Environmental Rules in Washington County, OR (2026)
4 county-level rules, plus city-specific rules for 2 cities in Washington County, Oregon.
Verified from official government sources
Stormwater Management
In unincorporated Washington County, any grading that blocks a drainage course requires a properly designed drainage device, minimum 12-inch culvert, under County Code Chapter 14.12. New impervious surfaces without a public storm system must use an on-site disposal system that doesn't concentrate runoff beyond natural conditions.
Unincorporated Washington County Stormwater Rules
Some RestrictionsWashington County Code § 14.12.310(B), (I)
Whenever a grading operation obstructs or impairs the flow of surface water run-off through a drainage course, a suitable drainage device, designed according to approved county design criteria, shall be constructed to convey the flow past the point of impairment or obstruction. The minimum size culvert to be used for such conveyance shall be twelve inches in diameter unless designed by the proj...
Erosion Control
Washington County requires a grading permit for excavation, fill, private road cuts and site grading anywhere in the unincorporated county, with detailed exemptions for small jobs, excavations under three feet or 150 cubic yards, minor fills, basements tied to an existing building permit, but not for larger or hazard-prone sites, which need a licensed engineer's geotechnical report.
Washington County Grading Permit Rules
Some RestrictionsWashington County Code §§ 14.12.010, 14.12.040
14.12.010 - Scope. The purpose of this chapter is to regulate all civil engineering related work including site grading inside the un-incorporated areas of Washington County in order to safeguard health, safety, private property protection, public welfare, and to minimize erosion caused by said activities within. ... 14.12.040 - Permit—Required—Exemptions. No person shall do, cause, permit, aid...
Flood Zones
Building in a mapped floodplain or drainage hazard area in unincorporated Washington County requires engineered hydrologic analysis under Section 421-7, and most new construction cannot raise base flood levels by more than 1 foot. The county enforces the Oregon Specialty Codes under ORS 455 alongside its own Section 421 review.
Washington County Floodplain Development Standards
Significant RestrictionsWashington County Community Development Code §§ 421-1, 421-7.4
The County administers and enforces the State of Oregon Specialty Codes pursuant to the requirement established in ORS 455. The Oregon Specialty Codes contain certain provisions that apply to the design and construction of buildings and structures located in Special Flood Hazard Areas. ... 421-7.4In areas where a regulatory floodway has not been designated, no new construction, substantial impr...
Flood Elevation Requirements
New homes and major remodels built inside a mapped flood area in unincorporated Washington County must have their lowest floor, including any basement, elevated at least 1 foot above the base flood surface elevation and anchored against flotation and collapse, per Community Development Code Section 421-8.2.
Washington County Flood Elevation Rule
Some RestrictionsWashington County Community Development Code § 421-8.2(A)
421-8.2 Construction standards for new dwellings and substantial improvements to existing dwellings in flood areas: A. All new dwellings and substantial improvements to existing dwellings shall have the lowest floor, including any basement, elevated to at least 1 foot above the flood surface elevation and shall be anchored so as to prevent flotation, collapse or lateral movement;
Environmental Rules by City in Washington County
2 cities in Washington County have their own environmental rules rules. Each link goes to that city's dedicated page with code citations.
All Washington County ordinances
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