Mr. Trott’s BDT offices look out on the Wrigley’s heavily ornamented clock tower. “It’s the iconic building of Chicago, and one of our most important clients owned it and trusted us to own it for the next 20 years and treat it as they wanted it to be treated,” he said, referring to Mars.

The project’s added retail space acknowledges how high ground-floor retail rents along Michigan Avenue are, topping out at about $500 a square foot. By contrast, office rents in the area are in the $30 to $40 a square foot range.

“The retail rents along Michigan Avenue are enormous, and the Wrigley organization had not taken advantage of that over the years,” said Michael Kaufman, managing partner of Goettsch Partners. “The arcade was a big financial plus for the project.”

But the company has discovered, according to Robbie Robinson, managing director of BDT, that despite its prominent location and status, the Wrigley Building remains something of a mystery to many people. The project partners have been reintroducing the building, which still has a 60 percent vacancy rate, to the business community.

“For every five brokers we have brought into the building,” he said, “at least three have said they’ve never been in here before.”

Several new retail tenants have been signed, the most prominent of which is Walgreens, which announced that it leased 28,000 square feet on two levels in the building’s north tower for what it calls a “flagship” store.

Tom Connolly, vice president for facilities at Walgreens, noted the appeal of the location. “Every tourist who comes to Chicago walks down Michigan Avenue,” he said. “You could not ask for better positioning.”