Fence height is measured from the lowest grade, so retaining walls stacked with a fence are counted together. Under the adopted 2024 IRC, retaining walls over 4 feet (bottom of footing to top) or supporting a surcharge require a building permit and engineering.
Castle Rock measures fence height from the lowest adjacent grade, so a fence on top of a retaining wall combines toward the height limit and taller combinations may need engineering review. Retaining walls themselves fall under the adopted 2024 International Residential Code: IRC R105.2 exempts retaining walls not over 4 feet measured from the bottom of the footing to the top of the wall, unless the wall supports a surcharge (a slope or load) or impounds Class I, II, or IIIA liquids. Walls above that threshold require a building permit and typically a stamped engineered design.
Building an over-height or surcharged retaining wall without a permit can result in a stop-work order, correction notice, and required engineered repair.
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