Times have changed in the 1940s-1950s, cottages and "granny flats" were common in most residential areas.  Subsequently, restrictions were often put in ;lace to encourage low-density development, which included outlawing this type of "second home" in an existing house or the backyard.  In some cases, homeowners continued to build ADUs anyway, without permits, including external, detached structures, as well as attached or interior second homes.  

Now, part of the push for changes in ADU regulations is an attempt to legalize (and register) existing ADUs that may have been constructed without the proper permits.  This can help local governments reach their affordable housing goals, while ensuring ADUs meet safety standards to protect inhabitants.

Changing demographics also play a role.  According to U.S. Census data, our population is aging, while the average family size has fallen, with a marked increase in single parents.  There are also more single persons living alone.

Many of these individuals have household incomes that don't match the "typical" housing prices in the area where they reside, or wish to reside --factors that have also contributed to growing interest in smaller home options, including ADUs.

(Excerpt from The SRES Professional)