The new 20-story apartment building at Assembly Row in Somerville recently started leasing units, with plans to begin occupancy in September. Montaje, the 447-unit facility, offers a mix of one-, two-, and three-bedroom residences and such amenities as a 20th floor sky lounge complete with an outdoor roof deck, a chef’s kitchen and dining area, and a media/living room. It will also feature a courtyard with a resort-style pool, grilling stations, an outdoor fireplace; as well as a bocce court; and a resident-only entrance to MidiCi, the Neapolitan Pizza Company restaurant that is set to open this fall. Montaje is one of the centerpieces of the second phase of Assembly Row, the massive mixed use development overlooking the Mystic River that is being undertaken by Federal Realty Investment Trust. The Phase 2 expansion will also include Assembly Row’s first hotel (Autograph Collection by Marriott), its first grocery store (Trader Joe’s), dozens of restaurants and retail shops, a new park, added parking, and FiTRow, a collection of five boutique studios. Six businesses have opened to date in Phase 2: Mike’s Pastry, Ann Taylor Factory Store, Columbia Factory Store, Club Pilates, The Fragrance Outlet, and most recently Yankee Candle. In addition to Montaje, the new housing will include Alloy, a 13-story, 122-unit building set to begin occupancy early next year.John Laidler can be reached at email@example.com.