Adverse possession has been characterized in the media as an "obscure law" but it has actually a fundamental tenet of property law and has been part of American law since the 1600s and part of European law since the 1400s.
  California (and every state) has an adverse possession law (delineated in the California Code of Civil Procedure). Details may vary from state to state but the law is fundamentally the same everywhere and is relatively unchanged from its first inception in Europe almost 600 years ago. In California, adverse possession requires five years of continued use which is "open and notorious" and "adverse" to the owner's interest. The maintenance and upkeep and improvement of the property is required and for the five years of use the property taxes must be paid for the property being adversely possessed. Homes Not Jails has met the requirements having squatted Page Street openly since February of 1993 to early 1999 and having painted the property, remodeled interiors and generally maintaining it.
 The concept of adverse possession was born from the belief that society's best interests were met when property and land were utilized productively rather than sitting fallow. This interest was especially noted 600 years ago in relation to food production. During this agrarian time, people were starving and lacked the land to produce the food for themselves or others. At the same time, many landowners had farmland sitting unused. Society's interests were served by letting the people who had no food grow food to eat on the land sitting unused and unproductive. Adverse possession was thus created to utilize unused land so people could be fed.
 That, of course, is very analogous to San Francisco where many people have no housing while housing sits empty and Page Street is a perfect example of unproductive land being made productive in a way that benefits society.
 Adverse possession was fundamental to the expansion of the United States in the 1800s and to the establishment of California as part of the United States. Homesteading laws and programs which produced the migration and settling of the west are based on the same concepts as is adverse possession, in that homesteaders had to agree to live on and improve and utilize productively tracts of land for (typically) five years in order to acquire ownership. Homesteading and adverse possession laws, though, were also perversely used as the legal justification for the seizure of property from native Californians and Americans.
 Urban squatters have often utilized adverse possession to acquire abandoned buildings, mostly in rundown neighborhoods in East Coast cities. The acquisition of Page Street via adverse possession would be a first in San Francisco and possibly its first use in an urban setting in California.