NEW YORK—Law firm Morrison & Foerster has agreed to take an additional floor at 250 West 55th Street in Manhattan.

Currently under construction by Boston Properties, 250 West 55th Street will total 1 million sq. ft. of space spanning 40 stories when it opens in 2014.

Morrison & Foerster entered into its original 15-year lease for approximately 180,000 sq. ft. about a year ago. The firm will be the anchor tenant of what will be the first new Midtown office building in three years when it opens. That initial lease put the firm on floors 17 through 23. With the addition, the firm will now also occupy the entire 24th floor, totaling 25,000 sq. ft. and bringing Morrison & Foerster’s total space to 205,000 sq. ft.

The firm expects to move into the completed building when it is ready for occupancy in the spring of 2014. Morrison & Foerster currently has more than 200 attorneys in its New York office and 150 professional staff.