Alpharetta, GA Environmental Rules: Stormwater Management (2026)

Section 3.3.1.E of the Unified Development Code applies its stormwater management standards to new development creating 1,000 square feet or more of new impervious surface, or one acre or more of land disturbance, and to redevelopment meeting parallel thresholds, plus any commercial or industrial hotspot use regardless of size. Section 3.3.1.G then sets the technical standards: for plans submitted on or after February 2, 2017, applicants must use Runoff Reduction, designing the system to retain the first 1.0 inch of rainfall on site to the maximum extent practicable, with a Water Quality alternative removing 80 percent of average annual total suspended solids only where runoff reduction is proven infeasible.

Stream channel protection requires 24-hour extended detention of the 1-year, 24-hour storm, erosion controls, and preservation of applicable stream buffers, while overbank flood protection requires controlling the post-development peak discharge to pre-development rates for the 2, 10, and 25-year, 24-hour storm events, and extreme flood protection covers the 100-year, 24-hour storm. Every covered project must complete a pre-submittal meeting, a stormwater concept plan, and a full stormwater management plan sealed by a Georgia-licensed engineer or landscape architect, and record an inspection and maintenance agreement against the property. The Directors of Community Development and Public Works jointly administer the program.