ADU Rules in Hillsboro, Oregon

Hillsboro, the seat of Washington County and Oregon's fifth-largest city, anchors the Silicon Forest — the Pacific Northwest's technology hub centered on Intel's massive semiconductor campus, which is among the largest Intel manufacturing sites in the world. Despite its industrial and tech identity, Hillsboro has robust residential neighborhoods and is subject to Oregon's housing reform mandates. ADU construction follows the Hillsboro Community Development Code in conjunction with statewide requirements.

Oregon Statewide ADU Law — Critical Context

Two Oregon laws establish the framework for ADU development in Hillsboro:

Oregon HB 2001 (2019) requires cities with populations over 10,000 to allow middle housing — duplexes, triplexes, fourplexes, and cottage clusters — in all residential zones. Hillsboro, with over 100,000 residents, is fully covered. Single-family-only zoning is no longer permitted citywide.

Oregon's statewide ADU framework is SB 1051 (2017) plus HB 2001 (2019) middle housing. HB 2098 (2021) is a rental-assistance bill, not ADU.

What Hillsboro Allows

Under the Hillsboro Community Development Code and statewide mandate, the following ADU types are permitted in residential zones:

• Detached ADU: A freestanding structure separate from the primary home, with independent kitchen, bathroom, and entrance.
• Attached ADU: An addition connected to the primary residence with its own dedicated entrance and self-contained living facilities.
• Interior Conversion: Conversion of existing basement, attic, or rooms within the primary structure into a separate, self-contained dwelling unit.
• Garage Conversion: Converting an existing garage — detached or attached — into habitable ADU space meeting Oregon building code standards.

One ADU per single-family lot is permitted by right. Hillsboro's updated code may allow greater density on eligible lots under HB 2001.

Size and Setback Standards

Oregon law prohibits Hillsboro from requiring ADUs smaller than 900 square feet or 75% of the primary dwelling's floor area, whichever is less. Detached ADU setbacks in Hillsboro's low-density residential zones typically include:

• Rear setback: 5 feet minimum
• Side setback: 5 feet minimum
• Height: Generally 15–20 feet for detached ADUs depending on zone

Legal nonconforming structures converted to ADUs may maintain existing setbacks.

Metro Regional Government and Urban Growth Boundary

Hillsboro is within the Metro regional government's jurisdiction and inside the Urban Growth Boundary (UGB) for the Portland metropolitan area. ADU permits apply only within Hillsboro city limits and the UGB. Washington County rural areas outside the UGB are not eligible for ADU development.

Tualatin River and Environmental Constraints

The Tualatin River runs through Washington County and Hillsboro. Properties near the river, its tributaries, wetlands, or associated floodplains may be subject to:

• Oregon Statewide Planning Goal 5 natural resource protections (riparian corridors, wetlands)
• FEMA flood zone requirements including elevated foundations and flood-resistant construction
• Washington County Clean Water Services (CWS) stormwater and stream corridor buffer requirements

Always check your specific property for environmental overlays before designing an ADU footprint.

Seismic Considerations

Hillsboro, like all of the Portland metropolitan area, faces seismic hazard from the Cascadia Subduction Zone offshore. Oregon DOGAMI maps significant liquefaction potential in Tualatin Valley lowlands. All new ADU construction must meet Oregon's seismic building code. Older structures being converted should be assessed for seismic vulnerability.

Permit Process

Oregon's statewide ADU framework is SB 1051 (2017) plus HB 2001 (2019) middle housing. HB 2098 (2021) is a rental-assistance bill, not ADU.

Contact: City of Hillsboro Community Development — (503) 681-6100 — hillsboro-oregon.gov

Disclaimer

ADU regulations in Hillsboro are subject to change as Oregon continues to implement housing reform legislation. Always verify current requirements with the City of Hillsboro Community Development Department. This page is for informational purposes only and does not constitute legal or planning advice.