Seattle is one of the most ADU-friendly major cities in the country. The city's landmark 2019 reform (Council Bill 119544, adopted as Ordinance 125791) eliminated owner-occupancy requirements, removed off-street parking mandates, and allowed up to two accessory dwelling units on most single-family lots. Washington State's HB 1337 (RCW 36.70A.680–681) has since locked those gains in statewide.
This guide covers what Seattle allows, the specific DADU and AADU rules under SMC 23.44.041 and 23.44.025, real-world costs, the Seattle DCI permit timeline, neighborhood opportunities, environmental considerations, and how to finance your project.
What Seattle Allows
Under the 2019 reform, a typical Seattle single-family lot can now have an attached ADU (AADU) and a detached ADU (DADU) in addition to the main house — effectively three units on one lot. There is no owner-occupancy requirement, so you can rent all units. These are the headline rules:
Max ADUs per lot
2 (one AADU + one DADU, or two in many zones)
Max DADU size
1,000 sq ft (SMC 23.44.041)
Max height
Up to 18–24 ft depending on lot width/setbacks
Owner occupancy
Not required (since 2019 reform)
Permitting authority
Seattle Dept. of Construction & Inspections (DCI)
Permit timeline
6–10 weeks (DCI review)
Parking near transit
None required within ½ mile of major transit
Permit + impact fees
$20K–$50K typical
Detached ADU Rules (SMC 23.44.041)
Size & Height
- Maximum DADU floor area is 1,000 sq ft (gross floor area, excluding garage in many cases)
- Height generally ranges from 18 to 24 feet depending on lot width, roof form, and setbacks
- Rooftop features and green-roof bonuses can add allowable height in qualifying cases
Source: SMC 23.44.041
Setbacks & Lot Coverage
DADUs must meet rear and side-yard setbacks, but Seattle's rules are comparatively flexible — a backyard cottage can often sit close to the rear property line. Total lot coverage (all structures combined) is capped by your zone, typically 35–50% depending on lot size. A DADU counts toward lot coverage; a basement AADU usually does not.
Source: SMC 23.44.041, SMC 23.44.010
Permit Process
DADUs are reviewed by the Seattle Department of Construction & Inspections (DCI) through a standard building permit. Most detached cottages do not require a separate land-use approval. DCI also publishes pre-approved DADU plan sets that can shorten review time and reduce design cost.
Source: Seattle DCI
Attached ADU Rules (SMC 23.44.025)
An attached ADU (AADU) is built within or connected to the primary home. This category covers three common approaches in Seattle:
- Basement conversions: Finishing below-grade space into a self-contained unit. Because the footprint already exists, basement AADUs typically add no lot coverage in SF5000 zones — the most cost-efficient path.
- Side or rear additions: Adding attached living space with its own entrance, kitchen, and bath.
- Above-garage units: Converting or building over an attached garage.
AADUs share structural elements and utilities with the main home, which keeps infrastructure costs lower than a standalone DADU.
Source: SMC 23.44.025
What an ADU Costs in Seattle
Costs depend on type, site conditions, and finish level. These are realistic 2026 Seattle ranges:
| ADU Type | Cost Range |
|---|---|
| Garage Conversion | $200K – $325K |
| Basement ADU (AADU) | $225K – $400K |
| Attached ADU | $300K – $500K |
| Detached ADU (DADU) | $450K – $800K+ |
Beyond construction, budget $20K–$50K for permit and impact fees, plus utility connections and site prep. At Seattle market rents of $2,000–$3,500/month, a well-placed ADU can carry a meaningful share of its own financing. See the full cost breakdown.
Seattle DCI Permit Timeline
Permits for ADUs in the City of Seattle go through the Seattle Department of Construction & Inspections (DCI). Plan for roughly 6–10 weeks of review for a typical project once a complete application is submitted, longer if corrections are required or if environmental review applies.
Using DCI's pre-approved DADU plans, submitting a complete and code-compliant set, and clearing critical-area questions up front are the three biggest levers for staying on the fast end of that range.
Source: Seattle DCI permit intake
Shoreline Management Act Consideration
Properties within 200 feet of Lake Washington, Lake Union, Green Lake, or Puget Sound may require Shoreline Management Act (SMA) review. The SMA does not prohibit ADU construction — but it can add 4–8 weeks and $5,000–$15,000 to permitting. Our feasibility study identifies SMA jurisdiction automatically.
Get a free feasibility studySeattle's Tree Ordinance
Seattle's tree protection ordinance regulates the removal of larger and exceptional trees on private property. If a significant or exceptional tree sits where you want to place a DADU, you may need to redesign the footprint, retain the tree, or in some cases provide replacement plantings.
This rarely blocks an ADU outright, but it can shape where the structure goes — which is why a site evaluation early in design saves money. Our design team maps tree constraints before finalizing a layout.
Transit Parking Exemption
Seattle does not require off-street parking for an ADU located within ½ mile of a major transit stop. Link Light Rail stations and RapidRide bus lines both qualify, which covers a large share of the city — Capitol Hill, Columbia City, Beacon Hill, Ballard, West Seattle, and more.
Removing the parking requirement frees up valuable lot area for the ADU itself and can be the difference between a feasible and an infeasible site. This protection is reinforced statewide under HB 1337 (RCW 36.70A.681).
Seattle Neighborhoods for ADUs
ADU feasibility and rental upside vary by neighborhood. Our design team tailors plans to each area's lot characteristics. Highlights:
Ballard
Large SF5000 lots, strong rental demand, and RapidRide D Line transit access. One of the most active DADU markets in the city — backyard cottages are common and well-received.
Capitol Hill
Dense, walkable, Link Light Rail station. Smaller lots favor basement and attached AADUs; the transit parking exemption applies across most of the neighborhood.
Columbia City
Light rail station, deep lots, and strong appreciation. Excellent DADU potential with rental demand from the rapidly growing Rainier Valley corridor.
West Seattle
Generous lot sizes and RapidRide C Line service. Detached cottages and garage conversions perform well; views can add significant rental value.
Wallingford / Green Lake
Established, walkable, high rents. Note: properties within 200 ft of Green Lake may trigger Shoreline Management Act review. Strong AADU and DADU demand near the lake.
Beacon Hill
Light rail access, larger lots than central neighborhoods, and comparatively lower land costs. One of Seattle’s best value plays for new DADU construction.
Financing Your Seattle ADU
HELOC / Home Equity Loan
Tap existing equity in your primary home. Flexible draw, but variable rates. Best for homeowners with substantial equity who want to start quickly.
Construction Loan
A short-term loan that funds the build in draws, then converts to a permanent mortgage. Underwriting considers the projected post-build value including ADU rental income.
ADU-Specific / Renovation Loans
Products such as Fannie Mae HomeStyle or renovation refinances let you finance the ADU against the improved property value. Some local credit unions offer dedicated ADU programs.
Cash-Out Refinance
Replace your existing mortgage with a larger one and take the difference as cash. Works well when current rates are favorable relative to your existing loan.
With Seattle rents of $2,000–$3,500/month, rental income is a core part of most financing plans. We can connect you with lenders experienced in ADU underwriting.
Ready to find out what your lot can support? Get a free feasibility study — we check your zoning, setbacks, tree constraints, and shoreline jurisdiction, then map the most cost-effective ADU for your property. Or explore our permitting service, where your permit investment is applied as a build deposit.
Related Guides
Last updated: June 2026. This guide references SMC 23.44.041, SMC 23.44.025, Seattle Ordinance 125791 (CB 119544), and HB 1337 (RCW 36.70A.680–681). Not legal advice.
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