Multnomah County, OR Environmental Rules: Stormwater Management (2026)
Key Facts
- Trigger threshold
- 500+ sq ft impervious surface
- Runoff standard
- No increase over pre-development rate
- Storm design event
- 10-year, 24-hour storm
- Reviewing office
- Planning Director
- Applies to
- Unincorporated county only
Summary
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.
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 other impacts and effects, including but not limited to regulation thereof in the SEC overlay.
(C) The provisions of this section are in addition to and not in lieu of stormwater and drainage requirements in the Multnomah County Road Rules and Design and Construction Manual, including those requirements relating to impervious surfaces and proposals to discharge stormwater onto a county right-of-way.
(D) The stormwater drainage system required in subsection (A) shall be designed to ensure that the rate of runoff for the 10-year 24hour storm event is no greater than that which existed prior to development at the property line or point of discharge into a water body.
(E) At a minimum, to establish satisfaction of the standards in this section and all other applicable stormwater-related regulations in this code, the following information must be provided to the planning director:
Full Breakdown
Multnomah County Code § 39.6235(A) triggers on new or replacement impervious surfaces exceeding 500 square feet (roof or shingle replacement on lawful structures is exempt). The required drainage system must, under § 39.6235(D), keep the 10-year 24-hour storm runoff rate at the property line or discharge point no greater than it was before development. Applicants must submit a scaled site plan, drainage documentation, and an Oregon licensed Professional Engineer's certification to the Planning Director under § 39.6235(E). This applies only in unincorporated county land. Portland, Gresham, Troutdale, Fairview and Wood Village run their own stormwater codes.
Violations & Fines
Development proceeds without a required drainage system or engineer certification on file; the Planning Director can withhold permits and require retrofit under § 39.6235(E).
Frequently Asked Questions
Does this apply if I live in Portland?
Is roof replacement exempt?
Sources & Official References
Other rules in Multnomah County
Compare Multnomah County to another location·View the Oregon environmental rules overview
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