Morrison & Foerster committed to about 180,000 square feet, floors 17 through 23, last year, in a 15-year lease that allowed the  building’s developer, Boston Properties, to proceed with construction of the one-million-square-foot tower, which was delayed because of the recession.

250 West 55th Street

The law firm exercised an option it had in that original lease to tack on another floor, 24, a 25,000-square-foot space.

"Boston Properties is making great strides with its new showcase building at 250 West 55th Street – with construction months ahead of schedule,” Charles Kerr, New York managing partner of Morrison & Foerster, said in a statement. “This is a tremendous project and we are excited to be coming in as Boston Properties’ anchor tenant for what will certainly be a marquee professional space in what is fast becoming one of the busiest corridors in Manhattan.”

250 West 57th Street is currently under construction and is slated to be finished by the end of the year.