Even 4 stories is pretty high density compared to the Bay Area norm.<p>More importantly, in SF, the vast majority of the city is zoned for 1 or 1–2 housing units per lot, with residences only (no mixed use), which means that in practice a huge part of the city is stuck at 1–2 stories. The lot shapes are these skinny awkward things that leave a little yard in the back, but many of those are basically unused dirt patches. Then add wide streets with huge amounts of street parking.