A 2% chance today, right? Increasing the supply of affordable housing should improve those odds.

What's at stake here is how much affordable housing the city for negotiate for with developers. Critics argue this state bill undercuts the city's negotiating position.

Doesn't the bill only apply to developments with at least 10% onsite affordable housing? Is the city really going to negotiate a higher amount than that?

It allows for as little as 5%. Some in the city are fighting for more. For example Jane Kim, the SF supervisor who Bernie Sanders just endorsed for state senate, is fighting for 25% (down from 30%).[1]

Basically this state bill is a developer move to undercut SF's negotiating position, as they have more political influence at the state level.

[1] http://www.bizjournals.com/sanfrancisco/blog/real-estate/201...