Environmental Rules in Multnomah County, OR (2026)
6 county-level rules, plus city-specific rules for 2 cities in Multnomah County, Oregon.
Verified from official government sources
Stormwater Management
Unincorporated Multnomah County requires an engineered stormwater drainage system for any new or replacement impervious surface over 500 square feet, sized to hold post-development runoff to pre-development rates.
Multnomah County Stormwater Drainage Rule
Some RestrictionsMultnomah County Code § 39.6235(A), (D)
Persons creating new or replacing existing impervious surfaces exceeding 500 square feet shall install a stormwater drainage system as provided in this section. This subsection (A) does not apply to shingle or roof replacement on lawful structures. (B) The provisions of this section are in addition to and not in lieu of any other provision of the code regulating stormwater or its drainage and ...
Erosion Control
Unincorporated Multnomah County requires a permit before any ground disturbing activity, and a stormwater drainage system once new or replaced impervious surface tops 500 square feet.
Grading & Drainage Permits, Multnomah County
Some RestrictionsMultnomah County Code § 39.6210(A); § 39.6235(A)
Unless exempt under this Code, whether under MCC 39.6215, 39.5080, 38.5510 or otherwise, no ground disturbing activity shall occur except pursuant to one of the following permits: a Minimal Impact Project (MIP) permit, an Erosion and Sediment Control permit (ESC), an Agricultural Fill permit (AF), a Geologic Hazards permit (GH), or a Large Fill permit (LF). ... Persons creating new or replacing...
Flood Zones
Unincorporated Multnomah County requires a Floodplain Development Permit before any construction, grading, or fill in FEMA-mapped special flood hazard areas, and variances from the standards are not allowed.
Multnomah County Floodplain Development Permits
Significant RestrictionsMultnomah County Code § 39.5015(A)
No structure, dwelling or manufactured dwelling shall be erected, located, altered, improved, repaired or enlarged and no other new development including but not limited to grading, mining, excavation and filling (see “Development” under MCC 39.5005) shall occur in areas of special flood hazard unless a Floodplain Development Permit specifically authorizing the proposal has been obtained from M...
Boat Dock Permits
Inside the Columbia River Gorge National Scenic Area, new private docks are capped at 120 or 200 square feet depending on how many families use them, under County Code section 38.7325.
Multnomah County Gorge Dock Size Limits
Some RestrictionsMultnomah County Code § 38.7325(A)-(B)
New private docks and boathouses serving only one family and one property may be allowed up to a maximum of 120 square feet in size. (B) New private docks and boathouses serving more than one family and property may be allowed up to a maximum of 200 square feet in size.
Defensible Space
Inside the Columbia River Gorge National Scenic Area's forest zones, new buildings must be surrounded by a maintained 50-foot defensible space fuel break clearing hazardous fuels to cut wildfire risk under Multnomah County's own Gorge overlay code.
Gorge Forest Zone Fuel Break Rule
Some RestrictionsMultnomah County Code § 38.7305(B)
All buildings and structural projections such as porches, balconies and decks shall be surrounded by a maintained defensible space fuel break of at least 50 feet to minimize the risks associated with wildfire, unless the County approves an alternative pursuant to (B)(2) or (B)(3) below. Hazardous fuels shall be removed within the defensible space fuel break area.
Flood Elevation Requirements
Unincorporated Multnomah County requires new and substantially improved homes in flood hazard areas to elevate the lowest floor at least one foot above base flood elevation, with manufactured homes elevated higher still.
Multnomah County Flood Freeboard Rule
Some RestrictionsMultnomah County Code § 39.5030(C)(1)
Have the lowest floor, including basement, elevated to at least one foot above the base flood elevation. All manufactured dwellings to be placed or substantially improved shall be elevated on a permanent foundation such that the finished floor of the manufactured dwelling is elevated to a minimum of 18 inches above the base flood elevation. The bottom of the lowest chassis frame beam for all ma...
Environmental Rules by City in Multnomah County
2 cities in Multnomah County have their own environmental rules rules. Each link goes to that city's dedicated page with code citations.
All Multnomah County ordinances
See every category we cover for Multnomah County: parking, noise, fences, fires, animals, pools, and more.
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