FORT WORTH, Texas - The Kimbell Art Museum, designed by Philadelphia's Louis Kahn in 1972, may be better known today for its building than for its collection - and its collection is pretty breathtaking, thick with the likes of Michelangelo and Caravaggio. Scholars consider the Kimbell to be Kahn's masterpiece and one of the great museums of the 20th Century.

If you've only seen photos of the exterior, a series of travertine vaults that some Texas wags have likened to a cow barn, it may be hard to appreciate the fuss. In person, the primitive forms exert an otherworldly pull. Approaching the main entrance, you become acutely aware of the shifting light, the mumble of gravel under your shoes, the music of the fountains and, once inside, the baroque shower of celestial light. It's as close as you get in the United States to experiencing the awe of an ancient temple.

Because the intimate museum is itself a great work of art, Kahn's partisans have long argued that the building should never be modified. The Kimbell's first attempt to expand, in 1989, produced such intense outrage that the venerable architect Philip Johnson took out a full-page ad in the New York Times to denounce the design by Romaldo Giurgola, a Kahn friend and fellow member of a group of innovative architects dubbed the Philadelphia School.

Having worked up the nerve to try again, the Kimbell is about to complete an addition by Renzo Piano, who has become the world's most sought-after museum architect. Piano's free-standing pavilion doesn't open until Nov. 27, and yet already there are rumblings among the keepers of Kahn's legacy that the Kimbell has been badly compromised, if not outright ruined. The damage is "irreparable," said Robert McCarter, a professor at Washington University in St. Louis, and author of a major book on Kahn's work.

Given such dire forecasts, I made a trip west to see what had been done to Philadelphia's gift to Texas. While some of the worst claims seem overstated, it is disappointing to report that Kimbell's new, $135 million gallery is a lackluster companion to Kahn's stirring building. Perhaps in an effort to show deference to the master, Piano has created an addition that is wan and dreary, and which ends up doing Kahn a disservice.

Piano has racked up so many museum commissions (next up: Manhattan's new Whitney) that the art world has come to think he can do no wrong. This is especially true in Texas, where he is responsible for two fine, small museums, the Menil Collection in Houston and the Nasher Sculpture Center in Dallas. Once the Kimbell decided to expand, it dispensed with the usual search process and simply hired Piano, who had done a brief stint in Kahn's office at 15th and Walnut before the Philadelphian's death in 1974.

Almost immediately, Piano shifted the location of the addition, from the parking lot at the rear of the Kimbell to the great lawn that rolls out from the main entrance. That decision, the partisans argue, upsets the careful balance between the man-made world and nature that Kahn worked so hard to achieve.

The two Kimbell buildings now stand eyeball-to-eyeball across the lawn, locked in an uneasy dialogue. But the dialogue Kahn intended was with the Texas landscape. In a letter to the Kimbell, the Dutch architect Wiel Arets (now dean at the prestigious Illinois Institute of Technology) complained that Piano's pavilion is the equivalent of putting "an addition in front of the White House in Washington." What makes it even worse, said William Whitaker, curator of Penn's architectural archives, Piano removed a grove of century-old oaks, which "inspired Kahn's whole design."

It is certainly true that Piano's pavilion, which includes three new galleries, classrooms, and a large auditorium, has changed the Kimbell. Kahn's building is no longer a lone temple on a hill and must operate as a mere piece in a campus ensemble. Yet Kahn's architecture is strong enough to live with the situation. In fact, Piano's siting choice may actually help restore the importance of Kahn's main entrance.

Kahn loved to infuse his buildings with a spiritual mystery, often by obscuring the entrance in some way. At the Kimbell, he set up a circuitous route from a parking lot behind the museum, to the front door. It's a magical walk that takes visitors past the fountains and a grove of yaupon hollies before obliquely revealing the front door. But hardly anyone makes the pilgrimage, according to director Eric M. Lee. Instead, most people duck in through the secondary entrance next to the parking lot, never seeing the lawn.

Locating the new pavilion across from the Kimbell's front door reverses the situation. As part of the expansion, Piano embedded an underground garage below the galleries. That means that Kahn's facade will be the first thing that visitors see as they come up the stairs or glass elevator.

Of course, that straight-on, axial view is not the way Kahn intended his facade to be seen. Yet it is better than not seeing it all. True aficionados can still park on the surface lot and take the long walk around.

Nathaniel Kahn, whose film My Architect is a biography of his father, said he remains concerned. "When you sat on the benches of the portico looking out through the big trees to the open park with the setting sun glinting on the fountains and projecting shifting patterns on the travertine wall, there was magic there."

My complaint with Piano's pavilion is its own architecture. If Kahn's building recalls a cow barn, Piano's bears too close a resemblance to a toolshed. It's self-effacing to an extreme: To avoid upstaging Kahn's masterpiece, Piano kept his pavilion low to the ground and opted for lifeless gray concrete walls, rather than repeat the Kimbell's creamy travertine. Minimalist architecture doesn't have to be this dull.

Inside, Piano seems to have run out of ideas. He mixes his own familiar tropes, like beautifully executed joints, with riffs on details from Kahn's building. The Kimbell is perhaps most famous for Kahn's bird-wing ceiling reflectors, which give the galleries their gorgeous silver light. Piano also produces a nice, diffused light in his galleries, but by the clunkiest of means, a kind of horizontal ceiling shade.

Some works of art should be left untouched forever - the paintings on the walls of the Kimbell, for instance. And the Parthenon. But modern art museums are living buildings. You can't fault the Kimbell for wanting more gallery space, only for the kind it settled for.