The Public Press developed this map showing areas in red where buildings are highly susceptible to earthquake damage.

City Hall sources have confirmed the basic details of a San Francisco Public Press report from Friday afternoon that the Board of Supervisors will consider requiring the owners of soft-story buildings of three stories or more to seismically retrofit them by 2020 – at the expense of building owners, something sure to rouse controversy.

The legislation was developed and introduced by the Mayor's Office and it's being sponsored by the board's two most prolific and effective supervisors, Board President David Chiu and Sup. Scott Wiener, which is probably a signal that city officials know this one is going to be “challenging,” as one source told us.

Details are still being hammered out before the measure is introduced at tomorrow's board meeting, including some of the financing options that would be open to property owners. But after voters in 2010 narrowly rejected Measure A, a bond that would have provided low-cost loans for the seismic retrofits, property owners could be forced to dig deep to ensure their buildings don't collapse in an earthquake.

Wiener confirmed that the legislation would be mandate on building owners without public money attached: "It would be a mandate that they within a certain time frame do an earthquake retrofit," Wiener told the Guardian.

As the Public Press reported, the legislation would apply to all wood-framed buildings of three stories or more built before 1978, with smaller buildings and single-family homes exempted. In the most recent print edition of the Public Press, extensive coverage of the city's earthquake vulnerabilities estimated that about 58,000 San Franciscans live in the nearly 3,000 soft-story buildings deemed dangerous places to be when the next big earthquake hits.

Wiener said city officials have been deeply involved with negotiations with various effected groups, including building owners and their tenants, who could face displacement as the work is done or higher rents if landlords pass through those costs. Wiener said the legislation is bound to evolve as talks and hearings continue: "There are a lot of variables and the introduction is really just a preliminary step."