The city of Boston is offering new details about Mayor Marty Walsh’s ambitious plan to rebuild the bridge to Long Island, in Boston Harbor, and create a “recovery campus” there for people grappling with addiction.

At a press briefing Tuesday, several members of Walsh’s administration discussed how the new bridge will be constructed and what the philosophy guiding the campus will be.

Chris Osgood, Boston’s chief of streets, said the new bridge will be built upon 13 piers that supported its predecessor, which was closed in 2014 due to safety concerns and later dismantled. That closure forced the shuttering of a homeless shelter and other social-service programs and the evacuation of hundreds of people.

The spans for the new bridge will be built off-site and then floated to the bridge for installation, Osgood said Tuesday, with design and construction costs ultimately totaling $92 million. He described this as a smart investment, saying the bridge will last approximately 75 years and will make the creation of the recovery campus possible.

“You’re looking at an upfront cost for something that basically unlocks a very large parcel of land, a set of existing buildings, and a very unique place where you can create a special sort of campus,” Osgood said.

Osgood also said the city will pursue the requisite local, state and federal permits this year — starting with a Boston Conservation Commission filing Wednesday — and begin construction in 2019, with the bridge slated to open in 2021.

Specific details about the recovery campus itself were in short supply Tuesday. According to Jennifer Tracy, who heads Boston’s Office of Recovery Services, it’s still too early to say what the campus’ make-up will be when it’s complete, how many occupants it will have, or how much time an individual receiving services might spend there.

“To put it bluntly, we’re taking our time,” Tracy said. “We’re organizing internally and beginning to reach out to the variety of stakeholders that we’re going to need to listen and engage and hear from, to really think about a robust community for recovery services that will meet the need four years from now.”

While the planning process may just be getting started, however, both Tracy and Marty Martinez, Boston’s secretary of health and human services, said the campus’ overarching mission is already established: provide a range of services, including some that individuals might have trouble accessing elsewhere, in a framework designed to maximize the chances of recovery from chemical dependency.

“Historically, the way services came on the island was sometimes sporadic and sometimes happenstance,” Tracy said. “We have the opportunity now to really think holistically — to really use the green space of the island, the water, the views, to incorporate wellness, to incorporate other supportive services that give folks a purpose. Workforce skills, education, training center. … At this, point, I think, sky’s the limit.”

“We’re not looking to put [homeless] shelters back on Long Island,” Martinez said. “What we’re looking to do is create the full continuum of care. We’re talking about step-down beds; we’re talking about residential programs, sober houses. We’re talking about an array of resources … even, probably, supportive: what it takes people once they get into recovery to stay in recovery.”

The proposal to rebuild the Long Island Bridge has sparked pushback in Quincy. While Long Island, in Boston Harbor, is part of Boston proper, the bridge’s span is split between those two communities, and anyone traveling from Boston to Long Island on a reconstructed bridge would have to travel through Quincy’s Moon Island first.

While Quincy is eyeing multiple steps that could complicate Walsh’s plans, the Walsh administration members who briefed the press Tuesday expressed confidence that Boston can alleviate any concerns that city’s residents and politicians may have.

“I think that we have an entire year of substantive discussions in front of us, essentially.” Osgood said. “That is really what 2018, from a bridge perspective as well as a programming perspective, is really all about. We are really focused on engaging.”

That said, while Quincy Mayor Thomas Koch has suggested providing ferry service for individuals seeking to reach a new recovery campus in lieu of a bridge, Osgood dismissed that idea as a non-starter.

“We’ve already seen, in this calendar year, ferry service by the MBTA halted four times,” Osgood said.

The officials who briefed the press Wednesday did not directly answer questions about whether rebuilding the Long Island Bridge could also pave the way for other new development not connected to the recovery campus. Instead, both Martinez and Osgood said the city’s focus is squarely on that project.

In a press release provided to the media afterward, Walsh — who’s described traveling to Long Island before the old bridge was shut down to counsel people in detox — touted Wednesday's filing with the Boston Conservation Commission as a small step toward a great end goal.

“For many people, including myself, Long Island played a vital role in Boston’s recovery landscape — and it will again,” said Mayor Walsh. “Tackling the opioid crisis means using each and every tool we have, and this is an important next step to ensure Long Island can serve as a resource for those in Boston, and those from surrounding areas, who are struggling with substance use disorders.”