Seattle ADU and detached ADU rules
Seattle ADU and DADU rules under the permanent 2026 code
Seattle's permanent neighborhood-residential code changed the old backyard-cottage playbook. Two ADUs are allowed, no parking is required, and a detached unit follows the same height framework as a principal unit. The lot's total unit density now drives FAR, setbacks, coverage, open space, and design requirements.
Reviewed August 14, 2026 against adopted city code and official city guidanceQuick definitions. An accessory dwelling unit (ADU) is a complete additional home on the same property. A detached ADU (DADU) is in its own building. FAR or GFA limits how much enclosed floor area the property may contain.
Bottom line
What the current rules mean
A Seattle lot may have up to two attached or detached ADUs. Most units are capped at 1,000 square feet with up to two bedrooms or 1,200 square feet with three or more bedrooms. In neighborhood residential zones, a five-foot rear yard, five-foot building separation, no required parking, and the current density-based FAR table can make a DADU workable, but protected trees, environmentally critical areas, amenity area, utilities, and total units must all be tested together.
What is allowed on one lot
Seattle permits up to two ADUs on a lot wherever housing is allowed, subject to the underlying zone and total-unit density. In neighborhood residential zones, maximum density is generally the greater of four units, one non-stacked unit per 1,250 square feet, or one stacked unit per 600 square feet; qualifying transit or affordable-housing projects may allow six units, while environmentally critical areas can reduce density. Shoreline rules can also change the result.
- Two attached or internal ADUs.
- One attached and one detached ADU.
- Two detached or stacked ADUs where the site and underlying density/form standards allow them.
- Conversion or replacement of a legal existing structure, with special size and Type I modification provisions for qualifying conditions.
Official sources: SMC 23.42.022: accessory dwelling units · Seattle Ordinance 127376: current NR development standards
Size, height, setbacks, and site capacity
- Maximum size
- An ADU with up to two bedrooms may contain up to 1,000 gross square feet; one with three or more bedrooms may contain up to 1,200. The calculation excludes up to 250 square feet of attached garage, underground portions, and up to 35 square feet of long-term bicycle parking. A rare 1,500-square-foot path and special existing-space exceptions apply only when every codified condition is met.
- Height
- A DADU now uses the principal-unit height rules. The basic neighborhood-residential maximum is 32 feet; specified three-or-more-unit, stacked, or tree-retention/canopy projects may reach 42 feet. A qualifying pitched roof may add up to five feet at the ridge. Grade and roof-form calculations remain site-specific.
- Setbacks
- In neighborhood residential zones, the front yard is generally 15 feet for one or two units and 10 feet for three or more. A DADU may use a five-foot rear yard or zero feet along an alley. Side yards are generally a five-foot average with a three-foot minimum, with a three-foot rule on certain small frequent-transit lots. A DADU and principal unit need five feet of eave-to-eave separation in NR zones; easements, street-improvement, shoreline, fire, and ECA standards can require more.
- FAR, lot coverage, or impervious area
- Seattle includes ADU and DADU area in FAR. In neighborhood residential zones the base FAR rises with achieved density: 0.6 below one unit per 4,000 square feet, 0.8 through one per 2,201, 1.0 through one per 1,601, and 1.6 at one per 1,600 or denser, with specified designs reaching 2.0. Lots below 5,000 square feet receive at least 2,500 square feet of chargeable floor area. Lot coverage is generally 50 percent, with 60 percent for specified courtyard or stacked forms. Amenity area, facade, entry, and pedestrian-path rules also apply.
Official sources: SMC 23.42.022: accessory dwelling units · Seattle Ordinance 127376: current NR development standards
Parking, occupancy, renting, and separate sale
- Parking
- Seattle requires no off-street parking for an ADU. Do not add a space merely because an older backyard-cottage guide says to do so.
- Owner occupancy
- Seattle has no owner-occupancy requirement.
- Short-term rental
- Short-term rental requires compliance with Seattle's business licensing and short-term-rental code. Long-term rentals generally enter the Rental Registration and Inspection Ordinance program, subject to its stated exemptions.
- Separate sale
- Seattle may not prohibit separate condominium conveyance solely because a unit is or was an ADU. That does not create an automatic fee-simple lot split; subdivision or unit-lot approval is separate.
Official sources: SMC 23.42.022: accessory dwelling units · Seattle Rental Registration and Inspection Ordinance · Seattle short-term rental rules
Utilities, fire access, and the permit path
A building permit and coordinated zoning, structural, energy, drainage, side-sewer, water, solid-waste, tree, ECA, and trade review may apply. Seattle Public Utilities generally uses one domestic water service per parcel, allows private submetering, and requires a side-sewer permit; service for two or more units generally needs at least a six-inch sewer. King County's wastewater capacity charge and site-specific utility work are separate costs.
- Confirm the current zone, overlays, shoreline/ECA status, legal lot and unit history, and any vested earlier application.
- Choose the total-unit count and attached, detached, or stacked form before calculating density, FAR, coverage, height, setbacks, amenity area, entries, and tree points.
- Map regulated trees and tree-protection areas, easements, grade, access, water, sewer, drainage, waste storage, and construction staging on a survey-based site plan.
- Decide whether a preapproved DADU design actually fits the parcel; it still needs site, foundation/geotechnical, utility, drainage, tree, and ECA work.
- Submit the building and every related trade, utility, side-sewer, site, tree, or land-use application and complete RRIO or short-term-rental steps if applicable.
Official sources: Seattle SDCI ADU permit guidance · Seattle Public Utilities water-service guidance · Seattle side-sewer permits
Rules that can change the answer for one property
A citywide ADU allowance does not prove that a detached unit fits on a particular lot. Resolve these items before buying plans or fixing the DADU location.
- The old 2019 backyard-cottage dimension tables were superseded by the permanent code effective January 21, 2026; vested projects can be different.
- FAR depends on achieved unit density, so adding an ADU can change both the numerator and the applicable FAR tier.
- Tier 1 heritage trees are generally nonremovable, and Tier 2 development removal requires narrow capacity, width, access, or utility findings.
- Environmentally critical areas are deducted or regulated under separate rules; a map hit can trigger reports, exceptions, or reduced density rather than a simple yes/no answer.
- Shoreline Chapter 23.60A can prohibit or materially restrict a DADU even though the general ADU section allows two units.
Official sources: SMC Chapter 25.11: tree protection · Seattle environmentally critical areas · SMC Chapter 23.60A: shoreline code
Prepared for a future address check
What a reliable DADU feasibility screen must verify
Seattle's DADU screen must model the proposed total unit count and form before calculating FAR, coverage, setbacks, height, amenity area, tree points, and entries. It also needs legal-unit history, ECA/shoreline, regulated-tree, alley, sewer, and utility inputs.
Confirm the address is actually governed by Seattle and select the correct base-zone standards.
Count the principal dwelling, existing legal ADUs, middle-housing units, and the proposed attached or detached configuration.
Use legal lot lines, front/rear designation, alleys, access easements, and existing structures rather than a tax-map rectangle.
Test the ADU against the rules that apply locally: FAR or GFA, building or lot coverage, impervious or hard surface, and landscaping.
Screen critical areas and buffers, shorelines, slopes, flood hazards, significant trees, drainage, and recorded easements.
Verify water, sewer or septic, fire flow, hydrant and apparatus access, power, meters, and trench routes.
The engine should never apply the former 2019 DADU table to an unvested project. It should route protected-tree removal, ECA or shoreline constraints, special density/FAR paths, legal-existing-space exceptions, and vesting to manual review.
Do not rely on the older rule
Seattle's interim code effective June 30, 2025 and the earlier 2019 ADU rules were superseded by permanent Ordinance 127376, effective January 21, 2026. Older charts with lower DADU height, owner occupancy, or parking should not be used for a new unvested application.
Primary sources
Official Seattle and Washington references
These are the sources used for this guide. Check them again before filing because ordinances, fee schedules, and permit forms can change.
A practical first-pass checklist
- 1
Confirm the parcel is inside Seattle city limits and identify its current zone.
- 2
Map the principal home, lot lines, easements, trees, utilities, driveway, critical areas, and buffers on one current survey.
- 3
Test the ADU footprint against setbacks, height, floor-area or coverage limits, hardscape, parking, and construction access.
- 4
Ask the utility providers to confirm water, sewer, power, meter, side-sewer, and capacity requirements before fixing the design.
- 5
Have the city confirm the complete land-use, building, fire, right-of-way, tree, and utility permit list in writing.