Image courtesy of CBT Architects

The first phase of a 2.6 million-square-foot Charlestown mixed-use development calls for nearly 878,000 square feet of office and lab space just outside of Sullivan Square.

Designs include new pedestrian connections to the MBTA’s Sullivan Square station, Boston-based developer RISE Together and TRAX Development of Newton said in a project notification form to the Boston Planning & Development Agency. The first phase – targeting groundbreaking in early 2023 and completion in 2026 – would begin at 10-40 Roland St. with construction of a 631,800-square-foot office-lab building.

The second phase, scheduled to begin in late 2023, would include two residential buildings totaling 112 units at 116 and 145 Cambridge St. The third phase calls for another 246,000 square feet of office-lab space at 78-100 Cambridge St., breaking ground in early 2025.

Developers said they will designate 20 percent of all residential units as income-restricted, exceeding the city of Boston’s current inclusionary development policy minimums.

Along with new construction, the project includes restoration of the existing Raybern building on Roland Street, where developers will offer subsidized rents for local businesses such as medical offices and local grocers, developers announced.

“The Inner Belt project is the first step in creating a more inclusive approach to development and relationship with an already vibrant neighborhood,” Herby Duverné, CEO of RISE Together, said in a statement.

A public comment period on the proposal expires May 2.

The first phase spans 6 acres of the site controlled by the development team, part of a larger project that could total 2.6 million square feet on 13 acres.

The western edge of Charlestown is attracting large-scale proposals that would redevelop industrial properties and parking lots as offices, labs and housing. The Fallon Co. recently announced plans an 812,000-square-foot redevelopment of 3.2 acres just east of Interstate 93.

The Flatley Co. is seeking approval for a 1.8 million-square-foot mixed-use development at the former Domino Sugar factory at 425 Medford St.

And Related Beal has proposed a 101,000-square-foot life science complex at 420 Rutherford Ave.