The California Land Conservation Act of 1965–commonly referred to as the Williamson Act–enables local governments to enter into contracts with private landowners for the purpose of restricting specific parcels of land to agricultural or related open space use. — By the definition from State of California, Department of Conservation
The Williamson Act of California is a law that provides relief of property tax to owners of farmland and open-space land in exchange for a ten-year agreement that the land will not be developed or otherwise converted for another use. The motivation for the Williamson Act is to promote voluntary land conservation, particularly farmland conservation. — By the definition from Wikipedia
Some real life examples,
John Doe (an anonymous name) purchased a 20 acres vacant land in Gilroy, California with a relatively low price of $680K last year. The land type is “Single Family Residential Land”. However, recently he found his land could not be subdivided and developed, and therefore, had a hard time finding a new buyer to take over.
The below picture display the said land in the red circle. This parcel is surrounded by large area of green color land, which is Williamson Act- Prime Agricultural Land. No one would make residential development in the center of indivisible farming land. Meanwhile, John soon found out that it was even harder to get water/gas/power line connected to his land. It’s no wonder why the land was sold at the price.