Though rising in popularity now, auxiliary dwelling units are not a new concept. These tiny homes have existed for many years and were a standard feature of single-family homes during the early 20th-century.


World War II
After World War II, there was an increased need for housing. This need led to a boom in the number of suburban communities. These suburban neighborhoods stressed nuclear family units and zoning codes were changed to reflect this change. The suburban communities conformed to
Euclidean-type zoning codes, a system of single-use zoning regulations that separate districts into residential, commercial and industrial areas.

The rise of popularity of suburban communities continued throughout the 50s and 60s, ultimately outlawing ADUs altogether.


San Francisco
Though the units were now outlawed, construction of these units continued in cities where the demand for housing outweighed the supply. The city of San Francisco experienced a defense boom during World War II, raising the demand for housing for workers. This demand resulted in a large number of construction of these ADUs, where most of the cities workers now resided. By 1960, San Francisco had between 20,000 to 30,000 ADUs, 90% of which were built illegally.

In the late 1970s to 1990s, some municipalities tried to adopt programs that promoted the use of ADUs. However, many of these programs failed as they did not have the necessary flexibility and scope needed for such a project.


Looking Forward
Today, a 
growing number of cities are changing zoning restrictions to allow the construction of these secondary units. ADUs are starting to be recognized as an affordable way to increase the housing supply.