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

At a glance

Building permit

Required in unincorporated areas; the county enforces the Oregon Building Code¹

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 Boundary⁴

Tiny home on a foundation

Generally allowed, 400 sq ft or less excluding lofts⁵

Owner-builder exemption

Available statewide for ceiling-height, room-size and temperature rules only⁶

Minimum lot size on resource land

80 acres for farm or forest use, 160 acres for EFU rangeland⁷

Water wells

Landowner permit required: $550 fee plus a $10,000 bond⁸

Who to call

Contact details are copied from each office’s official page and were checked against it on September 29, 2026.

Do you need a building permit in Washington County, Oregon?

Yes. Unincorporated Washington County, Oregon requires a building permit for new construction, and the county's Building Services division issues permits and enforces the Oregon Building Code¹. Washington County administers this authority under state law and its Home Rule Charter, enforcing the State of Oregon Building Code rather than writing a separate local code⁹. The current residential code is the Oregon Residential Specialty Code, based on the International Residential Code, whose construction provisions became mandatory statewide in 2024¹⁰. Commercial and other structural work follows the Oregon Structural Specialty Code, whose construction provisions became mandatory statewide in 2026¹¹. 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.

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 County¹³. Reach Building Services at 503-846-3470, at the Public Services Building, 155 N First Avenue, Suite 350, Hillsboro, OR 97124¹³. The same building houses Development Review/Current Planning, Washington County's zoning office, reachable at 503-846-8761¹⁴. Building Services runs an online portal for applying for permits, scheduling inspections and paying fees, plus an electronic plan-review system for plan submittals¹. Before scheduling a final building inspection, the final plumbing and final electrical inspections must be approved, and an erosion-control final inspection must be completed before occupancy; final approval from Current Planning may also be required¹⁵.

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, reviewing land-use applications for both². Urban unincorporated applications are processed within 120 days and rural unincorporated applications within 150 days¹⁶. Land inside an incorporated city such as Hillsboro, Beaverton or Tigard is governed by that city's own zoning, not the county's, so confirm a parcel is actually unincorporated before relying on county rules¹⁷. In the urban R-5 land-use district, the minimum lot area is 5,500 square feet and the target average lot area for new development is 6,000 square feet, with no maximum lot coverage for middle or non-middle housing; other districts set different standards¹⁸¹⁹. Rural land zoned exclusive farm use must be at least 80 acres, or 160 acres if designated rangeland, and forestland-zoned parcels must be at least 80 acres, subject to statutory exceptions⁷. A new urban partition or subdivision needs a development application, a preliminary plat and service-provider letters before the county will review it²⁰²¹.

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 counties²²²³. 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 Report²⁴. 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 backfilling²⁵. 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 trench²⁶. 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 sewer²⁷. A landowner drilling, altering or abandoning a water well must get an Oregon Water Resources Department permit and post a $10,000 bond before starting work, unless the landowner is a licensed and bonded well constructor; the permit fee is $550, the permit expires after 6 months, and a well report is due within 30 days of completion or expiration⁸.

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 documentation⁴. Oregon law lets a county allow one recreational vehicle as a rental dwelling on a rural lot alongside an owner-occupied primary home, but Washington County's own published rule is the prohibition above, so confirm with Current Planning before assuming that state option applies here²⁸¹⁴. 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 dwelling⁵. A tiny house on wheels is classified as a recreational vehicle under Oregon's RV code 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 years²⁹³⁰. A manufactured home may be placed inside the Urban Growth Boundary with a Type I application and floodplain, structural and design-standard materials, and the county will not issue a final inspection or certificate of occupancy for the home before an accompanying garage also gets final approval³¹. Outside the Urban Growth Boundary, Oregon law allows a manufactured dwelling on private land with the landowner's consent, provided the lot has adequate area for approved water supply and sewage disposal³².

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 location²⁴.
3. On rural resource land, confirm the parcel already meets Oregon's minimum lot size for its zone (80 acres for exclusive farm use, 160 acres for EFU rangeland, 80 acres for forestland) before assuming a house is allowed⁷.
4. Apply for a building permit through Washington County Building Services, in person, by mail or through the county's online permitting portal, or through Oregon's statewide ePermitting system where Washington County participates¹³³³.
5. Schedule required inspections in order; final plumbing and final electrical inspections must pass before the final building inspection, and an erosion-control final inspection must be completed before occupancy¹⁵.
6. Get a certificate of occupancy before moving in; the building official must issue one for a new dwelling unless a temporary certificate is issued, and the contractor or owner must first supply license information for the trades that did the work³⁴.
7. If doing the structural work yourself, ask about Oregon's owner-builder exemption, which excuses an owner-built home from ceiling-height, room-size and temperature-maintenance rules if you have not used the exemption in the last 5 years³⁵.

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 system²⁶³⁶. 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 minimum lot size ranges from a few thousand square feet in urban districts to 80 or 160 acres on resource land⁷. 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 it³⁷. 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 apart³⁸³⁹. 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%^40414243. The county sits about 8 miles from the Portland metro area and averages 55.7 inches of rain a year, with severe drought in about 8.6% of years⁴⁴⁴⁵⁴⁶.

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 county²⁹³⁰.
• Oregon law lets a county allow one recreational vehicle as a rental unit on a rural lot with an owner-occupied primary home, but Washington County's own published FAQ says RV living is prohibited outside an approved health-hardship permit; call Current Planning before assuming the state option applies to a specific parcel²⁸⁴¹⁴.
• 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 project¹⁴.

Sources

1. Washington County - Building Services (2026 county census)
2. Urban & Rural Land Use Districts (2026 county census)
3. Onsite Sewage (2026 county census)
4. Can I live in my recreational vehicle (RV)?, quoted and re-checked
5. Tiny Houses Frequently Asked Questions, quoted and re-checked
6. 2023 Oregon Residential Specialty Code - ORS 455.320 reprinted (2026 county census)
7. Oregon Revised Statutes, Chapter 215 (Oregon statewide rule), quoted and re-checked
8. Landowner Constructed Wells, quoted and re-checked
9. State building code timeline, quoted and re-checked
10. Residential Structures Code Program (Oregon statewide rule), quoted and re-checked
11. Oregon Structural Specialty Code adoption (Oregon statewide rule), quoted and re-checked
12. Oregon Revised Statutes, Chapter 455 (Oregon statewide rule), quoted and re-checked
13. Washington County Building Services (washingtoncountyor.gov)
14. Washington County Development Review / Current Planning (washingtoncountyor.gov)
15. Washington County’s Inspection Request Process, quoted and re-checked
16. Urban & Rural Land Use Districts, quoted and re-checked
17. Urban & Rural Land Use Districts, quoted and re-checked
18. Washington County CDC Assessment Appendix A, quoted and re-checked
19. Washington County CDC Assessment Appendix A, quoted and re-checked
20. Type II & III Urban Land Divisions Submittal Guidelines, quoted and re-checked
21. Oregon Revised Statutes, Chapter 92 — Subdivisions and Partitions (Oregon statewide rule), quoted and re-checked
22. Onsite Sewage, quoted and re-checked
23. Residential Septic Permitting (Oregon statewide rule), quoted and re-checked
24. Onsite Sewage Permits, quoted and re-checked
25. Final Inspection Request and Notice for Onsite Sewage Disposal System Permits, quoted and re-checked
26. Share of soils without severe limits for a conventional septic drain field (USDA SSURGO): Bolthole Index county data
27. What do I do with the finished material? (Oregon statewide rule), quoted and re-checked
28. Oregon Revised Statutes, ORS 215.490 (Oregon statewide rule), quoted and re-checked
29. Tiny Houses Frequently Asked Questions, quoted and re-checked
30. Tiny Houses Frequently Asked Questions, quoted and re-checked
31. TYPE I Manufactured Home Placement Inside the U.G.B., quoted and re-checked
32. Oregon Revised Statutes, ORS 446.125 (Oregon statewide rule), quoted and re-checked
33. Online building permit application process for homeowners (Oregon statewide rule), quoted and re-checked
34. 2023 Oregon Residential Specialty Code, Chapter 1, quoted and re-checked
35. 2023 Oregon Residential Specialty Code, Chapter 1, quoted and re-checked
36. Washington County Environmental Health (Onsite Sewage Program) (washingtoncountyor.gov)
37. Transportation Assessment, quoted and re-checked
38. Wooded share of land: Bolthole Index county data
39. Share of land at 15% slope or less: Bolthole Index county data
40. Typical home value (Zillow): Bolthole Index county data
41. Farmland value per acre (USDA census): Bolthole Index county data
42. Median property tax bill: Bolthole Index county data
43. Effective property tax rate: Bolthole Index county data
44. Distance to the nearest metro area: Bolthole Index county data
45. Annual precipitation: Bolthole Index county data
46. Share of years in severe drought: Bolthole Index county data

Written September 29, 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.