Longmont, CO Environmental Rules: Grading & Drainage (2026)

Longmont's grading and drainage rules sit at the intersection of three documents: LMC Title 15 (Land Development Code), which sets development standards for drainage as part of Chapter 15.05 Development Standards; LMC Chapter 14.26 (Stormwater Quality), which sets the water-quality side of the same equation; and the 2025 City of Longmont Design Standards (Section 100 and following), which contain the engineering criteria the Public Works & Natural Resources Department applies during Design Review. New development must submit a Drainage Study showing existing and proposed runoff for the 2-, 10-, and 100-year storm events; on-site conveyance (curb-and-gutter, swales, pipes, channels) must safely pass the 100-year event; and historic runoff rates and patterns to adjacent properties cannot be increased. Sites disturbing one acre or more also must install Permanent Stormwater Control measures sized to the city's water quality capture volume and detain the 100-year event for downstream flood protection. Sizing follows the Urban Drainage and Flood Control District (Mile High Flood District) USDCM criteria. Grading must respect minimum positive drainage away from foundations (typically 5% for the first 10 feet), match approved finished floor elevations, and avoid concentrating runoff toward neighboring lots. For drainage questions contact Public Works & Natural Resources at (303) 651-8416; for permit submittals, Planning & Development Services at (303) 651-8330.