The Suffolk County Planning Commission said it is exploring legal options that might overturn the Islip Town Board’s zoning change allowing the long-anticipated Heartland Town Square project in Brentwood to move forward.

The move comes after Islip leaders voted last week to override conditions on the Heartland rezoning set forth in the county commission’s review earlier this year.

Jennifer Casey, chairwoman of the county planning commission, said the three neighboring towns — Huntington, Babylon and Smithtown — have 20 days after Islip sends its decision to the county to raise objections to the rezoning. If any town objects, she said, it might allow the county planning commission to vote to overturn Islip’s rezoning.

Heartland is the brainchild of developer Jerry Wolkoff, who first presented the project to the Islip Town planning board 15 years ago. In it, the former Pilgrim State Psychiatric Center site in Brentwood, which Wolkoff purchased from the state in 2002 for $20 million, would be transformed into a massive mixed-use development.

The original vision for the 450-acre plot off the Sagtikos Parkway included 9,000 apartments, 3 million square feet of office space and 1 million square feet of retail.

The Islip Town Board on Tuesday voted unanimously to change the zoning for the site from residential to a newly established Pilgrim State Planned Redevelopment District — a crucial vote that allowed the project to move forward.

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The approved first phase will be built on a 113-acre portion of the site with the height of buildings limited to 10 floors — five stories higher than what the town’s planning board recommended — and will include 3,504 apartments, 560,000 square feet of retail space and 626,000 square feet of class-A office space.

But in its resolution, Islip included language that removed most of a series of conditions the county planning commission had attached to the project in March — including clauses about traffic, apprenticeship programs for contractors and a specific number of affordable housing units.

Islip Town Supervisor Angie Carpenter said after Tuesday’s vote that most of the county commission’s conditions “were not appropriate” to include on a land-use change, and that items like an apprenticeship program or a requirement to use union labor were better negotiated under any possible future town Industrial Development Agency deal. The Islip Town Board also comprises the five-member IDA board.

Casey said those conditions were crucial to protecting the region from potential impacts of the Heartland project, which sits in the northwest quadrant of Islip Town.

The conditions “looked out for the residents of Suffolk County because with a project of such a magnitude, it affects everyone in the county, not just the Town of Islip,” Casey said.

An objection by any of the three neighboring towns could trigger the county planning commission to hold another public hearing and possibly revote on its recommendation. A two-thirds majority by the county commission would be needed to overturn either part of or Islip’s entire zone change, Casey said. Suffolk County attorneys are currently reviewing their legal avenues and what a possible revote by the commission could do to the project, she said.

“It hasn’t been done before,” Casey said.

Huntington and Babylon town officials said they are currently awaiting communications from the county planning commission.

A.J. Carter, Huntington Town spokesman, said “the main concern is with the traffic elements of this project.”

“We’ve spoken about these issues at length and we feel that in the current form, they haven’t been addressed,” he said.

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While the Town of Babylon “currently doesn’t have any concerns or objections at this point,” said Kevin Bonner, a spokesman for the town, “we’ll certainly be reviewing the letter from the commission and be open to looking at all the options there.”

An email to the Smithtown Town supervisor was not immediately returned Friday.

Among Casey’s chief concerns is traffic during the construction phase. Islip’s approval bans construction traffic through Fish Path going west to Commack Road, instead directing construction vehicles to use Crooked Hill Road to the east or a temporary access route to be built from the Long Island Expressway service road to the north.

“We were told by the county Department of Public Works that there was no way Crooked Hill Road could handle this kind of traffic,” Casey said. “And the state would need to step in with approvals for the service road, which they haven’t.” The state Department of Transportation could not be reached for comment Friday evening.

In addition, a county law requires projects that connect to the Southwest Sewer District to provide at least 15 percent affordable housing, Casey said; Islip’s approval sets the mandatory amount for Heartland at 10 percent. County attorneys are also reviewing whether Islip Town can override the county’s requirement, Casey said.

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Labor leaders, including John Skinner, president of Local 46 Iron Workers, said without expressly written agreements for apprenticeship programs — which he said help train young workers and set them up for good careers that helps feed back into the local economy — promise of future inclusion “is a sham.”

Wolkoff after Tuesday’s meeting said he is “absolutely” open to working with the unions and has “been in talks” with union leaders in recent weeks.