Longmont, CO Environmental Rules: Stormwater Management (2026)

Longmont is a regulated Phase II Municipal Separate Storm Sewer System (MS4) community under CDPHE Permit COR090000 and channels its local enforcement through LMC Chapter 14.26 Stormwater Quality. The chapter prohibits illicit discharges (anything other than stormwater) into the storm sewer system, requires Permanent Stormwater Control (PSC) measures for any development disturbing one acre or more, and authorizes Environmental Services to inspect, sample, and enforce. The PSC permit moves through three phases: Design (technical documentation reviewed via the Design Review process using the PSC Submittal Checklist), Construction (facilities installed per approved plans with as-built drawings stamped by a licensed engineer and approved before construction closeout), and Operations & Maintenance (facilities must remain operational for the life of the development with an annual maintenance report submitted to the city). Construction-phase work also triggers the State CDPS Stormwater Construction Permit through CDPHE if disturbance is one acre or more. The 2025 City of Longmont Design Standards (Section 100) and the city's PSC Design Standards Guidance (rev. March 29, 2023) lay out the acceptable BMPs — extended detention basins, sand filters, grass swales, bioretention — sized using the Urban Drainage and Flood Control District (Mile High Flood District) criteria. Failure to install or maintain a PSC can result in a Stop Work order, civil penalties, and a withholding of the Certificate of Occupancy. Routine inspections of permitted facilities are conducted by Environmental Services at (303) 651-8416.