Do I need a permit for an ADU in
Hillsboro, Oregon?

Hillsboro ADU permits — the key details

Oregon Revised Statute ORS 197.312 (effective 2020) requires cities to allow ADUs on single-family residential lots and prohibits owner-occupancy requirements on the ADU itself. Hillsboro adopted this statute into HCC 20.08.40 but requires the PRIMARY RESIDENCE to be owner-occupied — meaning you cannot rent both the main house and the ADU to separate tenants. The ADU can be rented to a long-term tenant while you live in the primary dwelling, or you can live in the ADU and rent the main house, but one must always be owner-occupied. This is Hillsboro's most common misread: applicants think 'owner-occupancy waived' means they can ignore it entirely. They cannot. The permit application must clearly identify the owner-occupant and which unit they will occupy at time of certificate of occupancy. If you plan to rent both units, Hillsboro will deny the permit or require you to apply for a multi-unit rental conversion, triggering higher impact fees and design review.

Detached ADUs (free-standing structures) must comply with IBC Section 422 (accessory structures) and IRC R401-R408 (foundation and framing), plus HCC 20.80 setbacks: 5 feet from rear property line, 10 feet from side property line (if fire-rated exterior walls per IBC 707). The frost depth in Hillsboro's Willamette Valley zone is 12 inches; footings must go 18 inches minimum (6 inches below frost line per IRC R403.1.8). East of the Sunset Ridge (Forest Grove area), frost depth jumps to 30 inches, and footings must go 36 inches — a critical detail if your lot straddles the climate zone boundary. Attached ADUs (above-garage, side addition) and garage conversions follow IRC R310 egress rules: one operable window per bedroom (minimum 5.7 sq ft, sill height 36 inches or lower) or a secondary exit door directly to outdoor space. Many garage conversions fail Hillsboro's initial review because the applicant frames in a bedroom without planning egress before submitting. Submit egress details in your framing plan. Foundation inspection happens before framing; framing inspection before drywall. Rough electrical, plumbing, and mechanical inspections are bundled in the rough trades walk; final building inspection and final utility inspection must both pass before the city signs off the certificate of occupancy.

ADU size exemptions and parking are critical to Hillsboro's current code. Junior ADUs (legally separate kitchens + shared wall + owner-occupied primary only) must be under 800 sq ft. Full ADUs have no size cap, but if total lot square footage plus ADU triggers the sprinkler threshold (typically 5,000+ sq ft of building footprint on a single lot), sprinklers are required in both structures — a cost jump of $15,000–$25,000. Parking: Hillsboro's HCC 20.38 waives off-street parking for ADUs under 750 sq ft in single-family zones. If your detached ADU is 800+ sq ft, you must provide one dedicated off-street parking space (can be gravel or permeable); if attached above a garage, the underlying garage counts. Owner-builder permit: Oregon allows owner-builders for owner-occupied structures. Hillsboro honors this — you may pull an owner-builder permit for a detached ADU if you will occupy it and live on the lot; you cannot use an owner-builder permit if renting out both units. If you hire a contractor, they must be licensed by Oregon's Construction Contractors and Licensing Board (CCB). Plan review typically takes 15-21 days for expedited (staff) review; 30-45 days if Planning Commission review is triggered (required for detached ADUs on lots under 7,500 sq ft, per Hillsboro's design review overlay in downtown/gateway zones).

Utility connections and sub-metering requirements vary by scope. If the ADU has separate utility connections (own water meter, own electrical service, own gas line), permitting is straightforward — the utility companies (City of Hillsboro Water & Wastewater, Pacific Power, NW Natural) handle interconnection once the city issues the building permit. If the ADU shares utilities with the primary dwelling, you must install sub-meters (water and electrical sub-meters are standard; gas sub-meter is optional but recommended for billing clarity). Cost: $1,200–$2,500 for sub-meter installation plus utility company fees ($300–$800). The building permit application must show utility plan (site plan with meter locations and line runs). Many applicants forget to involve the utilities early — contact them 4-6 weeks before you submit permits to confirm capacity and connection points. Hillsboro's ePermitting portal allows you to upload a draft utility plan; the city's planner will flag any missing detail before official submittal, speeding review.

Timeline and fees for Hillsboro ADUs: Building permit application fee ranges from $800–$1,500 depending on project valuation (plan review is ~1.5% of construction cost). Plan review deposit (non-refundable portion) is typically $400–$800. Electrical, plumbing, and mechanical separate permits add $300–$600 each. Impact fees (transportation, parks, schools) for ADU are capped at 80% of single-family fees under Oregon law, typically $2,000–$4,000 for a modest ADU in Hillsboro. Total permit and fees package: $5,000–$12,000 depending on size and site complexity. Once submitted, expedited review (staff sign-off) takes 20-25 business days; conditional approval or request for information (RFI) adds 15 days. Full Planning Commission review (required for some detached ADUs) adds 45-60 days. After permit issuance, construction timeline is typically 4-8 weeks for a simple detached ADU, 6-12 weeks for a conversion, plus 2-3 weeks for inspection and certificate of occupancy. Total elapsed time from application to move-in: 3-4 months expedited, 4-6 months with Planning Commission review.

Three Hillsboro accessory dwelling unit (adu) scenarios

Oregon ADU Law vs. Hillsboro's Local Additions: What's Really Waived?

Oregon Revised Statute ORS 197.312 (2019) was landmark: it required all Oregon cities to allow ADUs on single-family lots and explicitly banned owner-occupancy requirements on the ADU unit itself. What this means in plain English: if you own a single-family house in Hillsboro, Oregon law now says the city cannot ban you from building an ADU, cannot require you to live in the ADU, and cannot require the ADU to be smaller than 800 sq ft (it can be larger). Hillsboro adopted ORS 197.312 into its municipal code (HCC 20.08.40) without objection and without imposing city-level restrictions that exceed the state baseline. This is NOT true of every Oregon city — some towns (particularly rural areas) still impose caps like 'no ADU over 600 sq ft' or 'ADU must be rented to family only' or 'only one ADU per lot.' Hillsboro enforces the state law cleanly.

However, Hillsboro retained ONE restriction that many ADU applicants miss: the PRIMARY RESIDENCE must be owner-occupied. ORS 197.312 does not prohibit this (it only protects the ADU from owner-occupancy rules), and Hillsboro's code holds that you cannot have two separate rental units on one lot — only one primary owner-occupied unit plus an ADU. This is the city's interpretation of state density rules. If you want to rent both the main house and the ADU as investment properties, Hillsboro will deny your ADU permit and suggest instead that you apply for a multi-unit conversion (which triggers full Planning Commission review, impact fee recalculation, and likely rezoning). The practical consequence: if you plan to move into the ADU and rent the main house, that is allowed. If you plan to occupy the main house and rent the ADU, that is allowed. If you plan to rent both, that is not allowed under Hillsboro's current code, even though the ADU statute itself does not prevent it.

Parking and setbacks are also more generous in Hillsboro than in many other Oregon cities. HCC 20.38 waives parking entirely for ADUs under 750 sq ft, and Hillsboro reduced setback requirements for detached ADUs to 5 feet from the rear property line (vs. 10 feet for primary dwellings). This makes infill ADUs on smaller Hillsboro lots feasible in ways they are not in, say, Beaverton or Oregon City, which retain stricter spacing rules. Utility connections and fees are not waived — you still pay full impact fees (though capped at 80% of single-family fees under state law), and you still must pull separate permits for electrical, plumbing, and mechanical. Hillsboro does not offer any fee waivers or discounts for ADUs, unlike some California cities that waive plan review on pre-approved designs.

The Hillsboro ePermitting Portal: Why Submitting Online Beats Walk-In Filing

Hillsboro's Building Department uses an online ePermitting system (accessible through the city's website) that fundamentally changes the ADU review timeline compared to walk-in, paper-based filing. When you submit your ADU application through the portal, you upload PDF plans and a checklist; the city's plan reviewer accesses them immediately and can flag missing items within 2-3 business days, before your application is officially deemed complete. In person (walk-in) submissions to the Hillsboro Building Department counter often involve a 10-15 minute in-person review where the checker marks up your papers with a pencil, and you don't see feedback until you return or get a phone call days later. The portal version shortens this loop: upload, get RFI (Request for Information) email within 48 hours if something is missing, re-upload revised plan, get deemed complete. This can save 1-2 weeks on the front end. Additionally, the portal allows you to track your application status in real-time — you can see which reviewer has it, how many days it's been in review, and when inspections are scheduled. This transparency reduces the number of applicants who call the city asking 'Where's my permit?' — which in turn reduces demand on the counter staff and speeds overall processing.

Submitting through the ePermitting portal does NOT bypass Planning Commission review if your ADU triggers design review (as in Scenario B and C). The portal just speeds up the staff-level intake and completeness check. Once deemed complete, your application is scheduled for Planning Commission (if required) by the same timeline as a paper submission — typically 30-45 days. However, for expedited-eligible ADUs (Scenario A: detached, owner-occupied primary residence, lot >7,500 sq ft, no overlay district), the portal system flags this automatically, and your application goes straight to the staff plan reviewer without Planning Commission scheduling. Staff review on these expedited cases takes 18-22 business days total, vs. 45-60 days for Planning Commission review. The ePermitting portal also integrates with Hillsboro's utility notification system: once you submit an application, the city can auto-notify Water & Wastewater and the electrical utility (via their portal) that new service is planned, which speeds up their pre-approval checks. Practical tip: Use the ePermitting system. Do not walk in. The online submission, even though it feels less personal, results in faster, more transparent review than in-person filing.

One caveat: the ePermitting portal does not accept handwritten or scanned low-resolution images. Your plans must be clear, digital PDFs with legible dimensions and notes. If your architect or draftsperson provides blurry scans or originals on vellum, you must have them re-scanned or re-drawn in CAD before uploading. The city's portal has a 25 MB file-size limit per upload, so large plan sets (more than 15-20 sheets) may need to be split into two submissions or zipped. Call the Hillsboro Building Department beforehand (or check their FAQ on the portal homepage) to confirm current file requirements; they update these occasionally.

Common questions