Early settlers often built a small home to live in while constructing their larger, primary house nearby. When farming was a source of survival for most of the nation’s households, families routinely constructed additional homes on their land when needed. People with wealth and acreage regularly populated their lands with secondary mansions and ancillary buildings independent of the main estate house.
The growth of Washington, D.C., after the Civil War led to the development of alleyway housing, sometimes built as workforce housing in the spaces behind rows of grander, street-facing townhouses. Most of the units were small (about 700 square feet) and poorly constructed. In the 1930s, many of D.C.'s alley homes were torn down. Those that remain have been renovated and modernized and are now popular places to live.
In fact, until the 20th century, people with land built as many homes as they wished. There were few or no zoning rules, municipal services or infrastructure (utilities, roads, schools, trash collection, first responders) to consider.
A historic precedent for the modern day accessory dwelling unit is the “carriage house,” or “coach house.” Originally built for horse-drawn carriages, the structures associated with grander homes were frequently large enough to double as living quarters for workers and stable hands.
Decades later, in response to housing shortages and economic needs, many surviving carriage houses were converted into rental homes. By becoming landlords, the owners gained income from their otherwise unused outbuildings.
Automobile garages have a similar history. Some were originally built with a housing unit upstairs. Over time, many garages were converted (often illegally or under zoning codes no longer applicable today) into small homes when the spaces became more valuable for housing people than vehicles.
With the rise of suburban single-family home developments following World War II, ADUs practically ceased to be built legally in the United States. Then as now, residential zoning codes typically allowed only one home per lot, regardless of the acreage and with no exceptions. Attached and detached garages occupied yard space that might otherwise have been available for ADUs.
Some cities, including Chicago, grandfathered in pre-existing ADUs — but only if the residences remained consistently occupied. In Houston’s historic and trendy Heights neighborhood, old and new garage apartments are common and desired.
But elsewhere, even in rural areas with ample land, property owners are often prohibited from creating secondary dwellings. Many communities today don’t allow new ADUs, even if they did in the past — and even if ADUs currently exist there. (Countless units in single-family homes or yards are technically illegal or are allowed simply because they were created when such residences had been legal.)
ADUs began making a comeback in the 1980s as cities explored ways to support smaller and more affordable housing options within single-dwelling neighborhoods. In 2000, in response to a growing demand for ADUsupportive guidelines, AARP and the American Planning Association partnered to release an influential model state act and local code for ADUs.
More recently, there’s been renewed interest at the state and local levels (see page 8) in legalizing and encouraging the creation of ADUs, driven by the increasingly high cost of housing and, in some places, the belief that homeowners with suitable space shouldn’t be so restricted in the use of their property.
This article is adapted from The ABCs of ADUs, a publication by AARP Livable Communities. Learn more below.