A German magnate has donated his 280-piece collection of art from the American West to the Tacoma Art Museum in Washington state, along with money to build a new wing that will more than double the institution’s size, The News Tribune reports.

The trove, amassed over 25 years by retail billionaire Erivan Haub and his wife, Helga, includes works by Georgia O’Keeffe, Frederic Remington, and other 19th- and 20th-century masters. The couple owns property in Tacoma and elsewhere in the U.S. Northwest.

Rock Hushka, the museum’s curator of contemporary and Northwest art, said the gift is the largest in the institution’s history. Media reports did not specify a dollar figure for the donation, but Stephanie Stebich said the Haubs are contributing a “significant” portion of the estimated $15-million cost for a new, 15,000-square-foot wing.

The museum currently has about 12,000 square feet of exhibition space.