Thomas Dougherty wrote:

American demographics are changing and so are housing needs. According to a 2017 exposition held by AARP and the National Building Museum, entitled “Making Room: Housing For a Changing America,” 28% of the American population is single people living alone. Yet almost 90% of the nation’s residential units consist of two or more bedrooms. Accessory dwelling units (ADUs) were highlighted as one of the greatest potential resources to address the need for smaller residential structures. For towns and cities built according to the traditional development pattern, this often means ADUs along alleys.

Over the last century, the vast majority of municipalities in America have severely restricted, or made illegal outright, the building of a second dwelling in the backyards of single-family homes. That is beginning to change. For example, in 2017 California began the process of legalizing ADUs statewide. For towns and cities across the country, ADU legalization is making inner block development possible for the first time.

...

Transforming American alleys into streets through the development of ADUs would begin to create the density needed to support highly desirable, walkable, mixed-use neighborhoods. Alley residential streets would be small, prompting the construction of narrow townhouses, coach houses, and apartments over garages or workspaces. They would help address the housing crisis, providing a home for those who do not want or cannot afford a larger, detached single-family home. The new narrow streets could form a secondary street network designed around walking and biking, providing freedom and independence for the young, the old, and those who do not have a car. Alley homes would be less likely to fall prey to rapid gentrification and land speculation, since the lots they are developed on are already privately owned. Slow incremental development with a range of building types and uses will be enabled through the patient capital of a homeowner. As secondary buildings, incremental development and flexible uses would be encouraged. The defined streets could provide a character of prioritizing (or perhaps only being accessible to) pedestrians.