> property owners who don't rent out their properties

The existence of property tax (which on a single house in some areas of the state is upwards of $15,000 a year already) already makes it absolutely ruinous to just sit on an extra house you don't need and not rent it out.

> or who don't make efficient use of land

A little more convincing. Though I suspect most empty land in places where anyone would be willing to live in California, is empty because our insane zoning laws don't allow what would otherwise make sense there (I don't care that it's like what most areas have -- it's insane to have laws that would make it illegal to build a place like San Francisco, Brooklyn, or Boston).

The funny thing about those zoning laws though is that they're held up by a rare case of bipartisanship:

- The MAGA Boomer set who doesn't want any more development near them because "it'll bring traffic" or noise or crime

- And most of the "progressives" who don't want any more development anywhere because "we hate greedy developers" and "Not enough of this proposed development is low-income housing for the government to dole out in lotteries to a few lucky families."

A property loses ~30% of its value when rented out and maintenance cost and insurance cost goes up significantly. Plenty of landlords rather have their property be empty and just collect on the equity gains over the years without putting in any effort.