WHITE RIVER JUNCTION — Supporters of Northern Stage gathered in the former Miller Auto repair shop building Thursday to ceremonially break ground on a new $9 million theater that will raise the curtain on its first show in August 2015. “It has taken the efforts of many on both sides of the footlights to get us here,” said Janet Miller Haines, chairwoman of Northern Stage’s board of directors.

Founded in 1997, the theater company is already in rehearsals for its 18th season, the last in which its plays will be staged at Briggs Opera House downtown.

Miller Haines and other members of Northern Stage thanked the Briggs family Thursday for their longtime support of the organization. Bonnie Briggs, owner of the Briggs Opera House, said she doesn’t yet know what will become of the upstairs theater space, which dates back to the vaudeville era, once Northern Stage completes its move around the corner next fall.

The speed of the new project, which began raising money in February and is already about 80 percent funded, astonished even those who have been in the thick of it, said Stuart Johnson, chairman of the building committee.

“Part of what made this possible is a computer program,” he said, highlighting the intricate paperless choreography that put all the plans and approvals onto “the cloud,” shaving months of routine back-and-forth from the design and permitting process.

The former Miller Auto showroom, which has housed Northern Stage’s rehearsal spaces, offices, scenery and costume shops for several years now, will remain in place. But the former garage building next door, which dates to at least 1907, will come down to make way for the new stage.

“We don’t build in the American theater very often,” Northern Stage creative director Carol Dunne told those who gathered Thursday.

“We close theaters all the time because modern culture has a lot of distractions. So thank you for believing in us and giving us a home to practice the art we all believe in so passionately,” she added to applause.

Managing director Eric Bunge said the new 250-seat theater will be built by Bread Loaf Construction using several local subcontractors for the various phases, including demolition of the repair shop, which is scheduled to begin this weekend.

“We are working to keep all this money in about a 15-mile radius,” Bunge said.

Of the groundbreaking, he said, “If I could smile any broader, it would crack my face wide open. This is a very special day.”