[rsynnott]

> One of the reasons people go there is to see the charming old houses and nice old neighborhoods

The vast bulk of SF is probably not of any particular interest, though. Most European cities will have areas of historical interest where buildings are protected, sometimes incredibly strictly (here's a 250 year old 8000sqft house which would require 1.5million renovation to make it habitable, due to strict controls on acceptable renovation methods: https://www.independent.ie/life/home-garden/homes/georgian-p...). But that doesn't mean that you can't densify anywhere. Areas of historical importance are normally a small fraction of a city.