Below Grade, Above Board: The Costs and Benefits of Floodproofing Retrofits for Basement ADU Legalization

The Waterfront Alliance supports initiatives to create resilient coastal communities in New York City and prepare residents for rising sea levels and increased flooding events. They primarily serve the vulnerable communities through education, collaboration, and advocacy. The client asked us to quantify the financial feasibility of building-level flood-mitigating adaptations for potential basement ADUs in areas with both a significant presence of illegal basement units and a high risk of pluvial, coastal, and/or groundwater flooding. We developed a cost-benefit model using FEMA and Census data to analyze the financial viability of different floodproofing intervention measures that can protect units from long-term flood damage and distress. While the benefits of these intervention measures are clear, a gap remains between homeowner financial capabilities and the funds needed to convert basements into flood-safe units. Government support is needed to bridge that gap and safely bring thousands of new units into the New York City housing market.