A developer foresees a changed First Street with a remake of the CambridgeSide mall area in East Cambridge.

The financial basis for an upzoning of the CambridgeSide mall area must be and will be examined closely, city councillors decided Monday, brushing aside Law Department concerns that could complicate a public discussion.

“Cities do this all the time. This is nothing unusual. You hire a consultant who evaluates it, and he or she reports back,” said city councillor Dennis Carlone, an urban planner.

The giant plan along East Cambridge’s First Street would decrease the mall’s retail space but add residential units, offices and labs – a minimum of 100,000 square feet of ground-floor retail and restaurants and 575,000 square feet of net new gross floor with buildings potentially as high as 155 feet. At least 30 percent of the net new gross floor area would be residential, and half of that would be required to be affordable; a Planning Board memo suggested in September that it would be an even mix of low- and middle-income units.

Though many residents support the remaking of CambridgeSide, some are daunted by the size of the project and worry about its impact on the neighborhood, including on traffic. “Before approving a redevelopment that sets a precedent and inflicts many costs on Cambridge’s historic neighborhoods and its residents,” Marlene Lundberg wrote in a September letter, “it seems wise and responsible for city government to perform [financial] due diligence.”

New England Development agreed to show city decision-makers financial documents that show projected costs and profits, but not let the city have a copy of the proprietary information, leading to a council order Oct. 7 to “conduct a confidential financial analysis of NED’s pro forma to inform the council in confidence on the value of the proposed upzoning.”

But could the financials be kept private, even as the analysis was made public? Doubts led to a “charter right” pause on the order for two weeks to seek legal advice, and what was reported back from city solicitor Nancy Glowa by Mayor Marc McGovern was not reassuring.

“As I understood it … it is her belief that we would not be able to keep the pro forma in confidence very easily, and that if there were a way to keep it in confidence, it would prevent us from even discussing it in public,” McGovern said. “The city has done [financial analyses] at other times; I think the question was around the confidentiality piece of it, and would all of it be public or not. So we could use some more clarification.”

Carlone, the author of the order, was fine with keeping it on the table while he got clarification from Glowa and consulted with colleagues in other cities. Quinton Zondervan, a co-sponsor, said there seemed no point to waiting, since the legal conversation could happen at any time.

The council agreed unanimously to ask the city manager to begin finding a financial analyst to look at whether the upzoning was needed as proposed for CambridgeSide’s survival.

“I don’t see how we can approve anything based on facts that we don’t see and understand –because that’s the whole justification for this upzoning before us,” Carlone said.

Aside from questions about how public the financial analysis will be, another question in the process is who is hired and how that work is overseen. After the council pushed for and approved a citywide master planning process, the City Manager’s Office brought on Boston-based Utile to run a process called Envision Cambridge; though enthusiasm was initially high, the team and its process drew criticism later, including for failure to communicate frequently with the council. The result, which looks to be decided by the next council after Nov. 5 elections, is seen with near universal skepticism.