Western Plaza, the grand space on the grand boulevard of Pennsylvania Avenue, is a place some people love to hate. Not me. I like it a lot.

My reasons are fairly simple. I have never been there without learning something about this great city. I have never visited the place without being moved by its beauty and the history that is incised in its paving stones. And I have never been able to blank out--as its detractors so often do--a sense of the magnificent space that it can become.

Architect John Wiebenson is one who thinks differently, so much so that he conceived a "redesign competition" for Western Plaza. Thirteen student groups entered; four were awarded prizes. All of the entries are on view through July at the School of Architecture at the University of Maryland in College Park.

As a purely educational experiment the competition is perhaps unobjectionable, although it is hard for me not to believe the whole procedure sadly missed the main point.

"Somehow," as Wiebenson mischievously said when he announced the competition, "Western Plaza came out not quite right."

Precisely so, and the principal reason it came out the way it did is that it was not built as designed. Western Plaza today is an unfinished symphony. It does not need the radical revisions envisioned by Wiebenson and the students who, not surprisingly, echoed his views. What it needs is completion.

Here's why and here's how: Western Plaza was commissioned five years ago by the Pennsylvania Avenue Development Corp. as half of a two-park package for the White House end of the "nation's Main Street." The other half, Pershing Park, between 14th and 15th streets on Pennsylvania, was conceived as a green, sheltering urban park, and so it has become. Western Plaza, between 13th and 14th streets, was seen as an open, monumental place in keeping with the monumental sweep of the avenue.

Responding to these rules, architect Robert Venturi, of Venturi, Rauch & Scott Brown of Philadelphia, devised a brilliant, layered design featuring a large, flat rectangular platform to serve as a base for numerous pieces that would order the space from afar and close up.

These pieces were to be of two kinds, two different but related scales. There was a giant scale, consisting of two tall, narrow marble pylons set diagonally in relation to the avenue, framing the vista from the Capitol and, for the first time, giving the western end of the avenue an appropriate visual anchor. And there was middle scale, consisting of stone models of the White House and the Capitol and a number of sculptures and "objets trouve's" to be scattered in a random-appearing, but carefully considered, pattern across the great open space.

Responding to a variety of internal and external pressures, the PADC board eliminated almost all of the vertical features from the plan and built only the bottom part, the almost unadorned platform we see today. In retrospect, it is easy to understand how this occurred. In Washington a plan of many parts, particularly a daring plan, is subject to the rule of decentralized decision-making. In the case of Western Plaza everybody--the city, the Hill, the critics, the reviewing commissions--was able to take aim at a piece of the Venturi pie, and we ended up with just the crust.

The violence this does to the space is, in some ways, obvious. Venturi's crisply designed Art Deco planters, set in rows along the northern and southern edges of the plaza, were allowed to remain. In the absence of the other, higher, less rigidly oriented elements these planters willy-nilly became the chief vertical markings of the place. This is clearly absurd, visually and symbolically, and it makes the flatness, regularity and openness of the place seem almost overwhelming.

But to truly understand the terrible degree of the mistake it is necessary to understand the excellence of Venturi's design. And to comprehend this it is necessary to reexamine the widely held prejudice against open, hard, monumental spaces in the midst of our cities. In Washington we like to call them "imperial" spaces, and not without justification.

In the long history of urban and landscape design in the West, this bias can be seen as part of a continuing debate between the "hards" and the "softs," between advocates of formal and informal, regular and irregular, geometric and organic methods of organizing spaces. In the shorter term it can be seen as part of a healthy reaction to the mania for ill-conceived, sterile urban plazas that affected our builders and planners during the 1960s. In Washington it has been directed specifically against the excesses of modernist urban renewal in the Southwest and classic revival urban renewal in the Federal Triangle.

I think it is safe to say--and this is perhaps Wiebenson's strongest point--that if we had it to do over again along Pennsylvania Avenue, we would like to do it differently. We would like, especially, to preserve much of the low-scale urban liveliness that used to be along the southern edge of the street. But nostalgia for the turn-of-the-century past won't make it come back. There are many good ideas in the entries devised by the students in Wiebenson's competition, including the insertion of bits of local, as well as national, history in the scheme of the plaza. But mostly they go way beyond the limits Venturi was given and mostly they are soft and shady--things that were more or less called for in the competition guidelines.

And even the best of them does not come close to the subtlety and strength of Venturi's effort. Venturi was acting as a real-world architect responding to a real-world set of rules, and he did so with a splendid comprehension of the history of Washington's city plan, of the nature of urban open spaces in general and of the rich possibilities of the particular space he was given to work with.

Venturi's park is flat, hard and open, just as it should be. Twenty years ago Nathaniel Owings conceived a grand National Square at the western end of the avenue. To make room for it he envisioned the obliteration of the Willard Hotel, the Washington Hotel, the National Press Building, the National Theater and much else. Everybody, Owings included, today appreciates the immense calamity this overblown square would have become.

But Owings had something right. The western end of the Pennsylvania Avenue, L'Enfant's neatest boulevard, does need some clear, monumental, symbolic emphasis. The PADC planners recognized this. Venturi recognized it, too, and he gave us a a design worthy of the space, the place and the city. It is shameful that only part of it has been built, but even as it stands today Western Plaza has a lot going for it.

The plaza is as beautifully made as it can possibly be, a hard space that is as rich underfoot as any in the world. The idea of paving it with a map of the central portion of L'Enfant's plan for the city, with the avenue on the map aligned on axis with the avenue in reality, is a stunning notion, so apt and so successful that it now seems obvious. Great designs--after the fact--often possess this sense of inevitability.

Venturi's whimsy frightened a lot of people off. Hence those models of the White House and the Capitol and the "scattered" sculptures were excised. His seriousness frightened others. Hence the great pylons were not built. But what is left is quite something--a history lesson with a lot of amazing, profound, pompous and even funny observations about the capital city incised in the paving stones, and a focusing space that is appropriately monumental and grand. It is hard to stand on that place, even in the summer sun, and not to be exhilarated by the grand dream--and the less than orderly reality--that is the nation's capital.

Still, Western Plaza as it is today is nothing compared to the place it could become. Venturi was audacious enough to believe it is possible to have it both ways--to be serious about our history and affectionately amused by it, to define a uniquely spacious environment and to make it work close up, to be elevated by the dream and be entertained by the reality--and he gave us a design that comes very, very close to accomplishing these ambitious, difficult, complicated ends.

Happily, there are some signs that PADC itself is rethinking its position on Western Plaza. Last year the board voted to build two flagpoles (to be put in place this fall), in weak response to the obvious need for vertical elements. But Henry A. Berliner Jr., the recent Reagan appointee as chairman of the PADC board, seems to be cautiously committed to completing the design: "I'm interested in restoring the original design of the plaza to the extent we can do so without offending the esthetic sensibilities of other people along the avenue."

Well, that's a start. The best thing the architects of the Washington area, and the rest of us, could do would be to insist that the whole design be built. Nothing less.