Opponents of a controversial expansion plan by Boston Children’s Hospital notified the state they will appeal its decision to allow the project.

The group, which objects because the project would build over Prouty Garden, a serene green space on the hospital’s Longwood Medical Area campus, is claiming that state public health officials who approved the $1 billion expansion in October “conducted a flawed and legally insufficient review.’’

“Our appeal is designed to stop this massive and unnecessary construction project, which will increase health care costs, reduce competition, and wipe out an essential natural, historic, and human resource that has brought solace to thousands of patients, families, and staff at Boston Children’s Hospital,’’ said Gregor McGregor, a lawyer for the opponents.

The appeal is expected to be heard in Suffolk Superior Court, where critics also have a separate lawsuit against Children’s to stop the project.

Tom Lyons, a Department of Public Health spokesman, said officials “will review and respond to any communication that they send us.’’ Children’s spokesman Rob Graham said the hospital earned the state’s approval because it “demonstrated a clear need for its clinical care.’’

Children’s is planning an 11-story building on the site of the Prouty Garden and an eight-story outpatient center in Brookline. The project has also raised concerns about higher health care costs. Among the nation’s premier pediatric hospitals, Children’s is also the most expensive one in Massachusetts, and critics worry the new addition will drive up costs by pulling more patients from lower-cost competitors.

To keep costs in check, Massachusetts health officials imposed conditions on Children’s, such as restricting the hospital from passing excessive costs on to patients and insurers.

Priyanka Dayal McCluskey can be reached at priyanka-.mccluskey@globe.com. Follow her on Twitter @priyanka_dayal.