Longmont does not publish a stand-alone retaining-wall guide; walls are regulated under the adopted 2021 International Building Code (IBC) and 2021 International Residential Code (IRC), enforced by Building Services. Under IRC R404.4 and IBC standards, a permit is generally required when a retaining wall exceeds 4 feet measured from the bottom of the footing to the top of the wall, or when the wall supports a surcharge (slope, driveway, structure).
Longmont adopted the 2021 family of International Codes effective January 1, 2022, under its Building Code Update; this includes the 2021 IBC and 2021 IRC. The IRC R404.4 (retaining walls) requires engineered design and a permit for any retaining wall that is more than 4 feet in height measured from the bottom of the footing to the top of the wall, or any wall (regardless of height) that retains a surcharge such as a slope, driveway, or structural load. Walls under 4 feet retaining only soil and no surcharge are generally exempt from permit but still must not interfere with drainage, easements, or sight-distance triangles set by LMC 15.05.100. Retaining walls in the sight triangle are limited to 36 inches solid (or 42 inches if 50% open). Building Services (385 Kimbark St., 303-651-8332) issues the permit. The city is moving to the 2024 I-Codes effective July 1, 2026.
Building a retaining wall over 4 feet, or any height with a surcharge, without a permit and engineered drawings can result in stop-work orders, required removal or re-engineering, and re-inspection fees. Walls within the sight triangle that exceed 36 inches solid must be lowered.
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