Mitchell Rales speaks at a ceremony on his property, Glenstone, in Potomac on Monday. Behind Rales is a newly completed sculpture by artist Jeff Koons that contains 27,000 plants. Rales plans to build a large museum on the site. (Bonnie Jo Mount/The Washington Post)

Mitchell and Emily Rales, whose private Glenstone museum on their Potomac estate is considered one of the world’s premier modern art collections, expect to begin construction soon on a 150,000-square-foot building that will display more of their growing number of artworks.

In a brief outdoor gathering Monday in a field at Glenstone, the Raleses outlined their expansion plans and said they hope to make what is considered one of the most extraordinary modern art collections in private hands more accessible to the public.

Their collection, which includes works by Alexander Calder, Henri Matisse and Mark Rothko, is now exhibited on a rotating basis in a 25,000-square-foot building designed by well-known modernist architect Charles Gwathmey. Glenstone’s admission is free, but the collection is open only a few hours on a few days a week. Visits are restricted to small groups, which are shown around by docents.

The second building, which is expected to be completed by 2016, would put Glenstone alongside the Barnes in Philadelphia, the Phillips in Washington and the Frick in New York as one of the world’s largest privately owned galleries. The new building’s size and scope would rival that of the National Gallery of Art’s East Building.

Earl A. “Rusty” Powell III, director of the National Gallery of Art, has called the Raleses’ collection “one of the world’s most important” of the post-World War II era. But most of the works are in storage.

Mitchell Rales, the billionaire co-founder of Danaher Corp. who is No. 118 on the Forbes 400 list of wealthiest Americans, said he expects the new museum to allow Glenstone to accommodate many more people than the 12,000 or so who have visited since the collection opened to the public in 2006.

But there are no plans to allow thousands of people to descend on the site at once. The Raleses, who have made no commitment about the extent of future public access, say they want visitors to have an unhurried and peaceful opportunity to view the art. It is also unlikely that two-lane Glen Road, which leads to the property, could accommodate a large infusion of traffic.

Plans for the second building moved ahead last year after Montgomery County granted a controversial sewer hookup. At the time, some neighbors and environmental groups objected, saying the Raleses were getting special treatment because of Mitchell Rales’s wealth and his connections — an allegation that he and county officials denied.

The new building will be constructed of stacked, cast concrete and will utilize natural light with large picture windows and other openings. It will be made up of a series of connected pavilions, which will either showcase a single artist or display rotating exhibitions. The rooms will surround a central water courtyard with a shallow pool filled with plantings that will change color depending on the season. There also will be a cafe.

Mitchell Rales, who grew up in Bethesda, declined to discuss the price tag, but people with knowledge of projects of this magnitude said that it could easily top $125 million.

The new building is designed by New York architect Thomas Phifer and Partners, and the 200- acre estate will be newly landscaped by PWP of Berkeley, Calif.

The publicity-shy couple spoke briefly to about 200 people who attended the invitation-only ceremony, which included only two news media organizations. Rising nearly 40 feet behind them was “Split Rocker,” a recently installed Jeff Koons sculpture with a stainless steel hull that is dotted with 27,000 flowering plants and its own irrigation system.

Emily Rales, the museum’s director, said the sculpture exemplifies what the couple are trying to accomplish at Glenstone. “It is possibly the best embodiment of our mission at Glenstone to combine art and architecture,” she said.

“Here we turn the ordinary into the extraordinary,” she said.

Glenstone already includes massive outdoor sculptures by Ellsworth Kelly, Richard Serra and Tony Smith, and an indoor art collection that includes works by Alberto Giacometti, Willem de Kooning and Jackson Pollock.

But the Raleses want to display more and have spent the past five years trying to design an expansion.

After the ceremony, Mitchell Rales sipped lemonade under a large tent and said that he wanted to be “an inspiration for those who are working on the project” and that he also wanted “to celebrate the end of the beginning.”

Montgomery County Executive Isiah Leggett (D), among a handful of politicians who were invited, said the project would provide a “unique and free museum open to any and all” and would preserve open space.

“We will not see all those houses, all that traffic on the road, and the environmental degradation that you see in other places,” Leggett said.

Former New Yorker architecture critic Paul Goldberger, who is working with the Raleses on the project, described the collection and setting as “more than a place of great and beautiful things. It is also at its very core, a place of ideas.”