Seattle has built one of the more landlord-friendly ADU frameworks in the country, and understanding it fully before breaking ground can save you significant time and money. An accessory dwelling unit (ADU) is a secondary housing unit on the same lot as a primary residence. You'll hear them called granny flats, in-law suites, backyard cottages, or carriage houses depending on the configuration. Landlords build them to generate rental income, house family members, or increase property value in a city where housing demand consistently outpaces supply.
Types of ADUs allowed in Seattle
Seattle permits two distinct ADU types on residential lots, and you can build both on the same property:
- DADUs (detached accessory dwelling units): Stand-alone structures separate from the primary home, most often backyard cottages or converted garages with independent foundations. Maximum occupiable space is 1,000 sq ft, or 60% of the primary unit's floor area, whichever is smaller.
- AADUs (attached accessory dwelling units): Units integrated into the primary structure, such as a basement apartment, above-garage suite, or internal conversion. Also capped at 1,000 sq ft.
Both types must include a kitchen and bathroom to qualify as independent units. Allowing one AADU and one DADU per lot means a single property can support up to two ADUs alongside the primary residence, a policy that took effect as part of Seattle's 2019 reforms.
Zoning and minimum lot requirements
ADUs are permitted across all Neighborhood Residential (NR) zones, NR1 through NR4. The minimum lot size for most ADU projects is 3,200 sq ft, though specific requirements vary by zone and lot configuration. Each NR zone carries its own rules on scale, placement, and allowable square footage, so reviewing your specific parcel's zoning designation early in the planning process matters.
The Seattle Department of Construction and Inspections maintains current zoning rules and parcel-level lookup tools that can confirm what applies to your property before you invest in architectural drawings.
Setbacks, height, and parking
For DADUs, the standard rear and side setback is 5 feet from the property line. Structures exceeding 18 feet in height require additional setback distance. Design guidelines updated in 2021 refined how DADUs integrate into existing neighborhoods, with attention to massing, materials, and how structures relate to adjacent lots.
Off-street parking is not required for ADUs in Seattle. The city removed that mandate as part of its 2019 reforms, reflecting transit-oriented development priorities. No additional parking spaces are required regardless of ADU type, lot location, or proximity to transit, which simplifies site planning considerably for smaller urban lots.
Owner-occupancy rules
Seattle eliminated the owner-occupancy requirement in 2019. Before that change, you had to live on the property to build or rent an ADU. That rule is gone. You can now build an ADU and rent it without living on-site, or rent both the primary home and the ADU as separate units. Investors and landlords who don't occupy the property can build and manage ADUs under the same rules that apply to resident owners.
This was one of the more consequential policy shifts in Seattle's recent housing history. It opened ADU development to a broader pool of property owners and contributed directly to the surge in ADU construction the city has seen since 2019.
The permitting process
All ADU projects require a building permit through the Seattle Department of Construction and Inspections. The process involves submitting plans, completing a plan review, passing inspections at key construction milestones, and demonstrating compliance with current building and energy codes.
Permit timelines vary. Simple AADU conversions involving existing space may move faster than new DADU construction, but both can take several months depending on application volume and project complexity. Pre-application conferences with SDCI staff are available and can surface compliance issues before formal submittal, which is worth considering for more complex projects.
Most single-lot ADU projects fall below the threshold for full State Environmental Policy Act (SEPA) review. The legal challenges to Seattle's 2019 ADU Environmental Impact Statement were resolved in the city's favor, and the current rules are settled.
Rental rules and how ADUs can be used
Long-term rentals have no special restrictions beyond standard Washington landlord-tenant law. You can rent an ADU month-to-month or on a fixed lease without additional licensing specific to the ADU designation.
Short-term rentals through platforms like Airbnb operate under a separate licensing framework. Seattle's short-term rental rules require operators to obtain a city license and meet specific operating standards. The city monitors short-term rental activity to prevent ADUs from being removed from the long-term housing supply, so if you're considering that use, review current licensing requirements before listing. Seattle's short-term rental rules are covered separately and should be read alongside these ADU guidelines.
HOA considerations
Seattle's ADU rules apply city-wide, but homeowners association rules operate as a separate layer of private agreements. If your property sits within an HOA, the association's CC&Rs may restrict or prohibit ADU construction regardless of what city zoning permits. Review your HOA documents before starting the permitting process. If the HOA's restrictions conflict with what the city allows, you may need to seek a variance or amendment through the HOA before proceeding.
Properties without HOA restrictions have more flexibility, but it's still worth confirming your lot's deed conditions, as some older properties carry restrictive covenants that predate modern zoning.
Costs and financial considerations
Construction costs for a new DADU in Seattle typically range from $150,000 to $300,000 or more, depending on size, site conditions, and finish level. AADU conversions of existing space tend to cost less, often $80,000 to $150,000, though that varies widely based on scope. Permit fees, design fees, and utility connection costs add to both categories.
ADUs cannot be sold separately from the primary residence. The lot must stay under single ownership, so any sale includes both structures. That limits one exit strategy, but it doesn't diminish the income potential from long-term renting.
Once you're renting, you take on landlord responsibilities including lease management, maintenance, and compliance with Washington tenant protections. Landlord insurance is an important part of managing that exposure. A Washington landlord insurance policy can cover the dwelling structure, loss of rental income, and liability tied to your ADU rental.
Seattle's ADU activity and housing context
Seattle has been permitted more ADUs than single-family homes in recent years, a trend tracked by the Seattle Times and driven directly by the regulatory reforms since 2019. Removing owner-occupancy requirements, loosening lot size minimums, and eliminating parking mandates all contributed to that increase.
The city's Mandatory Housing Affordability (MHA) program runs alongside ADU policy to ensure densification doesn't undercut lower-income residents. Whether ADUs contribute to affordability or accelerate displacement depends largely on how units are priced, managed, and where they're built. Those questions are worth considering as part of your investment planning.
Frequently asked questions
Can I build two ADUs on my Seattle property?
Yes. Seattle allows one AADU and one DADU on a single lot, giving you up to two ADUs alongside the primary residence. Both count toward the density permitted under your zoning designation.
Do I have to live on the property to build or rent an ADU in Seattle?
No. Seattle eliminated the owner-occupancy requirement in 2019. You can build, own, and rent an ADU, or rent both the primary home and the ADU, without living on-site.
What are the setback rules for a backyard cottage in Seattle?
DADUs must sit at least 5 feet from the rear and side property lines. Structures exceeding 18 feet in height require larger setbacks. Confirm current requirements with the Seattle Department of Construction and Inspections before finalizing your design.
Is parking required for an ADU in Seattle?
No. Seattle removed the off-street parking requirement for ADUs in 2019. No additional parking is required regardless of ADU type or location.
Can I sell my ADU separately from the main house?
No. ADUs are legally tied to the primary residence and cannot be sold as separate parcels. The entire lot, including the primary home and any ADUs, must be sold together under single ownership.
What's the minimum lot size to build an ADU in Seattle?
Most residential lots need to be at least 3,200 sq ft to support an ADU project. Requirements can vary by zone, so check your specific parcel's zoning designation through SDCI before planning.
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