The strange dominance of really small single-family homes in San Francisco, with its roots in pre-Quake planning and post-Quake building, has come at last in this century to bite the city in its housing-supply backside. Density doesn’t match need now and it is difficult to see how it ever can. It is as though San Francisco, to personify for a moment, never expected to become a real city. So it allowed builders to fill the thousands of residential blocks with one-story-over-basement structures that cannot reasonably ever be transformed into multi-unit, multi-story buildings—unlike, say, a Mission-District Victorian or a Brooklyn brownstone. And should you be inclined to try, zoning and/or neighbors will prevent you from rebuilding one as a four-story wart on the smooth skin of row-upon-row of SFHs.
In their vast inertial numbers, the Little Boxes will always win. The march of those attached four- or five-room homes, on their narrow 25×100 foot lots, across hundreds of city blocks can only ever be disrupted here and there—a few corner developments, a few big structures on old gas station lots, a few scattered replacements, or the odd added story or ADU.
The die was cast—getting on for a hundred years ago now—and the pattern will persist.