The indoor-outdoor plaza of Rockefeller Group’s 1221 Sixth Ave. is poised to undergo the most sweeping transformation since the former McGraw-Hill tower between West 48th and 49th streets opened in 1969.

A redesign by Italian architects Citterio-Viel & Partners, to cost in the “mid-to-high eight digits,” will replace the space-wasting sunken plaza with a new, public-friendly plaza at sidewalk level.

A large circular cutout in the center will allow natural light to reach 35,000 square feet of newly created store space on two below-grade levels.

The project will also include new staircases leading down from the outdoor plaza, an open circular portal between the two shopping levels, and generous public seating.

Rockefeller Group SVP Bill Edwards said construction will start when an anchor tenant is signed for all or most of the retail space. He was confident the job would be finished in 2019. Newmark Knight Frank’s Mitch Friedel is fielding offers.