By Tim Redmond

I teach a class in SF political history at SF State’s Osher Lifelong Learning Institute, and this morning my guest speaker was Calvin Welch, who came to talk about the emergence of the community-based affordable housing movement in the city.

But first he gave us some numbers – and they’re critical to understanding where we are now as a city and what needs to be done about the housing crisis.

As of 2011, San Francisco had 372,000 housing units. Among them:

9,661 are rented out with Section 8 housing vouchers

6,259 are in public housing

18,810 are in residential hotels

28,666 are community-based permanently affordable units

172,000 are (or were two years ago) rent-controlled apartments.

That means, if you do the math, about 237,000 units of housing – 64 percent of the entire stock in the city – are in one way or another under government price regulation.

That, Welch noted, is the only reason that people making less than $100,000 a year are still living in San Francisco today. (more after the jump)