As others have pointed out, zoning can be interesting and may not allow for it to be rented out at all, let alone if you intended to cash flow on the primary dwelling as well as an accessory unit.

Seattle does have a Accessory Dwelling Unit rules; however, it requires that the property owner live in one of the units. 

Some other supposedly housing-related bills that will end the housing crisis, signed into law by Governor Nuisance:AB 12 by Matt Haney (D-San Francisco) – Tenancy: security deposits.AB 84 by Christopher Ward (D-San Diego) – Property tax: welfare exemption: affordable housing.AB 281 by Tim Grayson (D-Contra Costa) – Planning and zoning: housing: post entitlement phase permits.AB 318 by Dawn Addis (D-Morro Bay) – Mobilehome Residency Law Protection Act.AB 319 by Damon Connolly (D-San Rafael) – Mobilehome Parks Act: inspectors: conflict of interest: enforcement actions: sunset.AB 323 by Chris Holden (D-Pasadena) – Density Bonus Law: purchase of density bonus units by nonprofit housing organizations: civil actions.AB 346 by Assemlymember Sharon Quirk-Silva (D-Fullerton) – Income tax credits: low-income housing: California Debt Limit Allocation Committee rulemaking.AB 434 by Tim Grayson (D-Contra Costa) – Housing element: notice of violation.AB 480 by Philip Ting (D-San Francisco) – Surplus land.AB 516 by James Ramos (D-San Bernardino) – Mitigation Fee Act: fees for improvements: reports and audits.AB 519 by Pilar Schiavo (D-Los Angeles) – Affordable Housing Finance Workgroup: affordable housing: consolidated application and coordinated review process.AB 529 by Jesse Gabriel (D-Encino)- Adaptive reuse projects.AB 548 by  Tasha Boerner (D-San Diego) – State Housing Law: inspection.AB 572 by Matt Haney (D-San Francisco) – Common interest developments: imposition of assessments.AB 671 by Christopher Ward (D-San Diego) – CalHome Program: accessory dwelling units.AB 812 by Tasha Boerner (D-San Diego) – Housing development approvals: reserving affordable units in or near a cultural district for artists.AB 821 by Tim Grayson (D-Contra Costa) – Planning and zoning: general plan: zoning ordinance: conflicts.AB 894 by Laura Friedman (D-Los Angeles) – Parking requirements: shared parking.AB 911 by Pilar Schiavo (D-Los Angeles) – Unlawfully restrictive covenants: affordable housing.AB 976 by Philip Ting (D-San Francisco) – Accessory dwelling units: owner-occupancy requirements.AB 1033 by Philip Ting (D-San Francisco) – Accessory dwelling units: local ordinances: separate sale or conveyance.AB 1114 by Matt Haney (D-San Francisco) – Planning and zoning: housing development projects: post entitlement phase permits.AB 1218 by Josh Lowenthal (D-Los Angeles) – Development projects: demolition of residential dwelling units.AB 1287 by David Alvarez (D-San Diego) – Density Bonus Law: maximum allowable residential density: additional density bonus and incentives or concessions.AB 1308 by Sharon Quirk-Silva (D-Fullerton)- Planning and Zoning Law: single-family residences: parking requirements.AB 1317 by Wendy Carrillo (D-Los Angeles) – Unbundled parking.AB 1319 by Buffy Wicks (D-Oakland) – Bay Area Housing Finance Authority: housing revenue.AB 1332 by Juan Carrillo (D-Palmdale) – Accessory dwelling units: pre-approved plans.AB 1386 by Jesse Gabriel (D-Encino) – Veterans housing: tenant referrals.AB 1449 by David Alvarez (D-San Diego) – Affordable housing: California Environmental Quality Act: exemption.AB 1474 by Eloise Gómez Reyes (D-San Bernardino) – California Statewide Housing Plan.AB 1485 by Matt Haney (D-San Francisco) – Housing element: enforcement: Attorney General.AB 1490 by Alex Lee (D-San Jose) – Affordable housing development projects: adaptive reuse.AB 1508 by James Ramos (D-San Bernardino) – Department of Housing and Community Development: California Statewide Housing Plan.AB 1528 by Mike Gipson (D-Los Angeles) – Housing authorities: property taxation.AB 1620 by Rick Chavez Zbur (D-Los Angeles) – Costa-Hawkins Rental Housing Act: permanent disabilities: comparable or smaller units.AB 1633 by Philip Ting (D-San Francisco) – Housing Accountability Act: disapprovals: California Environmental Quality Act.AB 1734 by Reginald Byron Jones-Sawyer Sr.

Schluter seems to have a better complete line of trim accessories than others. 

By the way, instead of a duplex, you might be able to find a single-family home with an accessory dwelling unit to use as a rental.

Some on this thread are taking a position that e-cig / vaping should be permitted.

Besides this option, are you also open for a house hack (finish of a basement, Accessory dwell unit, mother-in-law, .....)If you are open for the two options, you will have more houses to choose from.Always open to more conversation over phone or coffee if that's helpful!

I explain that on my condo, house and accessory dwelling unit alone here in Los Angeles will net approximately $100,000/year with 3% appreciation in 30 years.   

I primarily sell in Massachusetts, i sell other items (roofing, car charger, mini split heat pumps, battery storage, smart home items through freedom forever, other accessories through partners on the Powur platform.) as well.I joined the form because this is just an abundance of high value information thats collected on here, and this information will be crucial in my wealth building journey.

House hack may be by finishing a garage, get Accessory Dwell Unit (ADU), Monther in Law (MIL), Duplex, Triplex, Fourplex, ....House hack a property will allow you to put way less money down (5%).Some neighborhoods in Seattle and Tacoma are pretty good.