Washington County, OR Environmental Rules: Stormwater Management (2026)
Key Facts
- Governing chapter
- County Code Ch. 14.12, Grading
- Scope
- Unincorporated county only
- Minimum culvert size
- 12 inches diameter
- Terrace spacing
- Every 30 vertical feet
- Enforcing office
- Building Services / building official
Summary
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.
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 project civil engineer. ... Where there is not a public storm water conveyance system in place, all new impervious areas created shall dispose of excess storm water through on-site disposal system. All stormwater runoff from new roof and driveway impervious surfaces shall not increase the concentration of discharge at the place of entry into the drainage basin in a manner that prevents its natural diffusion onto the existing condition.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Community Development Code: Supplement 2 | Code of Ordinances: Supplement 25).
Full Breakdown
12) governs stormwater in the unincorporated area only, incorporated cities like Hillsboro and Beaverton run their own systems. 310(B), whenever grading obstructs or impairs a drainage course, the permittee must install a drainage device sized to county design criteria, with a 12-inch-diameter culvert as the floor unless the project civil engineer specifies otherwise, using only industry-standard materials. 310(A) requires a site-specific drainage analysis report before permit issuance, demonstrating the grading preserves predevelopment flow quantity, quality, rate and pattern to adjoining properties, and won't cause erosion or sedimentation beyond what would occur without the work.
310(I) covers on-site storm water disposal: where no public storm conveyance system exists, every new impervious area, roofs and driveways included, must dispose of excess runoff through an on-site disposal system built to Building Services' OSDS guidelines, and the discharge concentration at the drainage basin entry point cannot exceed what natural diffusion would allow. 310(C). Swales and ditches need a minimum 5% gradient and 1-foot depth, protected from erosion. 310(E). Clean Water Services, the regional stormwater agency, has overlapping jurisdiction for many developed areas.
Violations & Fines
Grading without the required drainage analysis or without the mandated disposal system is enforced by the Washington County Building Official under Chapter 14.12's permit process; the official can withhold final approval, require correction of plans, or deny a permit outright under Section 14.12.060(C) where adequate storm water facilities cannot be shown for a habitable building site. Undersized or unapproved culverts and drainage devices must be redesigned before final inspection.
Frequently Asked Questions
Does this stormwater rule apply inside Hillsboro or Beaverton?
What size culvert does the county require?
What if there's no public storm sewer at my site?
Sources & Official References
Other rules in Washington County
Compare Washington County to another location·View the Oregon environmental rules overview
See something wrong?
Help us keep this page accurate. If you notice an error or outdated information, let us know.