Red means single-family or duplexes only
Orange means triplexes or fourplexes are legal
Green means larger apartment buildings are legal
"Apartment building" is defined to be a building with 3 or more homes. It is illegal to build a building with more than 5 homes in 87% of San Francisco.
2,500 sq ft is a typical lot size in San Francisco
Assumes 2.5 apartments per 10 ft of height per 2,500 sq ft
Many apartment buildings already exist in the red and orange areas but would be illegal to build today
You can see why new development is concentrated in the Mission, Soma, and Tenderloin neighborhoods