Unlike an urban museum, which must scrounge for every last square foot of real estate, the Sterling and Francine Clark Art Institute in Williamstown, Mass., has room to breathe: 140 acres, to be exact, of bucolic fields and woodlands, which slope up from the museum’s main buildings to a hillside that affords views of Massachusetts, New York, and Vermont.

Still, like many museums, the Clark has recently found its facilities too cramped for its ambitions. It has embarked on a dramatic, two-phase expansion plan, designed by the architect Tadao Ando and the landscape architecture firm Reed Hilderbrand. The first phase — a wood-and-glass building called Stone Hill Center, which houses the Williamstown Art Conservation Center, as well as an outdoor café, galleries, and classrooms — is complete and will open to the public on Sunday.

Over the years, the Clark has expanded beyond its original mission, which was to preserve and show the collection amassed by Sterling Clark, an heir to the Singer Sewing Machine fortune, and his wife, Francine. The Clark has become a research center, with a large art history reference library, open to the public; it co-sponsors, with Williams College, a master’s degree program that has turned out many curators and museum directors. And while its exhibitions were traditionally based in the permanent collection, in recent years the Clark has mounted more ambitious loan exhibitions, such as this summer’s “Like Breath on Glass: Whistler, Inness, and the Art of Painting Softly.”

The Clark’s last expansion was in 1973, when it added what is now called the Manton Research Center, adjacent to its original Neoclassical building, which dates from 1955. During the tenure of the current director, Michael Conforti, part of the Manton Research Center has been devoted to special exhibitions, but the space is “really inadequate, and it’s hobbled us,” the Clark’s senior curator, Richard Rand, said.

The second phase of the expansion, scheduled for completion in 2013, calls for a freestanding Visitor, Exhibition, and Conference Center, which will have approximately 10,000 square feet of special exhibition galleries, as well as a conference center and amenities such as a restaurant and bookstore. (Although located in a town of only 8,000 people, the Clark gets 200,000 visitors a year, many of them during the summer and a third of them from the New York City area.) The new building will face a 1.5-acre reflecting pool, which will become the centerpiece of the campus. In addition, the 1955 building will be renovated to create 40% more gallery space for the permanent collection.

Although the museum’s collection emphasizes Old Master and Impressionist paintings, the new exhibition galleries “will be able to show, frankly, anything,” Mr. Conforti said in an interview. “The ceiling heights would allow [future directors] to expand into 20th- or 21st-century art if they chose,” he said.

“Many museums have made mistakes because they build with one particular leader or one particular board of trustees in mind,” he added. “For people of my generation” — he is 63 — “who are involved in doing a project like this, the ideas have to be not about me and my particular vision [for the institution], but about the visions that could come from generations in the future.”

One of the goals of Mr. Ando’s and Reed Hilderbrand’s design for the expanded museum is to connect the buildings to the landscape. Kulapat Yantrasast, a partner in wHY Architecture who worked with Mr. Ando on the Clark designs, said that the relation of Stone Hill Center to other buildings, which it overlooks, is based on the relationship in a Japanese garden of a tea house to the main residence. Although Mr. Ando typically works in concrete, Mr. Yantrasast said, he chose wood in this case, in order to suit the modest New England context.

Visitors who climb the slope to Stone Hill Center can not only enjoy the view, eat at the outdoor café, and look at works on display in the galleries; they can also peek through windows of the conservation labs and observe the conservators at work.

The Williamstown Art Conservation Center, which has a separate board of trustees from the Clark, is one of several regional conservation centers that were set up by the National Endowment for the Arts in the 1970s, to serve small museums that do not have their own conservation labs. The WACC, which employs roughly two dozen conservators, serves the Clark and 52 other member museums in the Northeast.

The director of the WACC, Tom Branchick, described Stone Hill Center as a vast improvement over the old conservation labs, which were located in a former service garage. The paintings lab on the second floor is “just a phenomenal workspace for paintings conservators,” he said, with floor-to-ceiling windows letting in northern light, which “allows us an even color temperature.” (The former paintings lab looked onto grass, he explained, which colored everything slightly green.)

The views are also “phenomenal,” he said. “It makes a big psychological difference.”

The Clark has raised $18 million so far of the Stone Hill Center’s $25 million cost. No budget has been set yet for the second phase of the expansion.