When the Lever House was completed on Park Avenue in 1952 it didn't take long for people, especially people in the urban building establishment, to sense that some key change had taken place.

The building, now imperiled, was an exquisite jewel of glass and stainless steel designed by Gordon Bunshaft of Skidmore, Owings & Merrill that announced the true coming of the curtain wall to American and, indeed, to world architecture.

Many had been its precedents, from the crystalline dreams of an all-glass skyscraper put on paper by Mies van der Rohe in the early 1920s to the slablike U.N. Building based on a sketch by Le Corbusier that had preceded the Lever House by two years. But the Lever House, the right building in the right place at the right time, signaled the coming of age of a new type of skyscraper and, as much as any other single event, the wholehearted adoption of modern architecture by the corporate world.

To say that the consequences were far-reaching is to understate the matter, although the consequences, as we now know, were by no means all for the better. In any case, the Lever House has aged so well in this its 30th year that last month it was nominated unanimously by the New York City Landmarks Preservation Commission to become a historic landmark, the first time in the commission's 17-year history a building has been so honored in its first year of eligibility.

The commission action, which requires approval (or at least beneficent inaction) by the city Board of Estimate, may have come just in time for the Lever House, which received another, dubious, distinction as a 30th birthday present: Powerful economic interests want to tear it down.

The same terribly backhanded compliment has been paid to other, older landmark structures by applying the same indisputable formula: Height plus bulk equals dollars. Quality -- esthetic, social or otherwise -- is not a factor in the familiar equation.

From the very beginning the size and positioning of the Lever House relative to its site, which runs a full block on the west side of Park Avenue between 53rd and 54th streets, have been its most alluring features. The main office building, 24 stories high, is a gleaming, slender slab perpendicular to the avenue that appears to rise from a rectangular base, two stories high and supported by columns, covering the entire length of the site. The pedestrian space underneath has not worked out well (it's dark and uninviting most of the time), but the proportions of slab to base remain as pleasing as they ever were, and the trees rising from the open, second-floor plaza still make a welcome accent along the avenue.

In fact, despite the wear and tear of three decades, most noticeable in the color changes of the glass spandrels (now three distinct shades of green due to replacements), the building retains its distinctive, airy elegance.

It is not, however, very big. It wasn't really very big when it was built, even though in height it noticeably superseded most of its immediate neighbors. The point, as architect Bunshaft recalled the other day, was that the corporate clients were thinking "more about a headquarters building they could be proud of than of zoning, rents, income from the building, or anything else." In other words, the client was acting with admirable restraint, for even then the building density was much less than permitted by New York City zoning regulations.

Today, who knows by how many skillions of dollars it falls short? (Fisher Brothers, a developer interested in the land, has a proposal for a building three times as big.) It is worth noting, it is in fact the main point, that zoning was originally intended not to establish the true size of a building but simply to set a limit beyond which it could not grow. How drastically this has changed, how great the increase in avarice encouraged by public policy, can be seen everywhere you look on the East Side in Midtown Manhattan these days, where skyscrapers have been sprouting almost overnight to block the sun.

True, the city recently revamped its zoning regulations to lure development westward from the fashionable East Side, but even by its own optimistic estimates the results will be negligible: Some 12 million of the 30 million square feet of office space anticipated in Midtown during the next 10 years will shift west of Fifth Avenue.

So, questions concerning the architectural merits of the Lever House, such as the report commissioned by Fisher Brothers from the architectural firm of Swanke Hayden Connell, which says the building is of insufficient "interest of value" to "risk disrupting the further evolution of upper Park Avenue," should be treated as the economic arguments that they are. Development pressures are so intense in the neighborhood that even St. Bartholomew's Church, with a Robin Hood rationalization, has proposed to partially self-destruct in order to take advantage of the dollars in the Manhattan air by building a tower where its refectory now stands and using the funds for social programs. This is the same sort of logic that has led another developer interested in the Lever House property to propose "saving" the building by adding to it, presumably by building over the open platform of its base.

Of course, there is a certain irony in the exercise of defending the Lever House, a building that started, if it did not directly cause, the systematic modern attack upon Park Avenue, resulting in a few admirable structures such as the Seagram Building, but many more urbanistic abominations such as the Pan-Am Building.

Nonetheless, as the Landmarks Commission realized, it should be saved. It becomes more of a beautiful Modernist bijou every day, and someday may allow one of the last rays of sun to reach the street within a 10-block radius.