How Longmont Handles Environmental Rules: A Practical Guide

Every city handles environmental rules a little differently. In Longmont, Colorado, there are 4 distinct rules that residents and property owners should be aware of. Some are stricter than what neighboring cities enforce, and others are more relaxed. Here is what you need to know.

Stormwater Management

Longmont's stormwater program is governed by LMC Chapter 14.26 (Stormwater Quality) and the city's NPDES MS4 permit issued by the Colorado Department of Public Health and Environment. Any development disturbing more than one acre — or that is part of a larger common plan of development — must design, construct, and permanently maintain on-site Permanent Stormwater Control (PSC) measures to keep pollutants out of the St. Vrain Creek watershed. Permits are administered by Environmental Services and reviewed in three phases (Design, Construction, Operations & Maintenance), with annual maintenance reports required for the life of the development.

Key details: Governing Code: LMC Chapter 14.26 Stormwater Quality. MS4 Permit: CDPHE Phase II Permit COR090000. PSC Trigger: 1 acre disturbance (or common plan). Permit Phases: Design, Construction, O&M. Receiving Water: St. Vrain Creek watershed.

Illicit discharges and PSC violations are prosecuted in Longmont Municipal Court under LMC general-penalty rules (up to $999 fine and/or 180 days imprisonment per LMC Title 1) plus restoration of the receiving water. The city may also revoke development approvals, suspend the building permit, withhold the Certificate of Occupancy, and bill the responsible party for cleanup. Repeat or willful illicit discharges can be referred to CDPHE for additional state Water Quality Control Act penalties of up to $10,000 per day under C.R.S. 25-8-608.

This is one of the stricter rules in Longmont's municipal code. If you are unsure whether your situation complies, it is worth checking with the city before proceeding.

Flood Zones

Longmont's floodplain regulations are codified in LMC Title 20 (Floodplain Regulations). St. Vrain Creek runs through the heart of the city and produced the catastrophic September 2013 flood that displaced hundreds of residents — making floodplain compliance one of the most active regulatory areas in town. Any construction or development inside a FEMA Special Flood Hazard Area (SFHA) requires a Floodplain Development Permit issued by the city Floodplain Administrator, with a $100 application fee.

Key details: Governing Code: LMC Title 20 Floodplain Regulations. Regulatory Standard: 100-year (1% annual chance) floodplain. Main Watercourse: St. Vrain Creek (Sept 2013 flood event). Mapping Source: Effective FEMA FIRM + latest LOMR. Permit Required: All construction/development in SFHA.

Building or filling in the SFHA without a Floodplain Development Permit violates LMC Title 20 and is prosecuted in Longmont Municipal Court at up to $999 and/or 180 days per LMC Title 1 general penalty. The city can issue a Stop Work order, require removal of unpermitted fill or structures, and refuse Certificate of Occupancy. Federal consequences are larger: noncompliant structures jeopardize the entire community's NFIP eligibility, can trigger FEMA Section 1316 denial of flood insurance to the specific property, and can disqualify the owner from federal disaster assistance. Insurance carriers may also refuse or rate-up coverage on unpermitted floodplain construction.

This is one of the stricter rules in Longmont's municipal code. If you are unsure whether your situation complies, it is worth checking with the city before proceeding.

Erosion Control

All construction sites in Longmont must install and maintain erosion and sediment control BMPs to keep soil out of the MS4 and the St. Vrain Creek system. Authority comes from LMC Chapter 14.26 (Stormwater Quality) and is implemented through the 2025 Longmont Design Standards. Sites disturbing one acre or more also need the State CDPS Stormwater Construction Permit through CDPHE. The building permittee is responsible for erosion control associated with any building permit, regardless of acreage.

Key details: Governing Code: LMC Chapter 14.26 + 2025 Design Standards. BMPs Required: All sites — any disturbance. State Permit Threshold: 1 acre disturbance (CDPS through CDPHE). Responsible Party: Building permittee personally. SWMP On Site: Required for 1+ acre sites.

Erosion-control failures are violations of LMC Chapter 14.26 and prosecutable in Longmont Municipal Court at up to $999 and/or 180 days. The city can issue a Stop Work order, require immediate stabilization at the permittee's expense, and bill cleanup of receiving waters. Sites of one acre or more without an active CDPS Stormwater Construction Permit face additional CDPHE penalties of up to $10,000 per day under C.R.S. 25-8-608. The Certificate of Occupancy will not issue until disturbed areas are stabilized to the city's satisfaction.

This is one of the stricter rules in Longmont's municipal code. If you are unsure whether your situation complies, it is worth checking with the city before proceeding.

Grading & Drainage

Longmont reviews grading and drainage through the Land Development Code (Title 15) plus the city's published Design Standards. Drainage Studies are required as part of the Design Review process for new development, and stormwater conveyance must be sized to safely pass the 100-year event without aggravating off-site flooding. Permanent Stormwater Controls under LMC Chapter 14.26 stack on top of the drainage requirements for any site disturbing one acre or more.

Key details: Governing Code: LMC Title 15 + LMC Ch. 14.26 + 2025 Design Standards. Design Storm: 100-year for conveyance and detention. Drainage Study: Required at Design Review. Sizing Criteria: Mile High Flood District USDCM. Foundation Slope: 5% positive grade for first 10 ft (typical).

Grading or drainage work that deviates from the approved Drainage Study or Design Review plans violates LMC Title 15 and the conditions of the development approval; it is prosecutable in Longmont Municipal Court at up to $999 and/or 180 days under LMC Title 1 general penalty. The Certificate of Occupancy will not issue until as-built grading is verified to match the approved plan. Aggravating off-site flooding can also expose the property owner to civil liability to downstream neighbors under Colorado common-law drainage doctrines. Permanent Stormwater Control violations are enforced under LMC Chapter 14.26 as described in the stormwater article above.

This is one of the stricter rules in Longmont's municipal code. If you are unsure whether your situation complies, it is worth checking with the city before proceeding.

The Bottom Line

Longmont is tougher than many cities when it comes to environmental rules. Out of the 4 rules covered here, 4 are rated strict. If you are a homeowner, renter, or business owner in Longmont, take the time to understand these requirements before they become a problem. Most violations come with fines, and some repeat violations can escalate.

These rules come from Longmont's publicly available municipal code. For complete penalty schedules, exemption details, and answers to common questions, see the individual ordinance pages throughout this guide.