Multnomah County, OR Fence Regulations: Retaining Walls (2026)

Multnomah County Building Division follows the Oregon Residential Specialty Code (OAR 918-480) and Oregon Structural Specialty Code (OAR 918-460) for retaining walls. A permit is required when the wall exceeds 4 feet in height measured from the bottom of the footing to the top of the wall, OR when any wall retains a surcharge load (driveway above, building, or slope with grade differential). Walls over 4 feet require engineered plans stamped by an Oregon-licensed Professional Engineer. Portland City Code 24.10 (Administration and Enforcement of Building Code) and PCC 24.70 (Earthquake Hazard Mitigation) impose the same 4-ft threshold but add mandatory geotechnical review for walls in Landslide Hazard Zones mapped under PCC 33.632 — most of the West Hills (SW Portland above Terwilliger, NW Hills above Cornell) and Portland Heights. Drainage behind the wall must include perforated drain tile, gravel backfill, and weep holes per ORSC R404. Setbacks from property lines follow the underlying zoning: typically 0-3 ft in residential, 0 ft at the front property line for walls under 30 inches. Multnomah County Floodplain Development Permit is required if the wall is within an SFHA (ORS Goal 7 / NFIP). Portland's 2023 PICM requires habitat assessment for walls in riparian setbacks along the Willamette and Columbia Slough.