The tax would increase in each of the next two years of vacancy, and again after five and 10 years of vacancy. The goal would be to encourage owners either to develop or sell vacant properties.
Clarke sponsored a bill to set up a similar tax in 2000. It passed but was never enforced due to questions of legality.
Mayor Nutter also might take a dim view of the current bill. As a councilman, he introduced legislation to repeal Clarke's 2000 effort.
Clarke's staff says the current bill differs from the 2000 version and would pass constitutional muster.
"It's very unfortunate that in this case, it may take such a tragic event to actually get the bill adopted," Clarke said.
Speculators often sit on vacant properties, doing little to maintain them, and the blight can have affect the surrounding area.
Some properties pose more immediate dangers from collapse, fires, and squatters. Last year, a vacant mill in Kensington went up in a spectacular blaze, and two firefighters were killed in a wall collapse.
Richard Basciano, the 87-year-old owner of the Market Street property that tumbled Wednesday during demolition, long has owned significant portions of the 2100 and 2200 blocks of Market Street, including the old Forum porn theater. He only recently announced plans to redevelop the area.
The collapse hovered over Thursday's Council meeting, which began with a moment of silence for the victims.
Maria Quiñones Sánchez, who chairs the Licenses and Inspections Committee, suggested that the chronically underfunded L&I may need to find a way to inspect ongoing demolitions.
"The good news is there's a lot of building going on," she said. "The bad news is, with that, we have to always look at our resources and ensure the department is funded to the level that it could secure public safety."
Any money collected from the non-utilization tax would be directed to L&I, Clarke said.
Councilman Bobby Henon said he feared the workers at the site were day laborers, paid under the table in cash, a practice he has sought to halt with legislation.
Henon, a labor stalwart who works for Local 98 of the electricians union, said his concerns were not "about union and nonunion."
"But I can tell you, when you have union affiliations . . . they are properly trained, especially on structural demolitions," he said.
Contact Troy Graham at 215-854-2730 or firstname.lastname@example.org, or follow on Twitter @troyjgraham.