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

At a glance

Building permit

Required for residential construction and alterations, subject to exemptions⁴

Zoning

Yes, with separate urban and rural unincorporated land-use districts²

Septic permits

Washington County Environmental Health, not the state DEQ³

RV living

Prohibited except an approved health-hardship permit outside the Urban Growth Boundary⁵

Tiny home on a foundation

Generally allowed, 400 sq ft or less excluding lofts⁶

Owner-builder exemption

Owners can pull permits for their own home; the exemption relaxes only ceiling-height, room-size and temperature rules, and can't be reused within five years⁷⁸

Minimum lot size on resource land

80 acres for farm or forest use, 160 acres for EFU rangeland⁹

Water wells

Landowner permit required: $550 fee plus a $10,000 bond¹⁰

Who to call

Contact details are copied from each office’s official page and were checked against it on October 14, 2026.

Do you need a building permit 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.

Who is the Washington County building department, and how do you reach them?

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 503-846-8761¹⁵. Required inspections vary by project, must occur in the listed order, and each must be approved before work proceeds past it¹⁶. 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 approval¹⁶. 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 fee¹⁷.

Does Washington County have zoning?

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 rules¹⁸. 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 smaller¹⁹. 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 apply²⁰²¹. 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 acres²². 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 standards²³; in urban areas the pre-application conference determines which Community Development Code sections apply, and floodplain, drainage-hazard and natural-resource rules may add requirements²⁴. Setbacks also depend on the parcel's district, so ask Current Planning for the figures that apply²⁵. Washington County does not regulate short-term rentals, though cities may²⁶.

How do septic permits work in Washington County?

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 counties²⁷²⁸. 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 Report²⁹. 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 backfilling³⁰. 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 trench³¹. 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 rules³². 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 sewer³³.

Can you live in an RV or tiny home on your land in Washington County?

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 prohibition³⁴⁵. 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 required³⁵. 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 years³⁶³⁷. A manufactured home may be placed inside the Urban Growth Boundary with a Type I application and required floodplain, structural and design-standard materials³⁸.

How do you build a home in Washington County, step by step?

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 location²⁹.
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 fees⁴¹⁷.
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 requested¹⁶.
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 information³⁹. After all structural-permit inspections are complete and fees are paid, a certificate of occupancy can be issued⁴⁰.
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.

What should you check before buying land in Washington County?

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 system³¹⁴¹. 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 it⁴². 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 apart⁴³⁴⁴. 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%^45464748. 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 year⁴⁹⁵⁰.

Watch-outs

• Washington County's own tiny-home FAQ treats a tiny house on wheels as a recreational vehicle: prohibited as a dwelling in the urban unincorporated area and limited to a 2-year temporary health-hardship placement in the rural area, so don't assume 'tiny home' rules are the same everywhere in the county³⁶³⁷.
• Washington County's public-property camping ordinance allows limited camping only when shelter is unavailable and expressly does not regulate privately owned property; it does not create permission to live in an RV on private land⁵¹⁵.
• Washington County does not publish one countywide answer for minimum dwelling size, building height or utility connection requirements; ask Current Planning at 503-846-8761 which standards apply to the parcel before designing a project¹⁵.

Sources

1. About Oregon residential building permits, quoted and re-checked
2. Urban & Rural Land Use Districts, quoted and re-checked
3. Onsite Sewage (2026 county census)
4. About Oregon residential building permits, quoted and re-checked
5. Can I live in my recreational vehicle (RV)?, quoted and re-checked
6. Tiny Houses Frequently Asked Questions, quoted and re-checked
7. Property Owner Statement Regarding Construction Responsibilities, quoted and re-checked
8. 2023 Oregon Residential Specialty Code, Chapter 1, quoted and re-checked
9. Oregon Revised Statutes, Chapter 215 (Oregon statewide rule), quoted and re-checked
10. Landowner Constructed Wells, quoted and re-checked
11. State building code timeline, quoted and re-checked
12. Oregon Revised Statutes, Chapter 455 (Oregon statewide rule), quoted and re-checked
13. What size shed can I put up in my backyard without a permit?, quoted and re-checked
14. Washington County Building Services (washingtoncountyor.gov)
15. Washington County Development Review / Current Planning (washingtoncountyor.gov)
16. Washington County’s Inspection Request Process, quoted and re-checked
17. Washington County Fee Schedule - Fiscal Year 2026-27, Building Services, quoted and re-checked
18. Urban & Rural Land Use Districts, quoted and re-checked
19. Land Use District Summary, quoted and re-checked
20. Washington County CDC Assessment Appendix A, quoted and re-checked
21. Washington County CDC Assessment Appendix A, quoted and re-checked
22. Land Use District Summary, quoted and re-checked
23. Plat Review, quoted and re-checked
24. Type II & III Urban Land Divisions Submittal Guidelines, quoted and re-checked
25. Site Plan Checklist for RURAL Properties, quoted and re-checked
26. Short-term Rentals, quoted and re-checked
27. Onsite Sewage, quoted and re-checked
28. Residential Septic Permitting (Oregon statewide rule), quoted and re-checked
29. Onsite Sewage Permits, quoted and re-checked
30. Final Inspection Request and Notice for Onsite Sewage Disposal System Permits, quoted and re-checked
31. Share of soils USDA rates "not limited" for a conventional septic drain field (SSURGO). A strict rating: the median county is about 6% and a tenth of counties are 0%, so a low figure means most lots need a site evaluation or an engineered system, not that septic is impossible. Cite it only with that caveat: Bolthole Index county data
32. New Construction/Installation Permit for a Septic System, quoted and re-checked
33. What do I do with the finished material? (Oregon statewide rule), quoted and re-checked
34. Oregon Revised Statutes, ORS 446.125 Occupancy on private land (Oregon statewide rule), quoted and re-checked
35. Application Instructions for Urban Accessory Dwelling Units (ADUs), quoted and re-checked
36. Tiny Houses Frequently Asked Questions, quoted and re-checked
37. Tiny Houses Frequently Asked Questions, quoted and re-checked
38. TYPE I Manufactured Home Placement Inside the U.G.B., quoted and re-checked
39. 2023 Oregon Residential Specialty Code, Chapter 1, quoted and re-checked
40. Issuing a Certificate of Occupancy, quoted and re-checked
41. Washington County Environmental Health (Onsite Sewage Program) (washingtoncountyor.gov)
42. Transportation Assessment, quoted and re-checked
43. Wooded share of land: Bolthole Index county data
44. Share of land at 15% slope or less: Bolthole Index county data
45. Typical home value (Zillow): Bolthole Index county data
46. Farmland value per acre (USDA census): Bolthole Index county data
47. Median property tax bill: Bolthole Index county data
48. Effective property tax rate: Bolthole Index county data
49. Straight-line distance from the county center to the nearest city of 100,000+ people: Bolthole Index county data
50. Annual precipitation: Bolthole Index county data
51. Ordinance Provisions, quoted and re-checked

Written October 14, 2026 from the rules quoted below, each taken from an official source and re-checked by an independent reviewer. This is research, not legal advice: rules change, so confirm with the office before you buy or build.