In unincorporated Boulder County a retaining wall over 48 inches (4 feet), measured from bottom of footing to top, requires a building permit. Retaining walls are not counted as fences and must meet International Building Code structural design standards.
Boulder County distinguishes retaining walls from fences: 'The term fence does not include retaining walls.' Per the county's building permit guidance, retaining walls over 48 inches (4 feet) in height require a building permit; shorter walls are generally exempt unless they support a surcharge (an added load such as a slope, driveway, or structure above) or impound Class I, II, or IIIA liquids. Retaining walls and masonry fences must meet the structural design standards of the International Building Code. Because Boulder County is mountainous, engineered wall designs are common. These standards apply to the unincorporated county; municipalities set their own thresholds.
Building a retaining wall over 4 feet without a permit is a code violation; the county may require a stamped engineered design, permit, inspection, or removal of noncompliant walls.
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