After a six-year planning process, Boston Children's Hospital has the green light to begin construction on a $1 billion clinical building.

The hospital got the final go-ahead on Thursday from the Massachusetts Department of Public Health's Public Health Council.

The project has been controversial because it will result in the demolition of a half-acre healing garden, which was bestowed to Children's Hospital 60 years ago, and because of concerns about how the expansion will affect health care costs.

Opponents of the project rallied outside the State House before the meeting and vote, hoping for a last-minute save for Prouty Garden.

The Prouty Garden, at Boston Children's Hospital, is seen in 2015. (Robin Lubbock/WBUR)

"When you are in the hospital day after day after day ... Prouty Garden is the only place, the only place at Children's where you can go and get outside and get some fresh air and get away from your hospital room," said Jonathan Swersey of Newton.

Swersey has a 5-year-old daughter who's been treated for cancer at Boston Children's Hospital.

"[The garden] is so critical to these kids spiritually, holistically, emotionally, to their families," he added.

Children's Hospital President and CEO Sandra Fenwick told the Public Health Council she understands how important Prouty Garden is.

"This has been a very difficult, a very hard decision for all of us," Fenwick said.

The hospital will move some elements of the garden to new, smaller green spaces inside and outside the hospital — including a rooftop garden.

Fenwick said the new building is critically needed to replace outdated facilities at the hospital and to create more space.

"This morning when we walked in, we had an occupancy of 99 percent," she explained Thursday. "There were two beds available at the hospital."

That means turning away patients and some patients spending the night in the ER.

The new building will add 71 new beds to the hospital's current 404. It'll include a neonatal intensive care unit to replace the outdated one where families share open bay areas without privacy. It will feature a cardiac center and operating rooms.

Critics worry the expansion will lead to higher health care costs in the state, as patients are drawn to the expanded Children's Hospital from lower-cost pediatric hospitals. That's a claim recently made by the Health Policy Commission, a state watchdog agency.

But Children's Hospital says most of its new patients won't come from Massachusetts — they'll come from out of state and overseas, so they won't drive up costs here.

Attorney Gregor McGregor, who represents some of the opponents, asked the Public Health Council how it could accept those claims.

"The hospital is premised on never-before-seen ballooning of international patient volume — a factor beyond the hospital's control. They can't control the international pediatric health care market!" McGregor told the panel.

McGregor said recent trends don't suggest the growth the hospital is predicting.

But the hospital could face fines or even get beds taken away if it doesn't keep health care costs in check, or if its projections in terms of patient volume are incorrect. The Public Health Council agreed on several of those conditions.

And after more than two hours of testimony and questions from the panel, the vote happened quickly. Ten council members voted in favor of the project, and one abstained.

Opponents say they will appeal the decision. And they're in the midst of a lawsuit to try to block the project.

Children's Hospital administrators say they will get going on construction.

"This is for the children, it's for the families, and it's for the future," CEO Fenwick said.

Pending the attempts to still stop the project, the building is slated to be finished in 2020.

Here's a look at renderings of the expected changes:

The planned Anne and Olivia Prouty’s Wishingstone Garden, a quarter-acre outdoor garden. Anne and Olivia were daughters of hospital donor Olive Prouty who both died in infancy. The current Prouty Garden was bestowed to the hospital by Olive Prouty in 1956. (Courtesy Boston Children's Hospital)

One of the interior “green spaces." Hospital executives say many patients can’t go outdoors to the existing Prouty Garden because of their medical conditions, so interior gardens will give them greater access to green space. (Courtesy Boston Children's Hospital)

A rendering of the rooftop garden planned for Boston Children’s Hospital’s main building. (Courtesy Boston Children's Hospital)

This story was updated with Lynn's Morning Edition feature version.