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F Cornerstone is turning the office space at 135 E 57th St. into residential units. After the holiday season, the Saks OFF Fifth store will close, and the building will welcome Chelsea Piers Fitness, which has signed a 47,000‑sq‑ft lease. The fitness brand already operates locations at 33 Bond St., 595 Dean St. in Brooklyn, and SL Green’s One Madison Ave. The redevelopment will add 350 apartments, with major construction slated to begin early next year. Steve Gonzalez, TFC’s retail‑leasing lead, said the site offers a unique chance to introduce a premier wellness experience that energizes the neighborhood. The three‑level retail wing opened in the late 1980s as Place des Antiquaires, later became Daffy’s, and most recently housed Saks OFF Fifth.