TACOMA — A German billionaire with deep Tacoma roots is giving his Western American art collection to the Tacoma Art Museum, and the museum will build a new wing to house it, doubling its gallery space.

Most of the new 15,000-square-foot wing and all of the 280-piece art collection come courtesy of Erivan Haub and his wife, Helga, who own property here. They amassed the collection over 25 years.

It's the largest gift in the museum's history, said Rock Hushka, curator of contemporary and Northwest art for TAM. The museum will formally announce the gift and expansion plans on Monday.

"It's amazing material. It pretty much records the history of Western art from the early expeditionary time (1820s) to the early 21st century," Hushka said.

Museum director Stephanie Stebich called the gift "transformative."

"It's a huge moment for us," she said. "It places Tacoma Art Museum among the top 12 museums that have this breadth and quality (of Western American art)."

Stebich declined to reveal how much of the building project is being financed by the Haubs but called their portion "significant."

"We still have some fundraising to do," she said.

The $15 million needed for the expansion is an estimate, she said. Construction will begin in about 10 months, with completion expected in spring 2014, Stebich said. Plans are being drawn up now.

The museum's current exhibition space is 12,000 square feet. While some of the new wing will house bathrooms and other auxiliary spaces, it will essentially double the gallery space. The museum plans to build the rectangular wing along Pacific Avenue, south of the existing structure, toward Union Station.

Like the existing building, it will cantilever over the parking lot on tall piers. A grand staircase will be built from the parking lot to a redesigned plaza on Pacific Avenue. The staircase will be an art space itself, much like the Chihuly Bridge of Glass, Hushka said.

TAM has hired Seattle design architect Tom Kundig to design the expansion and plaza project.

The Haub collection is a who's who of Western art: Frederic Remington, Charles Russell, Georgia O'Keeffe, Thomas Moran, Albert Bierstadt, John Clymer and Tom Lovell as well as contemporary artists such as Bill Schenck. It contains both paintings and sculptures.

The new wing will display anywhere from 100 to 150 pieces at a time, Hushka said.

"This is definitely one of the finest Western art collections in private hands," Hushka said.

Only the Denver Art Museum with its extensive collections of Native American art and Western painting and photography has a substantially greater collection, Stebich and Hushka said.

The Haubs have established an endowment to care for the art that they have been collecting since 1984.

The gift raises the stature of TAM in the art world and comes on the heels of a major gift of glass art from artist Paul Marioni.

Does it dilute TAM's mission of focusing on Northwest art? Stebich said no.

"Our focus continues to be on the Northwest. I think we broaden our definition of Northwest and the West as well," she said.

"The history of Northwest art can't be told unless you know the story of American art. A full story of American art has to include these images of the American West," Hushka said. "By keeping this collection intact and in Tacoma, it helps us understand who we are as Northwesterners," Hushka said.