A masterful mixing of art, architecture and nature, Glenstone has announced that Oct. 4 will be the opening day for a much-anticipated expansion that will immediately draw national attention as a unique and contemplative cultural destination.

Nestled in the rolling meadows of a former fox-hunting estate 15 miles outside Washington, the contemporary art museum in Potomac, Md., is the brainchild of billionaire Mitch Rales and his wife, Emily. First opened in 2006 with a 30,000-square-foot building, the expanded facility will boast five times the gallery space and double the natural landscape of the original.

It becomes one of the largest private museums in the country, joining the company of the Broad in Los Angeles and the Rubell Family Collection, which plans to open its new home in Miami next year.

Moving smoothly between indoor and outdoor spaces, the new museum builds on the original mission of providing guests with the chance to enjoy nature and art in quiet meditation.

“The idea is to get people to slow down and look,” said Mitch Rales, co-founder and chairman of Danaher, a science and technology firm. “Throughout this transformation, we’ve maintained a single mission, to create a seamless integration of art, architecture and landscape.”

Designed by architecture firm Thomas Phifer and Partners, the new building, called the Pavilions, features 204,000 square feet of galleries, a central water court, contemplative rooms, office space and a library. Groundbreaking was in 2013. The museum will not disclose the cost of the expansion.

Thirteen spaces will display installations from major artists that are part of the Raleses’ collection. Brice Marden, Martin Puryear, Katharina Fritsch and Michael Heizer are among the artists included in the couple’s holdings, which focus on the period after World War II and feature paintings, photographs, works on paper, sculptures, multimedia works and installations.

Two cafes, parking “groves” and an arrival hall dot the 100-acre landscape, where visitors can stroll among outdoor sculptures. Adam Greenspan at PWP Landscape Architecture worked on the natural setting, which includes wooded groves, meadows and a stream. Paths meander through the estate, connecting the buildings and offering views of sculptures by Jeff Koons, Tony Smith and others.

Mitch Rales said the design highlights the interplay of art and nature. Visitors will walk from the parking groves to the arrival hall and then enjoy a short walk to the Pavilions. A free-standing cafe will be found near the main set of galleries, while another cafe offering light refreshments is set beyond the original museum building, known as the Gallery.

The expansion will accommodate many more visitors than previous years, but the couple plans to continue to limit the number of daily visitors to enhance the intimate experience. Staff will discourage guests from taking selfies and suggest they put away their phones entirely to leave behind the rush of regular life, said Emily Rales, chief curator and co-founder.

“We’ve worked to create a visitor experience unlike any other, providing each visitor with an unhurried, contemplative engagement with the artworks,” she said.

The museum is set on 230 acres of woods and hills in Montgomery County. Funded by the Glenstone Foundation, the original building of 9,000-square-feet of gallery space was designed by Charles Gwathmey. It showcased six exhibitions, including shows focused on Roni Horn, Fred Sandback and Peter Fischli. It will continue to present changing exhibitions.

The Raleses expect the expanded museum to attract 85,000-90,000 people in its first year, a time for learning how visitors engage with the building and the grounds. It will continue to evolve, they said.

“This is a chance to build something lasting and of enduring value,” Mitch Rales said. “We’re at this 15 years. Give us another 20.”

The public may schedule visits to the expanded museum at glenstone.org, starting in early September, officials said. Same-day reservations will also be accepted. Admission is free. Museum hours are Thursday through Sunday from 10 a.m. to 5:30 p.m.