A renovated, expanded Tisbury School could cost more than $55 million, according to estimates presented by the architect to the school building committee this week. A more modest renovation with several thousand fewer square feet of space was estimated at about $51 million.

Both estimates include costs for administration, project management, architecture and design, construction, furniture and equipment, as well as $1.5 million for temporary classrooms and a $2.9 million contingency provision.

Construction costs are estimated to be about $600 per square foot in September, 2021, project manager Richard Marks told the committee during its Monday night meeting on Zoom.

“Those are high, but they are representative of both building on the Island [and] the current construction market around the commonwealth,” Mr. Marks said.

The estimates also assume that the building will be closed for 15 months so that the work can be completed in a single phase, he added.

“That’s the most efficient way of doing it, in our view,” Mr. Marks said.

The committee members who are teachers — Sean DeBettencourt, Rita Jeffers and principal John Custer — all said that $1.5 million will not be enough for the temporary classrooms.

“That’s a red flag,” Ms. Jeffers said.

Mr. Marks said the estimate intentionally provides for half the student body, with the idea that the other half would be educated off-campus, the way older grades were shifted to the high school while asbestos abatement was performed last year.

“I would look to as many existing facilities as possible,” he said.

Committee members expressed surprise that the price differential was not larger between the two design concepts, one of which includes two additions to the original building while the other has just one.

“This will allow me to be more objective,” said committee member Reade Milne. “To take a significant cost difference out of my decision-making is really helpful.”

Committee members unanimously preferred the $55 million option, also known as option 3, but stopped short of approving further design work until their meeting next week.

“The public will have notice that we are making a decided vote on picking an option,” chairman Rachel Orr said. The meeting will take place at 5 p.m. June 29, on Zoom.

Tisbury selectman James Rogers, who serves on the building committee, said he will ask his fellow selectmen next week to engage an independent financial expert to work on funding mechanisms for the project, including grants and bonding, to reduce the burden on taxpayers. The select board meets June 30 at 4 p.m. on Zoom.

While the tone of the school building committee meeting was optimistic, planning board member Ben Robinson injected a cautionary note.

“I would take off the rose-colored glasses that everybody seems to be wearing,” Mr. Robinson said. “You’re setting yourself up for a serious price hurdle here, and I think it could use a little more vetting.”

The town has struggled for years over plans to renovate, rebuild or replace the 1929 school. After finally qualifying for the Massachusetts School Building Authority (MSBA) program about four years ago, town and school leaders developed a $46.6 million plan to rebuild the school. After state reimbursement, the cost would have been about $32 million. In 2018 the school plan was approved at town meeting, but narrowly failed in the ballot box later. The town subsequently was forced to drop out of the MSBA grant program and start over, this time with no state grant assistance.

A new school building committee was formed last year.

Late last summer, an inspection of the building turned up asbestos, prompting a delayed opening of school. Students in the upper grades were relocated to the regional high school for months while the problem was remediated.

Designs and estimates for the renovation options are posted online.