The transformation of Chicago’s long-neglected Old Post Office building into shiny offices reached a crucial milestone on Monday as it officially took the wraps off its $800 million makeover. The three-year effort to revive the vacant 1921 Art Deco structure designed by Graham, Anderson, Probst & White represents the single largest adaptive reuse project in the nation.

Monday’s grand opening tour comes just days after members of the public got their first glimpses of the building’s restored lobby during Open House Chicago weekend. Starting November 4, the employees of the Old Post Office’s first tenant—Ferrara Candy—will do the same when they start working at 433 W. Van Buren Street.

The confectionery company will eventually be joined by other tenants including Cboe, Walgreens, and Uber—which could move in as soon as January. Roughly 1.4 million of the building’s 2.5 million square feet of space will be occupied by June, according to Telos Group, the firm leading the leasing effort.

“The Old Post Office is a once-in-a-lifetime building,” said Telos president and CEO Brian Whiting in a statement. “The sheer scale of this building, spanning three city blocks, allowed us to do what almost no other building can do in terms of amenities.”

The abundance of space allowed the Old Post Office to offer wide, sprawling floor plans and an impressive mix of public and tenant spaces. Improvements include a private 3.5-acre rooftop park with a jogging track, a 28,000-square-foot fitness center, and upcoming public amenities such as a food hall and an extension of the Chicago Riverwalk.

Overseen by the Chicago office of global architecture firm Gensler, the rehabilitation preserves elements of the building’s past including the lobby’s original marble and the refurbished spiral mail chutes found in Ferrara’s new offices.

“People respond to memorable spaces that tell a story, and tenants and companies can leverage that in their brand narrative and culture,” said Gensler’s Sheryl Schulze in a statement. “The more a building can stand out, the better. The Old Post Office exemplifies these facts by combining a historic design with carefully curated, modern amenities focused on the entire experience for a variety of uses.”

The flurry of recent activity shows just how far the once decaying building has come since developer 601W Companies acquired the property from British-born billionaire Bill Davies in early 2016. Three years later, the Post Office is a key piece of the larger revitalization occurring in the Loop’s southwest corner—joining the redevelopment of Union Station, a $500 million investment in Willis Tower, and the upcoming mixed-use megaproject known as The 78.