As part of my job, I have to look at old Sanborn Insurance maps which go back to the 1880 in SF. In the1880s and 1890s there were houses all over the place. In the 1913 maps (the first after the 1906 quakes), there are blocks and blocks where there are only a few new houses. There aren't too many buildings left from before the quake. If you look up deeds in City Hall, all records start with 1906, as the old City Hall was totally destroyed. They gave any building that survived a built on date of 1906 in the records.