Clear, accessible definitions for common urban planning terms.

What Are Accessory Dwelling Units?

3 minute read

Sometimes referred to as mother-in-law units or granny flats, Accessory Dwelling Units are a hallmark of contemporary planning as jurisdictions of all sizes and histories legalize the construction of these supplementary residential units.

Granny Flat

A newly constructed Accessory Dwelling Unit on a residential property in Clovis, California. | Nicolás Boullosa / Flickr

The term “Accessory Dwelling Unit (ADU)” describes a housing unit that supplements the primary residential unit on a single property. ADU is a more official, technical term for this kind of housing unit—other terms used to describe similar buildings offer more descriptive power, like granny flats, mother-in-law units, backyard cottages, and casitas. Some jurisdictions also use the term secondary unit in an official capacity. The use of any given term to describe this building type depends on local custom, the official definition of bureaucrats, and the style guide of media outlets.

Just as a variety of terms are used to describe the same kind of building, there is also a lot of variety among the kinds of units that fit the definition of Accessory Dwelling Units.

Many ADUs are built in a separate structure, away from the primary residence; others are built above garages or in a separately accessed part of the primary residence, such as a basement. Others are built inside a garage as conversions of the space previously reserved for automobiles and other kinds of storage. Zoning codes that allow Accessory Dwelling Units also vary to reflect the unique residential neighborhoods of each community. ADU-enabling zoning codes would set boundaries on everything from ADU square footage, lot size, primary unit square footage, and setbacks from the property line, among other considerations.

Accessory Dwelling Units have attracted new levels of public attention in recent years, as cities and states around the country have deliberated and approved ADUs in many single-family residential neighborhoods. ADUs have been referred to as a kind of “Missing Middle Housing,” and connected to benefits in improved housing affordability and options for multi-generation housing arrangements. Most recently, zoning that allows ADUs has been credited with curbing the proliferation of over-sized residential units commonly referred to as McMansions.

Many of these laws, in effect, legalize existing, unpermitted Accessory Dwelling Units. In other neighborhoods and cities, ADUs were legal in residential neighborhoods before they were prohibited, which means some existing ADUs are grandfathered into residential neighborhoods. Supporters of new laws to relax prohibitions on ADUs can still expect to encounter opposition in neighborhoods resistant to change. Political opposition to the legalization of ADUs usually faults the building type for adding density and associated ills, like insufficient parking and congestion, in single-family neighborhoods. The sum of those negative parts, according to critics, is the destruction of neighborhood character. 

Housing and planning policy reformers have managed to achieve progress on behalf of ADUs, as housing costs have risen beyond the reach of a larger and larger share of the population every year, and as a growing body of literature has found evidence of the purported benefits of the building type. That progress has been uneven, varying between the state and local levels (as of this writing, Illinois and Chicago offer the most prominent examples) and getting bogged down by legal challenges and the realities of the construction industry. In some places, however, like Los Angeles, ADU construction has ranked among the most common new building types ever since its legalization in 2017. 

Despite the setbacks and delays, more and more jurisdictions all over the country are changing their zoning codes to allow for Accessory Dwelling Units in single-family neighborhoods, in cities from every chapter of U.S. development history. When the history of U.S. planning is updated in the future, the current era will be described as unequivocally supportive of the construction of Accessory Dwelling Units, and the built environment of the future will reflect the legacy of the current trend.


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