Accessory Dwelling Units are a form of residential infill housing that are poised to revolutionize housing in the United States. Unlike other urban development trends, this one is being driven by homeowners, not professional developers. Through case studies, expert interviews, firsthand anecdotes, images, and data analysis, Backdoor Revolution reveals the opportunities, challenges, and best practices of ADU development for homeowners, including costs, financing, design, zoning barriers, and regulatory loopholes. With sections written for policymakers and small housing advocates, Backdoor Revolution offers insightful analysis and a succinct prescription for solutions to municipal and institutional barriers for ADU development.