Tiny home rules in Washington County, OR — covering tiny houses on wheels (THOWs), park model RVs, and tiny home on foundation builds — determine where they are legal and how they get permitted.
Washington County allows foundation-built tiny homes as dwellings or ADUs under CDC 430-89 (ADU) and ORS 197.312(5). Tiny homes on wheels are regulated as RVs or park model homes and cannot be occupied as permanent dwellings on residential lots.
Oregon is a national leader on tiny homes. Tiny houses on permanent foundations under 400 sq ft are built to IRC Appendix Q (adopted as Appendix AQ in Oregon ORSC). They can serve as a primary dwelling, ADU, or detached unit. Washington County allows one ADU per single-family lot in urban unincorporated areas under CDC 430-89, implementing ORS 197.312(5) and HB 2001 (2019): no owner occupancy requirement, no off-street parking requirement. ADU size generally limited to 800 sq ft or 60% of primary dwelling. Tiny homes on wheels (THOW) are regulated as RVs under ORS 446.003 and cannot be occupied as a permanent dwelling on a residential lot except in licensed RV parks. Park model RVs (ANSI A119.5) face similar limits. Hillsboro, Beaverton, and Tigard all allow foundation ADUs under parallel HB 2001 ordinances.
Occupying a THOW as permanent residence: zoning violation, code compliance order. Unpermitted ADU construction: stop-work, retroactive permit, possible removal. Short-term rental use of ADU may trigger additional restrictions.
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See how Washington County's tiny homes rules stack up against other locations.
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