Following a similar ordinance signed into law by Mayor Ed Lee last July that dealt with developments that are 100 percent affordable, the new housing density ordinance apples to market-rate developments that have 30 percent affordability.

The housing density ordinance approved by the San Francisco Board of Supervisor on Tuesday was not the first attempt at increasing building heights in exchanging for additional affordable units. It dates back to a proposal by Mayor Ed Lee and Supervisor Katy Tang to craft a density bonus policy to spur affordable housing, proposed to the Board of Supervisors in September 2015.

The Affordable Housing Bonus Program achieved its first success when Lee signed that 100 percent affordable housing bonus ordinance on July 29, 2016.

The resolution by Tang and Supervisor Ahsha Safai "allows developers to build taller residential structures in exchange for making 30 percent of the units affordable," reports Rachel Swan for the San Francisco Chronicle.

Known as HOME–SF, it passed 10-1 on May 23, and is viewed as "a win for developers and pro-growth advocates who say the only way to make San Francisco affordable is to keep building." Tang attributed the success in part to middle class families seeking housing in the city, who saw recent gains when housing for teachers was approved this month.