Another big project aims for Sullivan Square. This one has a seawall.

An artist's rendering of the view from the Mystic River of a proposed development on 20 acres along Medford Street in Charlestown.Nelson/Weston & Sampson

Developers on Wednesday filed plans for a 1.7 million-square-foot complex of housing and office space along the Mystic River in Charlestown, a big project that would plug an even bigger hole on the city’s sea-level-threatened waterfront.

The Flatley Co., longtime owners of the Schrafft’s building and neighboring parcels on Charlestown’s old Mystic riverfront, gave formal notice to the Boston Planning & Development Agency of plans to redevelop 425 Medford St., a 20-acre parcel that once housed the Domino Sugar refinery, into a large mixed-use project.

The highlight of the plan would be a waterfront park, elevated 22 feet above base sea level, that would both improve public access to what’s now an asphalt-dominated industrial zone and help to protect a large swath of town against future storm surge. It’s designed to protect against storms the city may see by 2070, if sea level rise continues.

“It’s a barrier wall and a whole waterfront park,” said Flatley chief executive Jon Roche. “And it really solves a lot of flooding issues for that whole area of lower Charlestown, Somerville, and Cambridge.”

Flatley, a Braintree-based firm that has owned property in the area since the 1980s, has been planning the project off and on for years. It took part in city planning for climate defenses in Charlestown in 2018 and sees the site as a key spot to fend off rising seas.

A rendering of a large new development being proposed on Medford Street along the Mystic River in Charlestown.Nelson/Weston & Sampson

It’s also in a hotbed for development.

Not far up Medford Street, work is starting on a plan to convert the Boston Housing Authority’s aging Charlestown public housing complex into a much larger mixed-use development, while on nearby Rutherford Avenue, the old Hood milk plant is becoming a campus of office space and housing. Just last week, a developer filed plans with the city for 13 acres of new construction on sites scattered around Sullivan Square. And now this.

Flatley has been talking with neighbors for some time, Roche said, and has a plan to address potential traffic and transit access, perhaps including water shuttles. The firm intends to emphasize how much the project — which will set aside half of the 20-acre site as open space — will dramatically improve public access to the waterfront. And both the letter and renderings of the project highlight how it will attempt to preserve views of the water from nearby Bunker Hill.

Flatley aims to file more detailed plans in the next few months, which will lead to neighborhood and BPDA review. Given the growing focus on climate defenses and the rapid development in Sullivan Square, the timing is good for the long-planned project to get started, Roche said.

“It seems like the right time for the community and for the city,” he said. “So we’re moving forward.”