Single family detached housing on fully separated lots of 1-2 storeys, and very often only a ground floor, is typical through much of the US. That describes a startlingly large fraction of San Francisco, and even larger "dense" neighbourhoods (Richmond, Sunset) are typically 2-3 floors, where ground level is garage parking ("soft story" construction, a major earthquake concern. Only in NYC and environs do you find generally denser construction.

A 5-6 storey standard would likely more thaan double SF's housing capaacity.