A block-long Back Bay office that was once New England’s largest building is set to become Boston’s largest office-to-residential conversion project.

Vanbarton Group, a New York real estate firm, this month applied to create up to 490 apartments at 31 St. James Ave. The 11-story, 540,000-square-foot property, known as the Park Square Building, stretches a full block between Arlington and Berkeley streets. If approved, the project would cost up to $410 million and be by far Boston’s largest office to residential conversion to date.

“The location speaks for itself as one of Boston’s most desirable neighborhoods,” Gila Cohen, managing partner of Vanbarton Group, said in a statement.

The building has struggled financially since the pandemic and in April sold at foreclosure for $96 million to LNR Partners, a Miami-based subsidiary of commercial real estate giant Starwood Property Trust. That $96 million almost matches its mortgage from two decades ago, property records show, though it’s a far cry from the $160 million mortgage New York’s Capital Properties Management took out in 2017. It’s also 19 percent less than the property’s assessed value, which itself is down 33 percent in the past four fiscal years.

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It’s not immediately clear whether Vanbarton will assume the mortgage, or whether it might buy the property outright from LNR. The conversion would be Vanbarton’s first development in Boston.

The ground-floor tenants at 31 St. James include French furniture retailer Roche Bobois and restaurant Fire + Ice. Vanbarton said it plans to keep about 30,000 square feet of ground-floor retail space.

The ground level will also include “amenities for residents,” wrote Adam Baron, the head of East Coast acquisitions for Vanbarton, in the property’s conversion application. Baron declined to comment for this story.

Vanbarton has completed a 588-unit office to residential conversion project at 160 Water St. in New York, and has another 1,511 units under construction in three other buildings there. One of those projects is located at the longtime headquarters of New York’s Archdiocese.

In Boston, developers have proposed converting some 2 million square feet of underutilized office space into residential projects. Less than 20 percent of the 2,331 units proposed for conversion have been built or are under construction.

At 31 St. James Ave., Vanbarton proposed a unit mix ranging from studios to three-bedroom units. It’s proposing a rent range of $6.50 to $7.50 per square foot for the 80 percent of the building that’s not set aside at affordable rents, equating to roughly $5,425 for a 775-square-foot unit.

Nearly two dozen office tenants occupy 31 St. James Ave. now, though they’re “expected to vacate” within a year, Baron wrote in Vanbarton’s conversion application.

Catherine Carlock can be reached at catherine.carlock@globe.com. Follow her @bycathcarlock.