The Somerville Board of Aldermen voted 9-1 this month to approve zoning changes that would allow a $1B, master-planned commercial development to begin in Union Square alongside the long-awaited Green Line Extension.

“Union Square’s proximity to Kendall Square, MIT and Harvard, one the densest innovation centers in the world, makes it poised for the next wave of economic growth,” Union Square Station Associates (US2) President Greg Karczewski said.

US2 is Somerville’s development partner on the state-approved Union Square Revitalization Project. The plan includes 2.3M SF of transit-oriented, mixed-use development with nearly 1.4M SF reserved for offices at full build-out.

US2 competed against developers from around the country to get the master developer rights for the neighborhood and was officially selected in 2014. It has spent the time since in talks with the surrounding community on how to proceed with the planning and rezoning process.

“One of the goals in the plan stems from how Somerville is one of the most densely populated residential communities in the country and how this development can take some of the tax burden off its residents,” Karczewski said.

Upwards of 85% of Somerville’s residents leave the city to work in areas like Kendall Square, where companies are increasingly getting priced out or face growing pains with nowhere to build in the neighborhood’s notably tight lab and office market. Karczewski sees this as an opportunity for US2 to build office, lab, retail and hotel space and make Union Square an employment center.

The first phase of the project will be the D2 block, just north of the site of the future Union Square Green Line station. The initial phase is expected to have 400 units of housing and 175K SF of offices or labs. Construction is planned to begin in 2018 and wrap up before the new Green Line station opens in 2021.

When the Green Line Extension was at risk of getting cancelled when its costs ballooned by $1B, Union Square’s development potential came into question. After all, a key component was the new Green Line station getting built in the middle of the neighborhood. The Federal Transit Administration finally approved a scaled-down $2.3B version in April.

The Somerville Board of Alderman voted in December 2016 to put $50M toward the transit link, and Cambridge is paying $25M. US2 will make a $5.5M public benefits contribution to the project.

“We couldn’t think of a better investment,” Karczewski said. “Train connectivity is key to our vision.”