Leggat McCall Properties hopes to redevelop East Cambridge’s former Edward J. Sullivan Courthouse into 20 stories of office with 24 units of housing.

The Massachusetts Appeals Court upheld a 2015 Land Court decision on Wednesday allowing developer Leggat McCall Properties to redevelop East Cambridge’s former Edward J. Sullivan Courthouse at 40 Thorndike St. into a mixed-use development: 20 stories of office with a side serving of 24 units of housing.

“We now look forward to working with the city and the East Cambridge neighborhood to transform the derelict former courthouse facility into a modern mixed-use project, contributing many benefits to those living around the building and in the broader community,” said Rob Dickey, the LMP executive vice president who has managed the project.

Dickey said he was pleased by the court’s decision and thanked the Cambridge Law Department and the Massachusetts Attorney General’s office, who were co-defendants in the case. The project remains on track, Dickey said.

The court’s decision was a long time coming – the appeals court typically turns around cases in two months, and “should” do so within 130 days by order of the Supreme Judicial Court. This case took 289 days to decide, with a 12-page unanimous opinion by Justice Diana Maldonado writing for a three-judge panel. It was the third-oldest case on the docket when settled.

The East Cambridge neighbors who brought the lawsuit – Michael Hawley, Graham Gund, Roger Summons and Marie Sacccoccio – will have to decide whether to appeal to the Supreme Judicial Court.

“We’re considering options for further review by the SJC” but haven’t had a chance to discuss the opinion, Hawley said.

They are concerned that the massive tower is out of scale with their neighborhood and was built so densely only because Middlesex County was immune from zoning when putting up its $90 million jail, District Attorney’s Office and Middlesex Superior and Cambridge District courts between 1966 to 1974. Aging and asbestos concerns sent everything but the jail to outlying communities in 2008-09, and the jail closed in June 2014.

The structure was opposed by neighbors when it was built, and they hoped to see the tower torn down when the building emptied.

But their chances of a second appeal aren’t good: In 2015 the court took 26 of the 697 applications to review Appeals Court decisions. That’s 4 percent.

Judge Robert Foster of the Massachusetts Land Court

Judge Robert Foster of the Massachusetts Land Court heard the original courthouse case.

“The sole issue on appeal is whether the courthouse, when it loses its governmental immunity by transfer to the developer, will constitute a preexisting nonconforming structure,” the court summarized, and called the Land Court decision by Judge Robert Foster “well-reasoned.”

The neighbors “argue that the only way the courthouse can be considered a preexisting nonconforming structure is if it complied with the zoning requirements when it was constructed and now fails currently to comply because the city adopted stricter zoning ordinance since the construction,” the court said.

But the court disagreed with that argument: “We discern no meaningful distinction in terms of the protections afforded nonconforming structures in the zoning ordinance between a structure that becomes nonconforming because of a subsequently enacted stricter ordinance and one that becomes nonconforming because of a loss of statutory immunity,” its decision said.

The court also suggested that perhaps the neighbors did not have standing to challenge the developer’s special permit, noting “the facts in the record are thin” but “it would appear the judge was satisfied.”

East Cambridge resident and real estate developer Richard McKinnon said he was not surprised by the decision, noting that courts often defer to lower court decisions. McKinnon said Kevin P. O’Flaherty, the developer’s attorney, was “tremendously talented.”

The neighbors have until Aug. 8 to file for SJC review. They could also choose to ask the Appeals Court to rehear the case if they believe it “overlooked or misapprehended” parts of the case; that would be due by Aug. 2.