Business & Tech

City seeks exemption to allow shadow cast by Winthrop Square tower

26winthrop - Millennium Partners' proposal for the site of the Winthrop Square Garage. (Handel Architects)
Handel Architects
Millennium Partners wants to build a 775-foot-tall tower on the site of the Winthrop Square Garage.

The skyscraper that Millennium Partners has proposed for Winthrop Square could be the last new building to cast a shadow on Boston Common, under a plan being floated by city officials to win support for the 775-foot tower.

The Walsh administration is willing to write tougher rules restricting the size of shadows that new buildings can cast on Boston Common, as long as there’s an exemption for Winthrop Square, said Brian Golden, director of the Boston Planning & Development Agency. That could effectively cap the height of future buildings in parts of Downtown Crossing.

The deal could allow the city to reap the $153 million that Millennium is offering to pay for rights to build on the site of the defunct parking garage. And, Golden said, it could ease worries from neighbors and parks advocates that the Winthrop Square Tower would be just the first of several to darken the historic park.

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“We think this is a meaningful, concrete step to address those concerns,” Golden said. “And it yields $153 million for the City of Boston.”

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A 1990 state law governing shadows on the Common created a “shadow bank,’’ limiting the shade cast in the historic park by future buildings to about an acre. The Ritz-Carlton on Avery Street, also built by Millennium, used about three-fourths of the available shadow bank when it was built in 2001. About a quarter acre remains available.

Under the BPDA’s plan — which it is floating ahead of submitting formal changes in the shadow law to the City Council and state Legislature — new shadows from Winthrop Square would be subtracted from the existing bank. An agency spokeswoman said it hasn’t yet calculated the exact size of the shadow that would be cast by the Winthrop Square building.

Parks advocates were not impressed by the proposal. Liz Vizza, executive director of Friends of the Public Garden, said it sets a bad precedent, and provides too little protection against additional shadows.

“It doesn’t solve the problem,” Vizza said. “These are laws that have really worked well for 25 years. It’s a pretty big deal to break them.”

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The move comes just months after BPDA chopped down the height of a condo tower proposed at 171 Tremont St. Developers of that project were seeking to draw from the shadow bank. After initially being pitched at 355 feet, the final version of that project, which is not yet under construction, calls for a building that will be just 175 feet tall, short enough that it won’t cast a shadow on the park across the street. Other landowners with long-term plans to build tall buildings near Boston Common could also find their height constrained if there’s no bank to pull from.

Golden acknowledged that developers might fret over the new rule, but said any tower that might tap into the shadow bank would meet an extra level of scrutiny.

“No one’s entitled to anything from the shadow bank,” Golden said. “In any proposal we’ll ask ‘is this shadow-bank worthy?’ ”

In the case of Winthrop Square, where city officials have already divvied up most of Millennium’s $153 million purchase price to fund parks and public housing, the answer, Golden said, is yes.

“This is a once-in-a-generation opportunity,” he said.

Tim Logan can be reached at tim.logan@globe.com. Follow him on Twitter at @bytimlogan.