Everything homeowners need to know about building an accessory dwelling unit in Seattle - size limits, setbacks, permits, and the state law changes that made Seattle one of the most ADU-friendly big cities in the country.
Last verified: August 2026
10/10 · Extremely ADU-friendly
1,000 sq ft
Max ADU size
32 ft
Max height
Not required
Owner-occupancy
By-right
Permit type
This page summarizes Seattle's ADU standards under Seattle Municipal Code 23.44.041 and the city's 2025 update implementing Washington's statewide ADU law, for general reference - it is not legal advice. Regulations change; always confirm current requirements with the Seattle Department of Construction & Inspections (SDCI) before designing or applying for permits. See sources below.
Zoning & size requirements
What you're allowed to build
ADU type
Max size
Max height
Notes
Detached (DADU / backyard cottage)
1,000 sq ft of floor area
Up to 32 ft in most Neighborhood Residential (NR) and Residential Small Lot (RSL) zones; lower near property lines
Up to two ADUs (any mix of attached and detached) are allowed on one lot
Attached (AADU, basement or addition)
Sized within the primary structure - no separate detached-style footprint cap
Same as primary structure
Common approach for a basement or above-garage conversion
Allowed zoning districts
ADUs are allowed citywide on residentially zoned lots, including the Neighborhood Residential (NR) zones that replaced Seattle's former single-family zones, as well as Residential Small Lot (RSL) and multifamily zones. Confirm your parcel's zone and exact height allowance with SDCI.
Additional standards
Setbacks, parking & other rules
Setbacks
5 ft from side and rear property lines for a DADU under Seattle Municipal Code 23.44.041; taller portions of a structure may require additional setback distance
Parking
No off-street parking required for an ADU anywhere in the city; Seattle dropped its citywide ADU parking mandate in 2019, and Washington state law now also bars parking mandates near transit
Units per lot
Up to 2 ADUs permitted in addition to the principal residence (one attached plus one detached, or two detached units)
Owner-occupancy
Not required. Seattle eliminated its owner-occupancy requirement in 2019, and Washington's 2023 statewide ADU law (HB 1337) permanently bars cities from reinstating it
Building code
Must meet Washington State Residential Code dwelling-unit standards
Rental license
Rentals to non-family tenants, including ADUs, must be registered under Seattle's Rental Registration and Inspection Ordinance (RRIO)
Ordinance history
How Seattle's ADU rules got here
2019Major ADU ordinance overhaul. City Council eliminated the owner-occupancy requirement, removed off-street parking mandates, and allowed up to two ADUs per lot - one of the most significant pro-ADU rewrites by any major US city at the time.
2022Permitting boom. The city issued permits for 988 ADUs that year (437 attached, 551 detached), up from roughly 280 in 2019, with ADU permits outpacing new single-family house permits citywide for the first time.
2023Washington passes HB 1337. The statewide law required cities to allow at least two ADUs per lot in urban growth areas, guarantee at least 1,000 sq ft of ADU floor area, cap ADU impact fees at 50% of what's charged to the primary unit, and bar parking mandates near transit.
2025Local code brought into compliance. Seattle adopted Ordinance 127211 to implement HB 1337, standardizing the 1,000 sq ft floor-area allowance citywide and raising DADU height limits to up to 32 ft in most Neighborhood Residential and Residential Small Lot zones.
2026Permanent zoning takes effect. Seattle's permanent Neighborhood Residential zoning, replacing the former single-family zones under the One Seattle Plan and state middle-housing law HB 1110, took effect January 21, 2026.
Seattle and Portland are routinely cited as the two US cities building the most ADUs per capita, and Seattle's own permitting data backs that up: ADU permits have outpaced new detached single-family home permits in recent years, a rare reversal nationally.
Permit process
How Seattle ADU permitting works
1
Pre-application zoning check
Confirm your zone (NR, RSL, or multifamily) and lot eligibility with SDCI before designing.
2
Choose a standard plan or custom design
SDCI's pre-approved DADU plans (via the ADUniverse program) skip much of the design review and can cut weeks off the timeline; a custom design goes through full plan review instead.
3
Prepare your plan set
Site plan, floor plans, elevations, and structural drawings meeting current SDCI standards; electrical service changes may need a separate Seattle City Light review.
4
Submit for permit review
Apply through the Seattle Services Portal for a construction permit; standard review commonly runs 8-14 weeks, faster for pre-approved plans.
5
Sewer capacity acknowledgment
Complete King County's Sewage Treatment Capacity Charge acknowledgment as part of the application - a separate charge from your city permit fees.
6
Construction & inspections
Once permits issue, build and schedule the standard framing, plumbing, electrical, and final inspections before occupancy.
Timelines and fees shift year to year as SDCI updates its fee schedule - confirm current numbers directly with SDCI before budgeting.
Budgeting
What it costs to build an ADU in Seattle
Garage conversion
Roughly $90,000-$180,000, the least expensive path since the shell already exists
Basement conversion (AADU)
Roughly $130,000-$230,000, depending on existing ceiling height and utility access
New detached ADU (DADU)
Roughly $180,000-$350,000+ for a code-compliant, fully finished freestanding unit
Permit & plan review fees
Commonly several thousand dollars depending on project valuation and size, on top of separate plumbing, electrical, and mechanical permits
King County sewer capacity charge
A one-time or financed charge (payable over 15 years) tied to King County's monthly capacity-charge rate, applying to most new ADU sewer connections
These are general ranges from local contractor estimates and regional reporting, not quotes - actual cost depends heavily on site access, utility connections, and finishes. Seattle's permit and utility fees run higher than much of the country, so budget for those as a real line item, not an afterthought.
Before you apply
Tips for a smooth approval
Check whether a pre-approved plan fits your lot. SDCI's ADUniverse program can shave weeks off permitting compared to a fully custom design.
Plan for two ADUs if you might want them. Since Seattle now allows up to two ADUs per lot, decide early whether you want one attached and one detached (or two detached units) so your site plan doesn't box you in.
Budget separately for the King County sewer capacity charge. It's billed apart from your city permit fees and can be a meaningful add-on cost.
Confirm your zone before finalizing height and size. Height limits vary between Neighborhood Residential, Residential Small Lot, and multifamily zones, so a design that fits one zone may not fit another.
Register as a rental property if you'll rent it out. Non-family tenants require registration under Seattle's Rental Registration and Inspection Ordinance (RRIO).
FAQ
Common questions
Do I need to live on the property to build an ADU in Seattle?
No. Seattle eliminated its owner-occupancy requirement in 2019, and Washington's 2023 statewide ADU law (HB 1337) permanently bars cities from bringing it back.
How many ADUs can I build on one lot in Seattle?
Up to two - any combination of one attached and one detached ADU, or two detached units, in addition to the principal residence.
Do I need extra parking for an ADU?
No. Seattle dropped its ADU parking mandate in 2019, and state law now also bars parking mandates for ADUs near transit.
What's the maximum size for a detached ADU (DADU)?
Up to 1,000 sq ft of floor area, with height limits of up to 32 ft in most Neighborhood Residential and Residential Small Lot zones.
How long does the permit process take?
Standard review commonly runs about 8-14 weeks; SDCI's pre-approved DADU plans can be faster. Confirm current timelines with SDCI.
How much does an ADU cost to build in Seattle?
Garage conversions run roughly $90,000-$180,000, basement conversions roughly $130,000-$230,000, and new detached units roughly $180,000-$350,000+, based on local contractor estimates and regional reporting - not counting permit and sewer capacity fees.
How many ADUs has Seattle actually permitted?
The city issued permits for 988 ADUs in 2022 alone (437 attached, 551 detached), up from around 280 in 2019, and ADU permits have outpaced new single-family home permits in recent years.
Does Washington's statewide ADU law change anything for Seattle specifically?
Yes. HB 1337 (2023) set a floor under Seattle's own rules - guaranteeing at least 1,000 sq ft of ADU floor area, capping ADU impact fees at 50% of what's charged to the primary unit, and barring parking mandates near transit - and Seattle updated its local code in 2025 to formally comply.
Can I rent my ADU out on Airbnb or a short-term rental platform?
Seattle regulates short-term rentals separately from standard rental licensing, with its own permit category. Confirm current short-term rental rules with the city before finalizing your plans.
