Available housing units frequently fail to match the needs of a city’s evolving household forms. In response to unmet demand and illegal units, some jurisdictions have altered regulations to permit the development of different types of housing, including both accessory dwelling units (ADUs) and micro-units. Developers in a variety of jurisdictions, however, have shown interest in both unit types. This Article provides the first comprehensive study of regulatory challenges to both ADUs and micro-units in a geographically diverse range of jurisdictions, focusing on micro-unit and ADU development in New York, Washington, D.C., Austin, Denver, and Seattle. The Article discusses how changing household composition is resulting in a mismatch between housing needs and existing housing supply, and it reviews the claimed benefits and potential criticisms of micro-units and ADUs. Finally, the Article evaluates whether demand for these units is a passing fad or signals a more substantial shift in housing and planning patterns.