors tiptoeing on eggs when they write: ". . . the determination of the capacity to accommodate possible future growth should be based upon the environmental capacity of Capitol Hill, rather than upon indeterminate future employment levels. . . . a ceiling for growth should be proposed, and any further growth beyond the established limit should be accommodated elsewhere."

Noble sentiments indeed, and sound planning principles, too, but what is permitted in the plan is a ceiling even Congress can live with -- two new Senate office buildings and nine new House office buildings -- "a future development capacity," as they say, amounting to half again as much congressional office space as currently exists and allowing something close to a doubling of the present office population.

Thus, taken simply as a blueprint for future expansion, the master plan presents Congress with a package it can hardly refuse. What happened is that despite good intentions to the contrary, the planners fell into the trap of designing building envelopes for unknown future needs. What almost certainly will happen is that those envelopes will be filled out with actual buildings. The example of the McMillan plan, even though it took more than half a century to complete (with a few relatively minor exceptions), foretells such a conclusion.

This would not be so bad if the designs themselves were not so uniformly, so bone-chillingly, official. The authors stress that the designs are conceptual and flexible. But here again good in CITYSCAPE, From C1 tentions go awry. In the geometric spirit that has governed the growth of the monumental core of the capital city from the time of L'Enfant's inspired first design, the authors of the plan propose two "gateways" to the Capitol area -- on the northern (Senate) side along First Street and on the southern (House) side along South Capitol Street.

So far so good, especially on South Capitol Street, which today is little more than a parking lot. To read a description of the proposed quadrangle is to imagine a lesson in enlightened urban design (if not in graceful prose): "The future buildings and open spaces in this sector form a varied rhythm and alternation for the passers-by along this axis -- the open, informal space of the lower plaza, followed by the enclosed, more formal space of the upper plaza, interspersed with buildings present the pedestrians with a comfortable scale as they gradually rise towards Capitol Hill."

But to look at that long, wide, straight alley on the plan and to visualize the rigidly rectilinear buildings is another, indeed an opposite, story. The principles are first-rate. The design is a formula for disaster.

This is a crying shame, and not necessarily an irremediable one. The master plan and its authors are enlightened enough to stand correction. Although it does not always practice what it preaches, the plan in many respects represents an admirable attempt to require Congress to do what it conspicuously and capriciously failed to do when it built the ignominious Rayburn building, the hideous Madison Library or the innocuous Hart building -- and that is to mind its urban manners.

This is no small obligation, and the plan proposes that Congress live up to it by placing its new buildings in the most sensible locations, by making them both smaller and less imposing (thus nine House office buildings instead of two or three blockbusters) and by enlivening them with open spaces and shops and restaurants for office workers and tourists. All they need is a sympathetic redesign.

At the neighborhood level the plan shines. The idea of creating a definitive "transition zone" along Second Street between the public and private sectors of Capitol Hill, with distinctive sidewalks, plantings and the like, is long overdue and should be welcomed by the residents, who can give a collective sigh of relief that the Congress no longer covets large chunks of their territory.

The single chunk it still does want, however, should be watched closely. This is the attractive block of row houses along New Jersey Avenue SE, behind which the plan proposes one of its House office buildings. The planners assure us only that the buildings that "flank or replace" (emphasis added) these structures will be carefully scaled. To suggest that these houses be destroyed is almost, but not quite, as mischievous as to suggest that the Supreme Court be moved, which, not incidentally, the plan also does (albeit at some unspecified future time).

Throughout, the plan pays respect to the needs of pedestrians, and the changes in the Capitol Hill landscape it foresees are significant. On the northern side it creates the possibility of resurrecting Union Station and its plaza as the "vestibule in keeping with the Capitol" that Burnham, its architect, intended, and it provides clarity and focus for the existing formal garden between the station and the Capitol. At the southern end it proposes to clean up the sloppy crosscurrent of bridge and roads in favor of a true entrance to both the Capitol and the Mall areas.

Even more exciting is the prospect at last of converting Frederick Law Olmsted's leafy platform for the Capitol itself into the spectacularly peaceful prelude it was supposed to be. To do this the plan proposes removing the 900-plus cars that now clutter the space and constructing a 500-car garage underneath the paved platform.

This is not an unreasonable concession, especially considering the potential side-effect. By providing 86,000 square feet of space for "support, storage and office use" in this submerged structure, the members of the planning committee may ingeniously have solved a problem they were told not to approach: the irresponsible, never-say-die idea of expanding the Capitol's West Front.

All of these desirable changes, and in fact the entire plan, depends in some measure upon the will and ability of the Congress to tackle the gut issue of parking. The planners do provide an expensive fail-safe program for a 2,500-car parking garage near Union Station, but, barring this extreme remedy, they point out the obvious: "a future parking problem is . . . indicated if current driving preferences are extrapolated." Then they provide a convenient list of ways Congress can curb its wayward habits in this respect.

In the necessary process of careful review and revision, there may be neither a better nor a quicker way for Congress to demonstrate its willingness to act in the best spirit of the master plan it commissioned than to respond to this challenge. The issue is orderly, reasoned growth on Capitol Hill, or its opposite.