A water garden is situated outside the Pavilions, an addition to Glenstone that will open to the public in October. (Iwan Baan/Glenstone Museum)

The Pavilions, the new building at Glenstone, the Montgomery County, Md., art museum situated on 230 acres of woodland and pasture, will open to the public on Oct. 4. The building’s inaugural display, which will catapult the museum to the front rank of privately owned art museums worldwide, aims to demonstrate the range of the collection, according to Emily Wei Rales, Glenstone’s co-founder and director.

Among the highlights are three hanging sculptures by Ruth Asawa; paintings by Willem de Kooning and Mark Rothko; an early “combine” by Robert Rauschenberg; a rare early painting by Sol LeWitt; sculptures by Eva Hesse, Lynda Benglis, Martin Puryear, Dieter Roth and Richard Serra; and paintings by Keith Haring, Jean-Michel Basquiat and Andy Warhol.

Rales established Glenstone with her husband, Mitchell Rales, a billionaire businessman. When the museum opened in 2006, their collection centered on postwar American art, with notable works by de Kooning, Rothko, Jasper Johns, Cy Twombly and Rauschenberg.

“One of the things we’ve done in the past 12 years,” said Emily Rales, “is to broaden our scope, looking at different continents and focusing on avant-garde movements from Japan and Brazil and Europe. We’ve branched off to look at artists who we feel are very important to the story of 20th-century art but may not have great representation in other collections — even museum collections.”

The Pavilions, designed by Thomas Phifer of Thomas Phifer and Partners, is a 204,000-square-foot building providing 50,000 square feet of indoor exhibition space. That is more than five times the space available in Glenstone’s original building, designed by Charles Gwathmey (and currently installed with an impressive Louise Bourgeois exhibition, drawn from the collection).

The inaugural Pavilions hang will feature a number of rooms each dedicated to installations of work by one artist only, as well as a group hang of 65 works by 52 artists. (Altogether, there are about 1,300 works in the collection).

The Raleses have forged relationships with many of the artists in the collection. Among the ­single-artist displays are several works commissioned for Glenstone, including a multi-panel painting by Brice Marden, a room-size installation by Robert Gober and a massive open-air sculpture by Michael Heizer.

The Marden painting took five years from the time it was commissioned; the Heizer sculpture 10. “There were fabrication issues and logistics and engineering obstacles,” said Rales of the Heizer piece. But with each commission, she said, “we assured the artists at every step of the way, ‘We are here with you, and we will dedicate whatever resources we need to and be supportive in any way possible to make this happen.’ I think that sets us apart from other places that either don’t have the resources, the time or the interest to see the artist’s vision through to completion.”

The work displayed in the Pavilions ranges from classic minimalism to examples of postwar German art by the likes of Martin Kippenberger and Sigmar Polke; from pieces associated with Japan’s Gutai and Italy’s Arte Povera groups to key works by Brazilian modernists.

The Pavilions’ galleries are situated around the perimeter of a large water court filled with water lilies. The building’s natural light, the spacious hang (one enormous gallery contains just four Charles Ray sculptures) and the minimal architecture all combine to create an atmosphere of serenity. But the art provokes conflicting moods, from Rothko’s contemplative lozenges of color to Marcel Duchamp’s urinal, signed “R. Mutt,” and Kippenberger’s sculpture of himself standing ashamed in the corner like an errant schoolboy (inspired by a vicious German art critic).

Rales double-majored in art history and Chinese studies at Wellesley College. She did an internship at the Guggenheim Museum, ran a short-lived nonprofit that gave young artists shows in Lower Manhattan, and worked for several art dealers. She turned her attention to contemporary art because, she said, “I wanted to understand how artists thought. I wanted to be involved in that process somehow.”

When it comes to collecting, Rales said, she and her husband have a “15-year rule.” “We don’t collect an artist unless he or she has been actively exhibiting their work for at least 15 years. That eliminates all the 20-somethings who are doing really interesting work. But we feel we need a track record that’s longer so we can assess where the arc of the career is going and whether it really fits with the ethos of our collection.”

Rales said the collection continues to grow. She is motivated, she said, by a desire to be active in creating art history. “And I do believe that’s what museums do: They elevate certain things to the level of public view and therefore they are actively contributing to one’s understanding of history. That to me is really exciting.”

Glenstone is at 12002 Glen Rd., Potomac, Md. The public may schedule visits to the expanded museum, opening Oct. 4, at glenstone.org, starting in early September. Same-day reservations will also be accepted. Admission is free. Museum hours are Thursday through Sunday from 10 a.m. to 5 p.m.