The City of Seattle governs ADU construction through SMC 23.44.041 (detached accessory dwelling units, or DADUs) and SMC 23.44.025 (attached accessory dwelling units, or AADUs). Seattle has been one of the most ADU-friendly cities in Washington since its landmark 2019 reform — and HB 1337 has only strengthened those rights.
This guide covers the key rules in plain English. For the state-level context, see our HB 1337 explainer.
What Seattle Allows
2
ADUs per lot
$0
Required parking
None
Owner-occupancy
In most Seattle residential zones you can build up to two ADUs per lot — typically one AADU (a unit inside or attached to the house, under SMC 23.44.025) plus one DADU (a backyard cottage, under SMC 23.44.041). Owner-occupancy has not been required since the 2019 reform (CB 119544).
All ADU permitting runs through Seattle DCI (Department of Construction and Inspections).
Source: SMC 23.44.041, SMC 23.44.025, CB 119544
DADU Size Limits
Up to 1,000 sq ft as of right
Seattle allows a detached ADU of up to 1,000 square feet of gross floor area as of right. HB 1337 reinforces this by prohibiting any Washington jurisdiction from capping ADU size below 1,000 sq ft — so this floor is now state-protected.
Gross floor area generally excludes certain below-grade and garage space depending on configuration. Detached ADUs are the most common new-build option in Seattle.
AADU Conversions
An attached ADU — such as a basement conversion — is governed by SMC 23.44.025 and counts against the structure's overall floor-area and height limits rather than the standalone DADU cap. Older homes with full basements are excellent AADU candidates.
Source: SMC 23.44.041, RCW 36.70A.681(1)(c)
Setback Requirements
In standard single-family zones (e.g., SF 5000), a DADU generally requires 5-foot rear and side setbacks. Front setbacks follow the underlying zone for the primary structure.
Reduced Setbacks for Existing Structures
Some lots can use reduced setbacks where an existing structure is being converted, or where the DADU is sited along an alley. Conversions of legal nonconforming structures may keep their existing footprint even if it sits closer than 5 feet to a property line.
Shoreline lots are different
Lots within 200 ft of Lake Washington, Lake Union, Green Lake, or Puget Sound face larger shoreline setbacks and extra review under the Shoreline Management Act. Learn more.
Source: SMC 23.44.041
Lot Coverage
Most SF 5000 zones cap lot coverage at 35% of lot area. In some situations, Seattle allows an additional 5% of lot coverage to accommodate a backyard DADU, recognizing that a cottage adds footprint a remodel would not.
Hardscape and impervious-surface limits also apply and can interact with stormwater code, especially on smaller lots.
Source: SMC 23.44.041, SMC 23.44.010
Permit Process Through DCI
Inside Seattle city limits, an ADU is permitted with a standard building permit through Seattle DCI. Because ADUs are an outright permitted use in residential zones, no separate land-use permit is required for a typical project.
Typical Timeline
6–10 weeks for review on a standard lot, longer if critical areas, shoreline, or tree review apply.
Where to Apply
Seattle DCI via the city's Seattle Services Portal. We handle the full submittal for you.
Source: Seattle DCI, SMC 23.44.041
Don't want to navigate DCI yourself? Our ADU permitting service handles everything — zoning verification, plan review, and building permits — and your investment is applied as a build deposit when you sign the LOI. See our full cost breakdown.
The 2019 ADU Reform (CB 119544)
Seattle's 2019 ordinance — adopted as Council Bill 119544 (Ordinance 125791) — made the city a national model for ADU policy, removing the barriers that had kept backyard cottages rare for decades.
Owner Occupancy Removed
Seattle eliminated the owner-occupancy requirement for ADUs — owners no longer have to live on-site to build or rent. This change predated and was later reinforced by HB 1337.
Source: CB 119544 (Ordinance 125791)
Two ADUs Per Lot
The 2019 reform allowed up to two ADUs on a single-family lot (one AADU + one DADU, or other configurations), ending the prior one-ADU-per-lot cap.
Source: CB 119544 / SMC 23.44.041
Parking Waived
No off-street parking is required for ADUs. Combined with proximity to Link Light Rail and RapidRide lines, this removes a major cost and space barrier.
Source: CB 119544
Relaxed Lot Coverage
The reform adjusted maximum lot coverage and floor-area rules to make a backyard DADU feasible on a wider range of standard Seattle lots.
Source: CB 119544
How HB 1337 Interacts With Seattle's Code
HB 1337 (2023) sets a statewide floor for ADU rights — two ADUs per lot, no owner-occupancy mandate, a 1,000 sq ft minimum size cap, and limits on impact fees and parking near transit. A city cannot drop below these state minimums.
Because Seattle's 2019 reform already met or exceeded most of these standards, the practical impact in Seattle is reinforcement rather than overhaul — but it permanently locks in the city's ADU-friendly rules. See our HB 1337 explainer for the full breakdown.
Source: RCW 36.70A.680-681
Other Key Rules
SEPA Environmental Review
Most single ADU and DADU projects are categorically exempt from SEPA. Review can be triggered on lots with critical areas (steep slopes, wetlands, liquefaction zones) or where multiple units are proposed.
Source: SMC 25.05 / WAC 197-11
Tree Protection Ordinance
Exceptional trees (generally 24+ inches in diameter at breast height) and Tier 1/2 trees require a separate assessment and may affect where a DADU can be sited. Tree removal may require replacement or in-lieu fees.
Source: Seattle Tree Protection Ordinance (SMC 25.11)
Existing Structure Conversion
You can convert an existing garage, basement, or accessory structure into an ADU — including nonconforming structures that do not meet current setbacks. See our garage conversion and basement ADU services.
Source: SMC 23.44.041 / HB 1337
Related Guides
Official Sources
- Seattle Municipal Code Title 23 (Land Use): library.municode.com
- Seattle DCI ADU information: seattle.gov/sdci
- Seattle DCI: (206) 684-8600
Last updated: March 2026. Not legal advice. Verify current regulations with Seattle DCI.
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