When the Philadelphia Orchestra performed last month at City Hall, the brass, percussion, strings, and other sections played in an awkward space near the northwest entrance.

"It was where there was a big concrete area, so that is what spot was used," said Alan Greenberger, deputy mayor for planning and economic development.

But if Paul Levy gets his way - and it seems that he will - the orchestra will soon be better positioned to play.

Levy, president of the Center City District, is overseeing the transformation of Dilworth Plaza, the 1960s-era span of concrete and benches on the west side of City Hall.

With a $15 million federal grant in hand, the $50 million project is ready to move out of conceptual design and into a construction phase.

When completed in the fall of 2013, it will include a large and concert-friendly lawn partially covered by a cluster of trees; a programmable fountain that can double as an ice-skating rink and that pedestrians can walk across when it is not in use; a café that will remain lighted at night; and glass pavilions that will lead to - and light up - the subway system beneath.

Led by SEPTA, the subway system is undergoing a $200 million renovation that will begin at the same time.

"It's one of those really transformative opportunities for the city to create a front yard to City Hall, a great civic space and a first-class entrance to transit," Levy said, comparing it to a smaller-scale version of Millennium Park in Chicago.

The new Dilworth Plaza will be about as wide as Rittenhouse Square, but narrower.

The project is two years in the works, with the early design phase paid for by the William Penn Foundation, the Knight Foundation, and the Center City District.

Construction dollars will come from $15 million in transportation funds that are part of the federal stimulus program, and $15 million from the state's Redevelopment Assistance Capital Program.

Levy said he hopes to raise from $8 million to $9 million from local foundations and private contributors. He said the Center City District can also fill any funding gap with a $20 million loan commitment it has from PNC Bank.

In addition, Greenberger said, the city hopes to contribute $5 million to $10 million from its capital budget in the next two years.

"It's going to remake Dilworth Plaza and bring life onto it, instead of the life being below or tucked into strange, dark places on the plaza," he said. Right now, the plaza's benches are populated day and night by many of the city's homeless.

When completed, the new Dilworth Plaza will function as a front door to the expanded Convention Center, which will extend to Broad Street. The café will also offer a clear view of the Parkway.

"The goal here," Greenberger said, "is to make City Hall into the kind of public space it should be."