The former Ellis School and North Cambridge Catholic High School on Norris Street could become up to 37 rental units, most of them high-end, but there are details to pin down first.

Confident that construction is stopped for now at the city’s former Ellis School and North Cambridge Catholic High School, the Historical Commission decided Nov. 4 to delay talks about landmark status until December.

The 24-name petition for landmarking the 40 Norris St. building — a towering, 112-year-old structure amid a block of single-, two- and three-family homes — was brought forward by nervous neighbors, who sought assurances about how far the developer could go toward converting the school into apartments while the city debated his proposal.

Somerville dentist and developer Mouhab Rizkallah has tentative plans for between 33 and 37 units of luxury housing, minus five to six units set aside as affordable housing, in the 47,000-square-foot building he bought Sept. 15 from the Roman Catholic Archdiocese of Boston for $3.6 million. But after some initial work, including laying hardwood floors, construction has halted while plans are hashed out.

“There’s a lot of ambiguity in the proposal about how many units there would be and how they would be configured,” preservation planner Sarah L. Burks explained to the commissioners. “I don’t get the sense there’s a lot of urgency on this based on the amount of materials [Rizkallah needs] to provide to the Planning Board.”

The board has scheduled a hearing for Dec. 7, bumping it from Tuesday.

The number of units alone might cause concern on the street, which otherwise holds a couple of dozen houses. But it’s also how many people might be put in those units that worries neighbors, and Rizkallah — who is waiting for the city to weigh in on how many units it want so see there — doesn’t want to be pinned down on a number.

“My work is not to identify number of people. My work is to identify how to use the space in a way that is practical,” Rizkallah said in a lengthy interview Nov. 3.

“My approach has been much less demanding of the property than the more typical developer would be. While I do want the property to be filled out — all usable space used, and for what is a different question — what I am doing is less than 5 percent alteration to the inside of the building to get to that,” Rizkallah said. “My approach has been to take the spaces that are there and to call them units.”

Vagueness worries neighbors

A one-bedroom market-rate unit would rent for $1,500 or $1,600, a two-bedroom unit for $1,800 or $1,900 and a three-bedroom unit for $2,100 or $2,200, he said. This approach helps avoid demolition and preserve the school’s grand stairwells and other architectural details, while also speeding the work. He hopes to be renting in about a year.

Rizkallah has brought neighbors through the building on three weekends to show them his work, started a Yahoo group to answer concerns and sworn to them he is interested in preserving as much of the building as possible, but they are still wary.

“We don’t trust this developer much,” neighbor Dan Bertko told the commission Nov. 4.

They cite the vagueness of his plans as a reason.

Most units in the building would be two to three bedrooms in size, according to Rizkallah’s September permit application, and there would be as many as eight to 10 one-bedroom units, but “if only 33 units are provided by the planning board, the auditorium area will be converted to two eight-bedroom units with large, open community spaces.”

He doesn’t plan to rent to students, he said Nov. 3, but he also declined to speculate as to what eight nonrelated people would want to live together.

If the board opposed such units, he offers to split them up into more, and to split the auditorium tower into more floors holding more units — again, he says, depending on how many units the city decides it wants. The city’s Community Development office wanted more units than the 33 he first proposed, Rizkallah said.

But pursuing this path sends him straight toward a landmarking roadblock: While declaring a building a landmark protects the outside of a building, not the inside, putting living space into the tower would call for carving windows into the slate. Several neighbors were alarmed by the possibility.

Either way, he used the potential eight-bedroom units as an example of how having more units could be better than fewer, since fewer units could mean more bedrooms and therefore more residential density on Norris Street.

He also believes there are 35 parking spaces in the parking lot; although it was audited for 20 two decades ago, he said he has “an affidavit from the high school attorneys” that the lot was used for the higher number — and that the spaces painted on the asphalt to accommodate the higher number would still be larger than legally necessary. City zoning law demands one parking space per unit.

City agencies involved

With Rizkallah and his plan already before the Planning Board, Office of Inspectional Services and City Council, which asked the city manager’s office to guide the neighbors through the process, the commissioners hesitated to start the landmark study, which would protect the site for a year — treating it as though it were already landmarked.

But although members were clearly interested in the building, they also warned neighbors not to get complacent or overconfident.

“We can’t accept all landmarking,” said William B. King, chairman of the commission. “We’ve heard one side of the case, and our general reaction may be sympathetic. But we’ve done no more than schedule a hearing when we can hear the other side. Don’t go away saying it’s a done deal.”

Rizkallah was not at the hearing. The night before he said he hadn’t been told the meeting was taken place and, because he was seeing patients, could not attend.

He would welcome a landmarking, he said, because he agrees the building is special — but he said that before commissioners made it clear their process could take up the full year in which Rizkallah planned to be building, and then renting.

“I am at a point where I’m afraid my men are going to go unemployed,” he said. “I really hope the Planning Board makes a quick decision either yea or nay, because I need to either move on or stay here. … if in fact, they go ‘nay,’ I can move to another project, I can go purchase something else and keep these guys busy.”