Some of the objections from the article: Sean Kieghran, president of San Francisco's Residential Builders Association, said he supports getting rid of single-family only zoning but doesn't think it will result in many new units. Kieghran said that building fourplexes requires either two staircases or a staircase and an elevator, which takes up too much of the site. And, unless the city streamlined the process of buil…
Also, small elevators are definitely something that could benefit from an economy of scale, and anyway make a big difference in accessibility for any building above one story. Likewise, the possible designs of multi-unit homes on small lots could become more standardized. And the regulations could improve, and the regulatory institutions could gain more experience handling these kinds of projects.
All of that starts with the zoning.