Building permits, zoning and septic rules in Washington County, Oregon

Yes. A building permit is required to construct, enlarge, alter, repair or move a residential building or structure in Washington County, subject to specific exemptions⁴. Oregon's statewide requirements also cover new construction and specified structural, plumbing, mechanical and electrical changes, with Washington County administering the permits locally¹. Washington County enforces the Oregon Residential Specialty Code, based on the 2021 International Residential Code, whose construction provisions became mandatory statewide on April 1, 2024¹¹. A qualifying agricultural building, agricultural grading, equine facility or dog-training facility is exempt from the structural code, though electrical, plumbing and mechanical work on it still needs its own permits¹². An owner who performs the work or acts as general contractor on a home they live in or will live in can apply for the residential building, electrical, mechanical and plumbing permits; hired subcontractors must be licensed⁷. A non-habitable one-story detached accessory structure of 200 square feet or less needs no building permit; the limit rises to 400 square feet on a parcel of 2 acres or more when it sits at least 20 feet from every property line¹³.

Oregon's owner-built exemption is narrower than the name suggests: it relaxes only the structural code's ceiling-height, room-size and temperature requirements for a dwelling the owner (with unpaid friends or relatives) builds and intends to occupy, the permit is still issued, and the owner must not have used the exemption in the previous 5 years⁸. If the home does not meet those exempted requirements, the owner-builder must file the permit with the county clerk for recording against the deed⁸.

Washington County's Land Use & Transportation department runs Building Services, the office that issues building permits and schedules inspections in unincorporated Washington County¹⁴. Reach Building Services at 503-846-3470, at the Public Services Building, 155 N First Avenue, Suite 350, Hillsboro, OR 97124¹⁴. The same building houses Development Review/Current Planning, Washington County's zoning office, reachable at 503-846-8761¹⁴. Required inspections vary by project, must occur in the listed order, and each must be approved before work proceeds past it¹⁵. Final plumbing and final electrical inspections must be approved before a final building inspection can be requested, the erosion-control final inspection must occur before occupancy, and Land Use may also need to give final approval¹⁵. The published fee schedule bases residential building permit charges on project valuation; the base fee is $236 for valuations from $1 to $2,000, plus the applicable state surcharge, and regular plan review costs 65% of the building permit fee¹⁶.

Yes. Washington County zones both urban unincorporated land inside the Portland-area Urban Growth Boundary and rural unincorporated land outside it, and processes land-use applications within 120 days for urban projects and 150 days for rural ones². Land inside an incorporated city such as Hillsboro, Beaverton or Tigard follows that city's zoning, so confirm the parcel's jurisdiction before relying on county rules¹⁷. Urban minimum lot sizes vary by district and housing type: detached-home minimums range from 5,500 square feet in R-5 to 2,100 square feet in R-15 and denser districts, while attached-home minimums can be smaller¹⁸. In R-5, a development must average at least 6,000 square feet per lot with no single lot under 5,500 square feet, and the county's assessment found no maximum lot coverage there; that finding is limited to R-5, and setbacks, density and overlays still apply¹⁹. In rural districts, Exclusive Farm Use, Exclusive Forest and Conservation, and AF20 parcels have an 80-acre minimum lot size, AF10 has 10 acres, and AF5 and RR5 have 5 acres¹⁸. Standard subdivisions and other land divisions in unincorporated Washington County require county review and a subdivision or partition plat under the applicable district, development, public-facility and land-division standards²⁰; in urban areas the pre-application conference determines which Community Development Code sections apply, and floodplain, drainage-hazard and natural-resource rules may add requirements²¹. Setbacks also depend on the parcel's district, so ask Current Planning for the figures that apply²². Washington County does not regulate short-term rentals, though cities may²³.

Washington County Environmental Health issues septic system permits for households not served by public sewer, rather than the state Department of Environmental Quality handling it directly, as is true of most of Oregon's 33 non-DEQ-permitting counties^3,24. Permits are required for installing a new onsite sewage system and for repairing or altering an existing system, septic tank or drainfield, and the system type and approved location come from a Site Evaluation Report²⁵. Before backfilling, the installer must notify the county, and Environmental Health then has 7 business days to inspect the completed work unless it waives inspection and authorizes earlier backfilling²⁶. Only 2.2% of Washington County's soils carry no severe limitation for a conventional septic drainfield, so expect Environmental Health's site evaluation to often call for an engineered or alternative treatment system rather than a standard gravel trench²⁷. Pits, privies and vault toilets appear in the county's permit packet as non-water-carried facilities for campgrounds, but approval may require a Land Use Compatibility Statement and compliance with local land-use rules²⁸. A composting toilet does not remove the need for an approved septic system for a home's other plumbing, and material removed from it must be buried under 12 inches of soil; a home in a sewer district must still connect to the sewer²⁹.

Living in a recreational vehicle as your home is prohibited in unincorporated Washington County, with an exception only for a specifically approved health-hardship permit outside the Urban Growth Boundary, which requires a physician's documentation⁵. Oregon's private-land camping-vehicle rule still requires compliance with applicable state and local sanitation, water, plumbing, electrical and sewerage standards, so it does not override Washington County's RV prohibition^5,30. A tiny house built on a permanent foundation, 400 square feet or less excluding lofts, is generally allowed anywhere a dwelling is permitted in unincorporated Washington County, classified as an R-3 occupancy and requiring an Alternate Materials & Methods form if used as an accessory dwelling⁶. In urban unincorporated Washington County, accessory dwellings are allowed only in listed districts; no more than 2 may accompany a primary detached dwelling, and Development Review approval and a building permit are required³¹. A tiny house on wheels is classified as a recreational vehicle and is prohibited as a dwelling in urban unincorporated Washington County; in the rural area it may be allowed only as a temporary health-hardship placement, limited to one accommodation for up to 2 years⁶. A manufactured home may be placed inside the Urban Growth Boundary with a Type I application and required floodplain, structural and design-standard materials³².

1. Confirm the parcel is unincorporated Washington County and whether it sits inside or outside the Urban Growth Boundary, since that changes the RV, tiny-home and manufactured-home rules².
2. Get a septic Site Evaluation from Washington County Environmental Health before designing the house, since soil type sets the septic system type and location²⁵.
3. Confirm the district's minimum lot size and whether the proposed home is allowed; rural resource-land minimums can reach 80 or 160 acres⁹.
4. Apply for the residential building permit through Washington County Building Services and budget for valuation-based permit and plan-review fees^4,16.
5. Schedule each applicable inspection in order and do not proceed past an inspection point until it is approved; final plumbing and electrical inspections must be approved before the final building inspection can be requested¹⁵.
6. Do not move in until the final building inspection is approved and the building official issues a certificate of occupancy for the new dwelling or townhouse, unless a temporary certificate is issued; the general contractor or owner holding the structural permit must first supply contractor license information⁸. After all structural-permit inspections are complete and fees are paid, a certificate of occupancy can be issued³³.
7. If acting as your own general contractor, use licensed subcontractors⁷, and note that the owner-built structural exemption covers only ceiling heights, room sizes and temperature levels⁸.

Get a septic Site Evaluation before you buy if the parcel is not on public sewer; only 2.2% of county soils carry no severe limitation for a conventional drainfield, and Environmental Health can tell you whether a lot needs an engineered system^3,27. Check whether the parcel is inside or outside the Urban Growth Boundary and whether it's zoned exclusive farm use, forestland or a residential district, since resource-land minimums can be 80 or 160 acres⁹. A land-use application affecting a Washington County right-of-way requires a Transportation Assessment, so confirm what it takes to get compliant driveway access before counting on it³⁴. About 71.6% of the county is wooded and roughly 49% of its land sits at 15% slope or less, so terrain and vegetation clearing can vary sharply between parcels even a few miles apart²⁷. Washington County's typical home value is about $568,575 and farmland runs about $22,445 an acre, with a median property tax bill near $4,852 a year at an effective rate of 0.9%²⁷. The nearest city of 100,000 or more is about 8 miles from the county center, and Washington County averages 55.7 inches of rain a year²⁷.