In a normal place this is accomplished by increasing building height, increasing the building's footprint and reducing the unit size. No additional land needs to be used and higher density increases efficiency of services such as mass transit as well reducing environmental impact.
Unfortunately SF has some of the most restrictive zoning rules anywhere and, as described in the article, construction usually involves evicting tenants paying well below market rent. Given the highly litigious and politicized environment it is much more likely to get worse than better.
