Let it serve as an axiom of architecture that no finished building ever looks better than its initial renderings and more often than not it looks far worse. Let it also be said, however, that no finished building ever looks uglier than it does under construction and, more often than not, it looks far better.

Within these two admittedly baggy parameters, I will try to come to a provisional assessment of the Morgan Library, which is scheduled to reopen to the public on April 29, after three years of massive expansion and reconfiguration.

Just the other day, after donning an unbecoming green hard hat and signing a waiver absolving the Morgan of any responsibility for my being crushed under a girder, I toured the temporary hell of what was once and soon will be again the Morgan Library. Aside from any merits the site may or may not have, it commands our attention because the newest additions are the work of Renzo “Starchitect” Piano, a fact that lends the entire project that whiff of historic consequence that would be lacking if it were the work of, say, Bart Voorsanger.

The latter, though respected in the profession, is little known outside it. He is the luckless designer of the cafe court that opened at the Morgan in the early 1990s and has now been obliterated completely by Mr. Piano’s work. Given that this latest campaign began in 2003 and was presumably conceived several years earlier, Mr.Voorsanger’s contribution couldn’t have been more than a few years old when the trustees saw fit to replace it. Even in Manhattan, such institutional inconstancy takes your breath away.

Though it is hard to say whether the differences between the two men’s tastes reflect any profound shift in architectural styles, those differences are unmistakable. At the Morgan, as in Mr. Voorsanger’s more recent interventions at the Asia Society on Park Avenue at 70th Street, he sought an almost vertiginous curvacity expressed in the grand rhetoric of marble and several other expensive materials that were not always deployed with the greatest tact.By contrast, Mr.Piano’s almost sullen machine aesthetic favors a strict rectilinearity whose undisguised metal cladding is only occasionally enlivened by cherry-wood accents.

But the biggest and most auspicious difference will be conceptual. The museum known as the Morgan Library began in the Italianate palazzo on 36th Street that was designed in 1906 by the firm of McKim, Mead, and White. To this was added, some 25 years later, a new and vaguely similar structure that reached to Madison Avenue. For some time the administrative offices were located in a Victorian brownstone on 37th Street, which Mr. Voorsanger’s courtyard was supposed to connect to the museum’s 36th Street galleries. Though it did fulfill that function, there was something messy about the whole affair, and you could never shake the feeling of inhabiting a half-digested goulash of styles that Mr. Voorsanger’s Modernist idiom did little to allay.

Both practically and conceptually, Mr. Piano’s design appears to be a great advance over its predecessor.The three pavilions he has designed to plug the interstices in the pre-existing structures probably will have the effect of at last creating a sense of unity, a sense of their belonging to a “campus,” as the Morgan has taken to calling itself.

In the process, Mr. Piano has expanded the exhibition space greatly, included a subterranean auditorium several stories below grade, and dug a post-September 11 vault deep into Manhattan schist to protect the museum’s holdings from an atomic bomb. The most dramatic element promises to be the new courtyard, a sheer glass box flooded with light. Best of all, it will be entered from a recessed main entrance centered between 36th and 37th Streets on Madison Avenue. This, more than anything, will revolutionize the urbanistic interaction between the institution and the citizens of New York. Such recessed entrances are rare on Madison, and this one looks more promising than most.

The ultimate success of the overall design – how well it coordinates its fractious components, how well it interacts with the city itself – cannot be known until we enter the completed structure 3 1/2 months hence. In the meantime, my main concern is the modular, off-white rectilinear walls on the exterior of the pavilions that Mr. Piano has added. Because he wouldn’t be caught dead designing in a classically contextual style, he clearly hopes the muted geometry of the walls will square with the classical idiom of the two pavilions on 36th Street. It is too early to say if he will succeed, but I have a bad feeling about them.

jgardner@nysun.com