PHILADELPHIA The developers who bought the Dow Chemical Co. Building on Independence Mall in October say they plan to spend $20 million on a renovation that will include a 110-space parking garage and a restaurant with outdoor seating.

Three companies - Keystone Property Group of Bala Cynwyd, Mack-Cali Realty Corp., and parking lot developer Parkway Corp. - jointly bought the building at Sixth and Market Streets from Dow for $45 million.

Dow will lease nearly 200,000 square feet there for its headquarters, Keystone president Bill Glazer said. The U.S. General Services Administration will take over about 135,000 square feet in July. Robert Fahey, who brokered the deal for Dow, said the chemical company would lease space for at least seven years and the government agency for at least 15.

Glazer said the three buyers planned to turn 55,000 square feet of basement space into public parking. Though he said the companies had not yet obtained the necessary permits to convert the space to parking, he said he did not expect garage traffic to clog already busy Market Street because cars could enter from Ranstead Street. The parking would be especially geared toward tourists visiting the nearby Liberty Bell and Independence Hall, he said.

Parking spots alone could not repay a $20 million investment. For instance, if each of the 110 spots brought in $125 per month, it would take more than 120 years to make back the money. Glazer said, "It's not all about parking. It's an investment in the entire property."

He talked about creating "a new pedestrian experience" on the sidewalk outside the building, formerly owned by Rohm & Haas, which Dow bought in 2009, but he would not describe that plan more specifically.

The only space in the building available for lease is a 3,000-square-foot corner on the ground level, which Glazer said might be leased to a restaurant.

"We have plans for a very cool scheme right at that corner. There's a lot of space set back between the building and the sidewalk on Sixth Street," Glazer said. "We're rooting for a lot of things that we can do outside as well as inside to have an indoor-outdoor experience."