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ADU Building Requirements & Cost Breakdown for Portland, OR & the Pacific Northwest (2026 Guide)
Portland was building accessory dwelling units before most of the country knew what an ADU was. The city started waiving system development charges for ADUs back in 2010, more than a decade before most West Coast cities caught up. Vancouver, Washington and the rest of Clark County are newer to the game, only required to open the door to ADUs statewide after Washington passed HB 1337 in 2023. That gap in maturity matters if you're planning a build on either side of the Columbia River, because the rules, the fees, and the real costs are genuinely different depending on which side of the state line your lot sits on.
This guide covers both. First, the actual zoning and permitting requirements in Portland, OR and in Vancouver, WA / Clark County, side by side. Then a real, itemized 2026 cost breakdown for building an ADU in Portland, broken out by ADU type, because how much an ADU costs gets you a different answer from every website you check, and almost none of them explain why.
Portland ADU Cost Breakdown (2026)
ADU Type
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These figures reflect 2026 material and labor costs in the Portland metro. Site work and foundation typically run 15 to 20 percent of the budget, framing and shell 25 to 30 percent, and interior finish-out 30 to 35 percent. Portland's SDC waiver can cut roughly $19,000 off a qualifying ADU, covered below.
What Is an ADU, and Why Is the Pacific Northwest Building So Many?
Detached ADU: a standalone backyard cottage, fully independent from the main house
Attached ADU: built onto or as an addition to the main house, sharing at least one wall
Conversion ADU: an existing garage or basement turned into a self-contained unit with its own kitchen and bathroom
Oregon HB 2001 (2019) required most Oregon cities to allow ADUs on residential lots and barred cities from requiring owner-occupancy or off-street parking as a condition of building one
Washington HB 1337 (2023), codified at RCW 36.70A.680, requires fully-planning cities and counties, including Clark County, to allow two ADUs per residential lot, caps ADU impact fees at half the fee charged to a primary home, and removes owner-occupancy requirements
The result: it's now legal in most residential zones on both sides of the Columbia River to add a second, or in Washington a third, home to a lot that used to allow only one, though Portland and Vancouver, WA apply different rules for doing it
Portland vs. Vancouver, WA: ADU Rules Compared
Portland ADU rules fall under Portland Zoning Code 33.205 and Oregon HB 2001. Vancouver, WA and unincorporated Clark County both operate under Washington's HB 1337, codified at RCW 36.70A.680.
Portland caps ADUs at 75 percent of the main home's size or 800 square feet, with basement conversions in older homes allowed up to 1,000 square feet. Vancouver allows up to 1,000 square feet, and Clark County allows 800 square feet or 40 percent of the main home, up to 1,500 square feet on large lots.
All three jurisdictions now allow up to two ADUs per residential lot, though Washington's rollout under HB 1337 is still phasing in on Vancouver and Clark County's own timelines. Portland's two-ADU allowance came out of its Residential Infill Project.
None of the three jurisdictions require owner-occupancy. Portland requires no off-street parking. Vancouver's parking requirement varies by site, and Clark County may require on-site parking where street parking isn't available.
Portland's system development charges apply in full unless you use the SDC waiver program, covered below. Washington caps ADU impact fees at half the fee charged to a primary home under HB 1337, and Clark County cut its ADU impact fees 75 percent under a 2018 county ordinance.
Cost Drivers & How to Save Money on Your ADU Build
Permitting timelines run 3 to 6 months in Portland and Vancouver, so factor holding costs into your budget
Wet climate foundations and drainage add cost, so budget extra for site work on tight or sloped lots
Construction and renovation loans are common ADU financing, since most lenders now recognize their rental income potential
Work with a local contractor like Salcans Remodeling for honest pricing and help applying for the Portland SDC waiver
Top ADU & Remodeling Builders in the Portland & Vancouver, WA Area (2026)
Salcans Remodeling is a full-service remodeling contractor serving both Vancouver, WA and Portland, OR, with direct experience navigating permitting on both sides of the Columbia River. From detached backyard cottages to garage and basement conversions, the team handles design, permitting, and construction so homeowners get one point of contact from first sketch to final inspection.
Lamont Bros. is a Portland-based design-build firm with a strong focus on Oregon ADU projects and designs that comply with the city's Residential Infill Project standards. Their in-house architecture and construction teams work through Portland's permitting process together, which can shorten the timeline on detached and attached ADU builds.
Wolf Industries is a Clark County, WA contractor with dedicated ADU permitting guides and hands-on experience with the county's urban and rural ADU categories. They are a solid option for homeowners outside Vancouver's city limits who need a builder familiar with Clark County's specific setback and parking rules.
Do I Need a Permit to Build an ADU in Portland, OR or Vancouver, WA?
Yes, in both. Portland requires a building permit reviewed under Zoning Code Chapter 33.205. Vancouver, WA and Clark County require permits reviewed against their own ADU size, setback, and parking standards. Permit timelines run roughly 75 days in Portland and 4 to 12 weeks in Clark County depending on season.
How Much Does It Cost to Build an ADU in Portland in 2026?
It depends on the type. A garage or basement conversion typically runs $80,000 to $160,000. An attached ADU runs $180,000 to $320,000. A detached new-build ADU (600 to 800 sq ft) typically runs $220,000 to $400,000. Applying for Portland's SDC waiver can reduce the total by close to $19,000.
What Is Portland's SDC Waiver for ADUs?
It's a program that exempts a qualifying ADU from system development charges for sewer, parks, transportation, and water, worth roughly $19,000 based on the city's current fee schedule. In exchange, you sign a 10-year covenant agreeing not to use the ADU as a short-term rental. Salcans Remodeling has handled ADU permitting and construction on both sides of the Columbia River and offers free feasibility estimates.
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