An architect's rendering of a planned apartment building that would rise in place of office space on what's currently part of the Watertown Mall parking lot. Image courtesy of Gensler

A 1.2 million-square-foot makeover will replace the bulk of the Watertown Mall property with life science buildings and multifamily housing.

Developer Alexandria Real Estate Equities revised its initial proposal this summer, adding 88 residential units to replace a previously-proposed office building.

Alexandria submitted the original master plan in 2021 without any proposed housing, but recently updated the application in what the development team characterized as a response to local feedback.

The 103,727-square-foot housing component will span four stories above 53,580 square feet of ground-floor retail in a new building replacing the Best Buy store. The Target store on the eastern end of the property will remain.

The Watertown Planning Board approved a master plan for the property on Nov. 8. Alluding to the current glut of lab space in Greater Boston, Watertown officials pressed developers on whether buildings completed later in the phasing would include residential space as well as labs. 

Victor Pechaty, design director at architects Gensler, said lab buildings’ requirements for large ventilation structures makes it hard to accommodate housing under the same roof.

“It is a question we get in our firm, and clients are interested in that,” Pechaty said. “It has not been done successfully, at least in the work that we have done.”

Alexandria Real Estate Equities, the nation’s largest life science developer, owns 75 properties in Greater Boston totaling nearly 13.2 million square feet. But it has slowed its construction pipeline amid a glut of lab space in Greater Boston far exceeding current tenant demand.

A plan view of Alexandria Real Estate Equities’ approved Waterfront Mall redevelopment. The newly-added residential block is in the bottom-right of the site, next to the property’s existing Target store. Image courtesy of Reed Hilderbrand

In October, the company revealed it will sell a local development site by the end of the year and take a $90.8 million impairment charge, citing the macroeconomic outlook.

The Greater Boston suburban lab market had a 9.6 percent vacancy rate and 36.8 percent availability rate at the end of the third quarter, according to CBRE.

Alexandria’s “Watertown Mall Transformation” master plan spans a 24-acre site, including the shopping mall and neighboring commercial properties at 480 Arsenal Way and 446-458 Arsenal St. and 500 Arsenal St.

Three lab buildings totaling 848,500 square feet, a parking garage and food and beverage amenities building are planned for the neighboring parcels located west of the mall property. The square-footage includes an existing 135,000-square-foot life science building at 480 Arsenal Way.

Community benefits include $8 million in affordable housing linkage payments, $2 million to the city’s complete streets and bicycle/pedestrian plan, creation of a shared use path along Arsenal Street and a series of upgrades to nearby intersections.