Like others at Somerville-based Walnut Street Center, John Mahoney was crushed when the nonprofit for adults with developmental disabilities learned two years ago that its Union Square home was being taken by the MBTA for the Green Line Extension project.

Walnut Street Center had been in Somerville for 40 years, and the agency had no plans to leave.

“It was kind of a shock,” said Mahoney, president of the agency’s board of directors. “We had just moved into the building a few years ago, renovated it, added parking. We wanted to stay in Somerville so bad.”

But the MBTA said it needed the agency’s 35 Charlestown St. property to build a new station at Union Square, one of six new stations proposed as part of the 4.7-mile rail extension into Somerville and Medford.

MBTA spokesman Joe Pesaturo told the Somerville Journal last year that the state needed a portion of the land to construct a retaining/noise wall, drainage and an emergency egress ramp for the new Union Square station.

On May 31, 2015, the MBTA officially acquired the property.

A few months earlier, powerboat dealer Russo Marine announced it was selling its flagship Boston-area store in Medford and moving into a smaller building in Danvers as part of an effort to cut operating costs.

The dealer put its 42,000-square-foot property on Mystic Avenue on the market, and it didn’t take long to find a buyer.

“As you can imagine, finding a building that is 40,000 square feet with 50 parking spots in this area is no small task,” said Carolyne Guffey, Walnut Street Center’s chief operating officer, during a groundbreaking for the agency’s new location in Medford on May 26. “But we did it. And we’re here.”

As of now, the building’s walls still feature vintage maps alongside paintings of boats and boating slogans. “Boating … the family vacation that never ends,” remains written in large letters above the main entrance.

But come October, Walnut Street plans to open a completely renovated, fully handicap accessible facility that will serve as the agency’s hub, housing administrative officers and all of its daytime services, while continuing residential services in Somerville, Cambridge and Arlington.

Currently, Walnut Street serves about 100 people through its daytime programs. But the new building, which is slightly bigger than the agency’s former home, will help Walnut Street expand its services north of Somerville and Medford.

“This building came online — we looked at it and said, ‘Perfect,’” said Mahoney, whose daughter participates in Walnut Street Center programs. “I think it will open up horizons for us a little more. It’s off the highway. The building has good curb appeal. We’re going to make a lot more people happy, we hope.”

The facility will feature spaces for music/movement, horticultural and art, along with a gymnasium, computer lab and training kitchen. It will also allow the agency to offer new services, such as job assistance for adults who have suffered serious brain injuries.

“We hope to expand to include adult day health programming, prevocational and vocational services for adults with acquired brain injury and a space that’s specifically designed to meet the needs of those with autism and behavioral health needs,” Guffey said.

During the groundbreaking, Mayor Stephanie M. Burke said the city was happy to welcome Walnut Street, adding she started meeting with Guffey and other representatives from the agency just a week after being elected in November.

“It’s such a wonderful time to know that you’re expanding your services,” Burke said. “There’s so many people in our community that require them. And we also want to bridge the gap with our youth at the high school — we have several vocational programs that I think could assist from an internship perspective and from a learning perspective. So we’re looking forward to being partners with Walnut Street in the future.”

Although the move was unexpected and took Walnut Street out of Somerville, the agency is pleased with the new location, which is just a few miles from its former Union Square home.

“It’s different, but I think this is going to be a good location,” said Stephen Prescod, a member of Walnut Street’s board of directors. “It’s bigger. We’re going to be able to offer more.”

Ongoing negotiations

Walnut Street Center continues to negotiate with the MBTA on costs related to the agency’s former Union Square property, along with its relocation to Medford.

Last year, Walnut Street Center Executive Director Joe Churchill told the Somerville Journal the MBTA acquired its Somerville property for $5.6 million. At the time, the property was assessed at $3.2 million, a number reflecting its taxable land and buildings — not its value as developable land.

But Guffey said last week that Walnut Street was still in negotiations with the MBTA.

“We’re still going back and forth with them,” she said. “It’s been really collaborative to date, but we haven’t gotten any additional funds. We’re cautiously optimistic, but I don’t think it’s bad to get the word that they haven’t quite [delivered].”

Walnut Street Center history

According to its website, Walnut Street Center was founded in 1970 by a group of parents who became frustrated with the lack of services and activities for their children and decided to take action.

The agency began as a small operation providing recreational and educational services during the day. But over the years, Walnut Street has grown to provide comprehensive services including residential programs and individual support, in addition to day programs.

“None of this would have existed at all without the pioneering families, friends and advocates who recognized years ago that changes to the status quo had to be made for those individuals suffering and dealing with developmental disabilities and the families that care for them,” Churchill said at last week’s groundbreaking event.

— To learn more about Walnut Street Center and its services, go to the agency’s website, www.wscinc.org.