A developer has notified Somerville officials that it’s interested in building a 1.2-million-square-foot mixed-use project for a new phase of the Assembly Square redevelopment.

Somerville Office Assoc. has proposed a hotel, housing and 1 million square feet of office space at the former Assembly Square Cinemas site.

Somerville Office would break ground as soon as next summer if it finds a major tenant for the office space, Principal Michael Ades told the Boston Globe. The company has owned the property since the 1970s, including a four-story office building. Somerville Office is working with Newmark Grubb Knight Frank on the project.

The first 300,000-square-foot phase of the massive mixed-use project on the Mystic River spanned 66 acres and includes the Assembly Row retail outlets and 450 AvalonBay apartments.

Master developer Federal Realty is beginning a $350-million additional phase that will include 447 luxury apartments, 117 condominiums, 167,000 square feet of retail shops on the lower floors of the 700,000-square-foot Partners Healthcare office complex and a 155-room hotel.

The city last began planning for redevelopment of an additional 73 acres located between Assembly Row and Mystic Avenue. The area contains a mix of small parcels including a La Quinta hotel, 99 Restaurant and industrial users including an iron works and auto repair garage.