ADU Rules: Portland vs Seattle (2026 Comparison)
Compare ADU regulations, permit fees, size limits, and timelines between Portland and Seattle. Both Pacific Northwest cities allow 2 ADUs per lot under state law.
Side-by-Side Comparison
Portland vs Seattle
| PortlandOregon | SeattleWashington | |
|---|---|---|
| ADUs Allowed? | Yes, by right in all residential zones | Yes, by right in all residential zones |
| Max Detached ADU Size | 800 sq ft | 1,000 sq ft |
| Max Attached ADU Size | 75% of primary dwelling up to 800 sq ft | 1,000 sq ft |
| ADUs Per Lot | 1 attached OR 1 detached (middle housing rules allow up to 4 total units) | 2 ADUs per lot (1 attached + 1 detached) |
| Estimated Permit Fees | $3,000–$8,000 in permit fees plus SDCs | $4,000–$12,000 in permit fees; school impact fees additional |
| System/Impact Fees | SDCs typically $10,000–$25,000 for transportation, parks, water/sewer | School impact fees ~$3,700–$7,400 per new residential unit |
| Permit Timeline | 4-8 weeks plan review (2-4 weeks with pre-approved plans) | 4-8 weeks standard (1-3 weeks with pre-approved plan) |
| Owner Occupancy Required? | No (OR HB 2001 prohibits) | No (WA HB 1337 prohibits) |
| Parking Required? | No (eliminated 2013) | No (eliminated 2019, confirmed by WA HB 1337) |
| State Preemption? | Yes (Oregon HB 2001, 2019) | Yes (Washington HB 1337, 2023) |
Key Differences
Portland and Seattle are two of the most ADU-friendly cities in the United States, and both benefit from strong state preemption laws that override restrictive local regulations. However, they differ in important ways. Seattle allows two ADUs per lot — one attached and one detached — as codified by Washington HB 1337 (2023). Portland, by contrast, typically allows only one ADU per single-family lot (either attached or detached), though Portland's middle housing rules under Oregon HB 2001 allow up to four total units on a lot through duplex/triplex/fourplex structures. For homeowners specifically wanting an ADU, Seattle's two-ADU allowance is a meaningful advantage. Size limits also favor Seattle: both attached and detached ADUs can be up to 1,000 sq ft, versus Portland's 800 sq ft cap. This 200 sq ft difference can mean the difference between a functional one-bedroom and a comfortable two-bedroom ADU. Cost structures differ significantly. Portland's System Development Charges (SDCs) for transportation, parks, and utilities can add $10,000–$25,000 to project costs on top of permit fees. Seattle's primary added cost is school impact fees (~$3,700–$7,400). Both cities offer pre-approved plan sets that can meaningfully reduce review time and cost.
Cost Comparison
ADU construction in Portland typically costs $150,000–$300,000 for a new detached unit, with permit fees of $3,000–$8,000 and SDCs of $10,000–$25,000 totaling $13,000–$33,000 in government fees. Seattle ADU construction is typically $180,000–$350,000 due to higher labor costs, with permit fees of $4,000–$12,000 and school impact fees of $3,700–$7,400 totaling roughly $7,700–$19,400. Portland's SDC burden can make it more expensive from a fees perspective despite lower baseline permit fees. Both cities offer SDC/fee deferrals for affordable housing ADUs.
Our Verdict
Seattle edges out Portland for most ADU builders due to higher size limits (1,000 sq ft vs 800 sq ft), the ability to build two ADUs per lot instead of one, and a somewhat lower total fee burden in many cases. Portland's SDC system, while deferred for affordable housing, can add $20,000+ to costs that Seattle doesn't charge. That said, Portland remains an excellent ADU market with one of the longest track records of ADU-friendly policy in the US, a large pool of experienced ADU builders, and a history of permitting innovation. If you want a second rental unit on the same lot, Seattle's two-ADU rule is a clear winner.
Explore Each City
Frequently Asked Questions
Which city allows more ADUs per lot — Portland or Seattle?
Seattle allows 2 ADUs per lot (1 attached + 1 detached) under Washington HB 1337 (2023). Portland typically allows 1 ADU per single-family lot (attached or detached), though middle housing rules allow up to 4 total units via duplexes, triplexes, or fourplexes.
Are ADU permits faster in Portland or Seattle?
Both cities offer similar timelines — 4-8 weeks for standard plan review and 1-4 weeks with pre-approved plan sets. Portland BDS and Seattle SDCI both have pre-approved ADU plan programs that significantly reduce review time.
Which city has lower ADU permit costs?
Portland's permit fees ($3,000–$8,000) are slightly lower than Seattle's ($4,000–$12,000), but Portland's System Development Charges (SDCs) for transportation, parks, and utilities can add $10,000–$25,000. Seattle charges school impact fees of $3,700–$7,400 instead. Total government fees are often comparable, though Portland's SDC burden can be higher.
Do both Portland and Seattle require owner-occupancy for ADUs?
No. Both cities have eliminated owner-occupancy requirements — Portland since 2016 (ahead of state law) and Seattle since 2019. Washington HB 1337 (2023) and Oregon HB 2001 (2019) both prohibit local owner-occupancy mandates for ADUs.
Source: PropertyZoned Editorial Research. Last verified April 5, 2026. View source