Multnomah County, OR Environmental Rules: Erosion Control (2026)
Key Facts
- Permit trigger
- any ground disturbing activity
- MIP permit fill cap
- under 10 cubic yards
- Stormwater system trigger
- 500+ sq ft impervious surface
- Design storm standard
- 10-year, 24-hour event
- Applies to
- unincorporated county only
Summary
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.
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 existing impervious surfaces exceeding 500 square feet shall install a stormwater drainage system as provided in this section.
Full Breakdown
MCC § 39.6210(A) bars ground disturbing activity without a Minimal Impact Project, Erosion and Sediment Control, Agricultural Fill, Geologic Hazards, or Large Fill permit, whichever fits the scope. The Minimal Impact Project tier covers small jobs: under 10,000 sq ft disturbed, under 10 cubic yards of fill, pre-development slopes of 10% or less, and finished unsupported slopes under 33% and under 4 feet tall (§ 39.6220(B)(1)-(4)). Separately, § 39.6235(A) requires a stormwater drainage system for any new or replacement impervious surface over 500 square feet (roof/shingle replacement excepted), sized so 10-year, 24-hour storm runoff at the property line doesn't exceed pre-development rates. This governs only unincorporated county land. Portland, Gresham, Troutdale, Fairview, and Wood Village enforce their own grading and stormwater codes.
Violations & Fines
The planning director can stop work on-site if inspection finds erosion exceeding permit limits, and no certificate of occupancy or final approval issues until all ground disturbing and stormwater requirements are satisfied (MCC § 39.6210(F)(2), (G)).
Frequently Asked Questions
Does this apply inside Portland?
Do I need a permit for a small garden project?
Sources & Official References
Other rules in Multnomah County
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