The group behind it, Friends of Prouty Garden, has received enough donations to hire a lawyer and a public relations firm.
But while the garden has a powerful emotional tug, the group has also raised a practical question: Will a bigger, higher-tech hospital thwart state and national efforts to get spiraling medical costs under control?
Boston’s prestigious teaching hospitals, Children’s included, have helped make Massachusetts one of the most expensive places in the country for medical care. A four-year-old state law seeks to slow the growth of health costs, partly by setting an annual target, the first of its kind in the nation, that hospitals are not supposed to exceed.
In February, as the campaign to save Prouty Garden gained attention, state regulators took the unusual step of asking Boston Children’s to seek an independent analysis of the project’s cost. The cost-control law allows the state to require that extra step in its approval process for major new hospital construction. Once the analysis is complete, state officials will move toward deciding whether to allow the expansion.
But Friends of Prouty Garden could further delay the project. The group has already sued, claiming that the hospital illegally began preconstruction before the state signed off. And it has warned of further legal action if the expansion is approved.
The group says that the hospital is expanding mainly to attract more international and out-of-state patients, many of whom may pay more for its services, and that it should pursue a more modest expansion elsewhere. The hospital has seen an overall drop in patients in recent years, according to state data.
“This is about the hospital’s desire to increase revenue,” said Jim McManus, a spokesman for the group.
The hospital says it considered other potential sites, but concluded that building over the garden on its main campus — which stands alongside Harvard Medical School — would be the least costly and disruptive.
That leaves Prouty Garden, with its meandering paths, fountains, towering redwood tree and rabbits that nibble the grass, in the cross hairs. The expansion plan calls for three new gardens, including one on the roof of the new building, which hospital officials say would provide 25 percent more green space over all. But Prouty supporters are unimpressed.
“We need that green escape where the air is open to the wild sky, the trees are old and lovely,” Susan Record, whose daughter spent time at the hospital this year, said at the hearing on the expansion in February. “No smaller green space or rooftop garden can quite duplicate this unique jewel.”
But other parents described the lack of privacy they endured, either in the N.I.C.U. or hospital rooms their ill children had to share with others, at the most trying of times.
Rob Graham, a hospital spokesman, said advances in treatment and improved survival rates for childhood diseases had brought a steady increase in complex patients, many of whom require long stays. Too often, he said, those children have to wait for beds or surgery. Doctors and nurses at the hospital have described rationing intensive care beds and turning away transfers because there was no room.
The hospital wants to add 71 beds, mostly for intensive care patients, to the 396 it has now, as well as four operating rooms and three magnetic resonance imaging machines.
In addition to the new building on the hospital’s main campus, the expansion would include an outpatient building with 60 exam rooms about a mile away. Two buildings on the main campus would also be renovated.
Boston Children’s is also planning to add 48 inpatient beds to a satellite location in Waltham, a Boston suburb, that is now used only for day surgery. But it is not currently seeking state approval for that project, estimated at $300 million.
Dr. Paul Hattis, a professor at the Tufts University School of Medicine and a former member of a commission created under the 2012 law to study health care costs, said the omission seemed to be “an effort to obscure the total cost growth impact” of the hospital’s building plans.
Mr. Graham said the Waltham project was “still under development.”
Financial filings show that Boston Children’s earned $113 million on operations in 2014, a 28 percent increase, largely because of a surge in international patients. The hospital also recently acquired a big primary care practice that serves New York City suburbs.
Dr. Alan Lassiter, an expert on children’s hospitals at the Advisory Board Company, an analytics and research firm, said the work by Boston Children’s to attract international and wealthy suburban patients reflected “a seismic shift” in survival tactics for children’s hospitals as they faced increased competition from general hospitals and static local demand.
Patients from other countries, he said, often “will pay out of pocket with cash for very high-end intensive services,” while suburban patients tend to have private insurance that reimburses well. About a third of Boston Children’s patients are covered by Medicaid, the government insurance program for the poor. That is fewer than most children’s hospitals, Dr. Lassiter said, but more than most hospitals that treat adults.
Dr. Hattis said the expansion would probably drive up the hospital’s operating costs, not least by requiring a larger staff. Hospitals often cover such increases by raising the amount they charge private insurers, he said.
But Mr. Graham said serving more international and out-of-state patients who needed complex care would help the hospital generate revenue without charging Massachusetts insurers more.
Such assurances matter little to people whom Prouty Garden helped endure tremendous loss.
Elizabeth Richter’s brother, David Horton, died of a brain tumor at Boston Children’s in 1973, when he was 12. His family scattered his ashes in the garden, Ms. Richter said, after spending countless hours with him there while he was sick.
“Those times we spent together as a family in that garden, just laughing and enjoying each other’s company, those were precious moments,” she said. “So this goes very deep for me.”