Washington County, OR Environmental Rules: Flood Zones (2026)
FEMA flood zone rules in Washington County, OR, also called floodplain regulations or special flood hazard area (SFHA) rules, determine flood insurance requirements and elevation standards for new construction.
Key Facts
- Governing maps
- FIRM and FIS, effective Oct. 19, 2018
- Engineer delineation trigger
- Within 250 ft of mapped boundary
- Max flood-level rise (non-floodway)
- 1 foot, per § 421-7.4
- Regulatory floodway standard
- Zero increase allowed, § 421-7.1
- Review tiers
- Type I, II or III by use
Summary
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.
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 improvements, or other development (including fill) shall be permitted within flood areas on the Flood Insurance Rate Map (FIRM), unless it is demonstrated through hydrologic and hydraulic analysis, performed in accordance with standard engineering practice by a licensed professional engineer, that the cumulative effect of the proposal, when combined with all other existing and anticipated development within the basin based upon full development of the basin as envisioned in the applicable Community Plan or the Rural/Natural Resource Plan, will not increase flood levels during the occurrence of the base flood discharge more than 1 foot.
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
Section 421 of the Community Development Code governs development in floodplains and drainage hazard areas, administered alongside the State of Oregon Specialty Codes adopted under ORS 455. S. 1. 1. 1 sets a zero-rise standard for encroachments into an already-designated regulatory floodway. Simple activities such as recreation trails, property line adjustments, and wildlife-viewing platforms on pilings can proceed through the streamlined Type I procedure under Section 421-4, while a single detached dwelling on an undersized lawful parcel, substantial improvements, and subdivisions move through Type II review under Section 421-5, and floodway culverting tied to private development requires the more demanding Type III Planned Development process under Section 421-6.
Violations & Fines
Development within a mapped floodplain or drainage hazard area without the required engineering analysis and permit type under Sections 421-3 through 421-7 violates Section 215-1's bar on developing land contrary to the Code. The Director can deny a building or development permit under Section 215-2 where an existing floodplain violation is unresolved, and the county prosecutes continuing violations under Chapter 1.14 of the Washington County Code per Section 215-3.
Frequently Asked Questions
Can I build a house in a Washington County floodplain?
How do I know if my property is in a mapped floodplain?
What is a regulatory floodway and why does it matter?
Sources & Official References
Other rules in Washington County
Compare Washington County to another location·View the Oregon environmental rules overview
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