Federal Realty Investment Trust announced details Tuesday for Phase 2 of Assembly Row. Montaje, a 20-story, 447-unit apartment building, will offer a 14k SF rooftop terrace complete with a pool and indoor/outdoor sky lounge. The 20th floor expanse will overlook the Boston skyline and eventually the nearby Wynn Boston Harbor upon the casino’s completion. Alloy, which recently topped off, brings 122 condos to the area and will sit atop a boutique hotel by Marriott’s Autograph Collection, a luxury brand that also includes the highly popular Envoy Hotel in the Seaport. The total number of restaurants and retailers at the Somerville transit-oriented development will surpass 100 by the end of this expansion.

Federal Realty also announced it has begun design work on a new 250k SF office building at 100 Foley St. near the Assembly MBTA station. Planning for the building comes after the company fully leased 100k SF of Class-A office space at 450 Artisan Way and 700k SF at the Partners Healthcare building.