The Green Area Ratio: Nature-Based Solutions Regulation in Washington DC

Nature-Based Solutions (NBS) have emerged as a key strategy to address complex and intensifying urban sustainability challenges, yet empirical assessments of regulatory frameworks that operationalize these strategies remain limited. This study addresses that gap through an empirical evaluation of the Green Area Ratio (GAR), a hybrid performance-based zoning regulation for sustainable landscapes in Washington, DC, using more than a decade of regulatory implementation data. Regulatory plan approval records from 1329 private properties subject to GAR between 2015 and 2025 were analyzed to examine trends in landscaping Best Management Practice (BMP) selection, relationships with parcel characteristics, and interactions with stormwater management requirements. Over this period, approximately 184 ha of multifunctional NBS were approved through GAR plans primarily consisting of landscaping areas, plantings, and tree planting and preservation, often combined through BMP stacking and amplified by a native plant bonus. BMP selection varied significantly according to zoning district, lot area, and GAR Score Requirement. Importantly, GAR requirements influenced BMP selection differently than stormwater requirements. The findings suggest that simplicity, flexibility, and adaptability of the GAR make the regulation a versatile policy instrument for DC and a useful model for cities seeking to expand NBS through performance-based zoning regulation.