Hillsboro is Washington County’s largest city, and building here is different from building on a Portland inner-east lot. Orenco and the MAX corridor sit next to farm-edge plats. Reedville and Jackson Bottom drain toward the Tualatin. South Hillsboro is still filling in. Wolf’s factory is in Battle Ground, in Southwest Washington, and the drive is I-5 across the Interstate Bridge to Highway 26 or the TV Highway corridor. Permits go through City of Hillsboro Development Services rather than Portland DevHub, and sewer and storm questions often start with Clean Water Services.
Three things matter most at the start: the size cap, the CWS letter, and whether the lot is in the city or in unincorporated Washington County with a Hillsboro mailing address.
How Hillsboro regulates ADUs
Inside city limits, ADUs are allowed on lots with a legal single detached dwelling or townhouse. Hillsboro’s zoning ordinance (Chapter 12) is the first document we read. The number that surprises people coming from Portland: a new ADU generally has to be at least 250 square feet and not more than 750 square feet. Converting a level of the primary house (basement, attic, second story) can use that entire level even if it’s larger than 750, with a ceiling of 50 percent of the primary on that conversion path. A detached Wolf cottage falls under the 750-square-foot limit, which is tighter than Portland’s 800 square feet / 75 percent.
The published pattern is one ADU per qualifying dwelling. Confirm the current section when you file, since older articles online may be out of date. Height, setbacks, lot coverage, and side yards follow the zone, whether it’s a Reedville lot or a South Hillsboro tract. If a number isn’t in the current code or our Oregon ADU rules guide, it depends on your lot. Ask with the address.
Statewide, ORS 197A.425 and HB 2001 still apply: the city must allow an ADU on qualifying single-family lots inside the UGB, and it can’t require owner occupancy or extra off-street parking for a long-term unit. Hillsboro still sets the siting rules, the 750-square-foot cap, and the fees.
Unincorporated Washington County lots with a Hillsboro ZIP code go through county land-use and building, plus CWS. Aloha is the classic example: people say Hillsboro and file with the county. On a county lot, CWS may be the sewer reviewer while the county handles the building permit.
Models that fit under 750 square feet
Wolf doesn’t sell HUD code (the federal standard for manufactured homes) manufactured homes. These are modular dwellings built under Oregon’s BCD prefabricated-structures program, leaving Battle Ground with the Oregon insignia and placed on a local foundation under a city installation permit. Hillsboro still inspects the foundation, the set, and the utilities.
- Model K (~432 sq ft). For rental or guest use when one bedroom is enough.
- Model F / Model E (~616 sq ft). Two bedrooms that still fit under 750. This is the two-bedroom option in Hillsboro.
Model J is about 828 square feet, which is over the 750 cap, so it doesn’t work on a Hillsboro lot even though it might in Portland. A conversion exception would apply to an interior project, not a factory-built cottage. We check the current Chapter 12 language for your parcel.
Permits: ProjectDox and OpenHillsboro
Hillsboro’s electronic plan review runs through ProjectDox, and the city has also been rolling out a new OpenHillsboro permit system. Development Services is at the Civic Center, 150 E Main Street, fourth floor. The published contact is 503-681-6153, permits@hillsboro-oregon.gov. Walk-ins still work at this counter, which is a different experience from Portland’s upload-and-wait DevHub queue.
Before the city will issue a permit, Clean Water Services requires an Environmental Review. For Beaverton, Cornelius, Forest Grove, Hillsboro, Sherwood, Tigard, and Tualatin, CWS’s steps are: submit the Prescreen, obtain a Service Provider Letter (or a form saying none is required), then bring that letter to the city with the permit application. Without the SPL, the file stalls. The requirement exists because of the sensitive areas and vegetated corridors around the Tualatin and Jackson Bottom.
Our process is feasibility first (city versus Washington County, the 750 calculation, the CWS prescreen, trees, utilities, and the truck route), then a scoped proposal, then a ProjectDox submittal. South Hillsboro has extra SDC-credit and area-specific forms, so if you’re in Reed’s Crossing, Butternut Creek, or Rosedale Parks, tell us. Those packets are different from an older Reedville lot.
Electrical, plumbing, and mechanical permits go with the building permit. Inspections are scheduled through the city’s system.
SDCs: city charges, CWS, and Washington County TDT
Hillsboro charges its own system development charges. Clean Water Services sets sanitary and storm rates, which the city typically calculates and collects. Washington County’s Transportation Development Tax (TDT) is a county charge that Portland lots don’t have. Construction excise tax may also apply. We run the current SDC, CWS, TDT, and CET schedules for your project instead of publishing a combined figure here.
Salem has eliminated ADU SDCs and Tigard has published a transportation-and-parks ADU exemption packet, but our Oregon guide doesn’t show either for Hillsboro. Assume fees apply until someone has checked your project against the current methodology. South Hillsboro SDC credit forms work differently from an infill cottage downtown.
What we check on Hillsboro lots
- Size. People often arrive with an 800-square-foot Portland plan. Hillsboro allows 750 for a new detached unit, which rules out Model J and keeps E and F in play.
- CWS and drainage. Jackson Bottom, Dawson Creek, and the Tualatin floodplain edges drain very differently from Orenco Station’s engineered plats. The SPL is where a simple backyard project can become a vegetated-corridor review.
- Delivery. Highway 26 and TV Highway get the module west. Older Reedville streets, South Hillsboro cul-de-sacs, and overhead power lines decide whether we use a crane or rollers. A 12-foot Model K is easier to deliver than a 14-foot F.
- Utilities by era. Newer subdivisions have laterals where the plot plan says they are. Lots from the 1940s and 1970s bring surprises: leftover septic systems, undersized panels, and water meters that can’t serve a second dwelling without an upgrade.
- City or Aloha. If the lot is unincorporated, the county rules apply.
Utilities
Water in Hillsboro is a city utility. Sanitary and storm often run through Clean Water Services even when the city issues the building permit, and power is typically PGE. With three parties involved in one trench, “just connect to the house” isn’t a bid. Meter size, backflow, and an existing storm facility on the lot can each add a drawing.
How people use these homes
Hillsboro ADUs house parents of families who work in the city’s employment core, adult children priced out of buying, long-term renters, and guests. We see a lot of tech-shift and multigenerational households, and not much vacation use. Short-term rental rules are local, so don’t count on Airbnb income.
“Tiny home,” “granny flat,” and “ADU” all describe the same building here, with the same 750 cap, the same CWS letter, and the same factory insignia. If it’s a dwelling, Hillsboro treats it as one. Family, rental, and guest use are occupancy questions on top of that.
For a nearby example, see the historical Model B investment rental in Tigard. Your Hillsboro project is still evaluated under Hillsboro’s rules and your lot’s conditions. Model B is currently available only within City of Portland limits.
Factory build and the westside set
The home is built in Battle Ground while Hillsboro review and CWS screening move ahead. Hillsboro is wetter in the shoulder seasons than people expect, and Jackson Bottom doesn’t drain like an engineered South Hillsboro plat. Factory construction keeps the house out of the weather, but the lot still deals with saturated silts, a high winter water table, and CWS comments about impervious area. We plan foundation and trench work around the forecast.
Highway 26 and TV Highway are the main freight routes. Orenco’s structured streets and downtown’s older grid need different approaches for the last mile, and Intel shift changes and school pickup on Cornell affect set-day timing. On set day we handle neighborhood traffic control, bring a crane if needed, and weather-seal the same day. Dual-wide plans add a day to join the two halves. Hillsboro is a local market for us, farther than Vancouver and closer than Salem. Warranty and punch-list visits come from the same field crews that set the home and run the same corridor.
What a Hillsboro proposal covers
Site work, Clean Water Services stormwater requirements, meter upgrades, transportation charges, and city SDCs can make the total differ between two Hillsboro properties with the same model. Factory construction makes the home price more predictable than a site-built project, but local reviews and service-provider requirements still add cost. See financing.
Neighborhoods and nearby cities
Downtown Hillsboro’s older grid is different from Orenco’s mixed-use blocks, Reedville is different from South Hillsboro, and a lot next to Jackson Bottom is different from a Cornell Road tract. Forest Grove has its own city hall, and Aloha is often county. Tigard and Beaverton are Washington County neighbors with their own permit systems (Tigard has its SDC exemption packet, and Beaverton uses BEPS). If your lot falls in one of those cities, we work from that city’s code and fee schedule, and the Hillsboro 750-square-foot cap no longer applies.
What a Hillsboro evaluation looks at
Bring the address and a sketch of the backyard. If you have a CWS letter from a prior addition, a plot plan from the builder, or a South Hillsboro SDC-credit form, send that too. An address alone is enough to start. We look at:
- City of Hillsboro or Washington County.
- The 750-square-foot cap.
- The CWS prescreen and Service Provider Letter.
- Zone, overlays, and South Hillsboro area forms if they apply.
- Water, sewer, storm, and electrical: laterals, meter size, and panel.
- The truck route from Highway 26 or TV Highway to the curb.
- The TDT and city SDC schedule.
- Use: family, long-term rental, or short-term.
If the 750-square-foot cap rules out the plan you had in mind, you’ll know within a week, before a floor plan is locked in.
Send the address. Tell us city or county and whether you need one bedroom or two under 750 square feet, and we’ll match a model to Chapter 12.
