I think there are a lot of ways to in-fill housing in SFBA without affecting open space. Allowing tall skyscraper condos in downtown SF makes a lot of sense (and has been one of the major changes from 1995 onward); the continued replacement of light industrial with lofts (from the 1980s), and subdividing victorians into apartments (1920s?).

Most of this has been focused on SF itself. I'd be happy to see more apartment complexes in the 2-5 story range replacing single family homes in parts of the peninsula. Improving transit can also make the east bay and San Jose itself more viable. HSR will arguably make Stockton/Modesto an option for a lot of people.

Actual home space isn't the limiting factor I think. I'd like to see the supporting infrastructure be more dense and efficient (roads/transit, schools, utilities).

Of course, once I bank my first $250mm, I'd like a $20mm house in Palo Alto as it is right now, and a $10mm loft and $10mm house up in SF. :)