Four years after welcoming its first shoppers and residents, one of Boston’s most popular transit-oriented developments is growing up — and out.

“The initial investment and vertical development was intended to seed the environment and grow it incrementally to test what worked and what didn’t,” Federal Realty Investment Trust Vice President Patrick McMahon said. “Once we began to see success on each front, we took to the planning of Phase 2 and really went vertical.”

Federal Realty developed Assembly Row in Somerville and opened the project in 2014. The initial phase, including outlet stores, restaurants and apartments, became known in Boston real estate circles as much for its guest experience as it was for the public-private collaboration that linked it directly to the MBTA Orange Line subway. A combined $100M investment from Federal Realty and government bodies boosted infrastructure in the area, and the transit component is driving the developer to take a more upscale and local approach in its latest phase.

A 10K SF Polo Ralph Lauren outlet store opened this summer as part of Assembly’s second phase. There was more investment from both the tenant and developer in the store than a typical outlet. Federal specifically invested in an enhanced exterior that gives the space more the feel of Newbury Street or the Prudential Center than that of an outlet mall, and McMahon said other tenants have agreed to do something similar when they open at Assembly Row.

“Ultimately the market will dictate what exactly we insert into the balance of the merchandising programming,” McMahon said. “The opening of the Polo store is a nod in the direction where we’re heading for the balance of the retail shops here.”

“By creating an inviting streetscape, it invites people to be here and encourages them to stay here,” he said. “By investing in storefront design to make it look more like downtown Boston, that helps to underscore the quality of the place.”

Another way it is becoming more like downtown Boston is the arrival of a luxury boutique hotel and more neighborhood amenities like medical services, a convenience store and even full-priced retail stores like DavidsTea. The Row, a 158-room Marriott Autograph Collection hotel, opened in early August, and Assembly’s second phase also attracted local celebrity chef Andy Husbands to open an outpost of his popular barbecue restaurant, The Smoke Shop.

It wasn’t that long ago Somerville was known as “Slummerville” due to its cheap housing stock attracting many recent college graduates. The residential evolution of Assembly has contributed to the city shaking its former image.

Montaje, a 20-story, 447-unit apartment building, is more than 90% leased, and rent for the remaining available units starts at over $2,700/month for a penthouse studio, according to Apartments.com. Alloy at Assembly Row, a 122-unit condo building, is completely sold out, with some units selling for over $1K/SF.

“The rapid ascendancy of Assembly Row is astonishing and, in some ways, is the embodiment of some pronounced development trends in the Greater Boston market,” Perry Brokerage Director of Intelligence Brendan Carroll said via email. “It is the resounding success of a place that had long been envisioned as an opportunity but was sluggish in becoming realized — and then WHAM! In that respect, it somewhat reflects the Seaport District.”

One way it has begun to reflect the Seaport is its pivot to an employment center. Partners Healthcare, the largest employer in Massachusetts, moved into a $465M, 825K SF office at Assembly in 2016 next to the Orange Line station, and more offices are planned. A 250K SF office is scheduled to go up at 255 Grand Union Blvd., and later development blocks could also cater to office and life science tenants.

“Kudos to Federal for getting Partners there,” said JLL Executive Vice President Ben Coffin, who works extensively on Assembly Row. “That solidified it as a destination. If it’s good enough for Partners, it’s good for everyone.”

Developers and brokers have pitched projects along the Orange Line from North Station to Assembly as natural relief valves to Kendall Square given their proximity and growth opportunities at a time when parts of East Cambridge have a 0% lab vacancy rate. Coffin thinks Assembly is poised to benefit as a much-needed life science relief valve.

“We’re very bullish,” he said. “Given how tight and constrained sites are in East Cambridge, it has to go somewhere. I really think Somerville is on the path of growth for life science.”