Basement additions through remodeling has become much more common on the Peninsula over the past decade or two.

There are height and density limits, so among the few options is to go down.

Not a whole lot of coverage that I'm finding, though there's a brief mention in this Palo Alto Weekly article (2018):

<https://paloaltoonline.com/news/2018/01/12/groundwater-spurs...>

If you search "palo alto basement remodel" you'll find a heck of a lot of contractor / services pages (enough to frustrate me looking for actual news coverage or mentions).