ADU Permit in Seattle, WA
Permit Summary
ADU Types & Property Eligibility
ADU Type Rules
Seattle allows attached, detached, and stacked ADUs. No lot may have more than two ADUs, and the ADUs must be on the same lot as a principal dwelling unit. The two-ADU allowance can include two detached ADUs, subject to the standards of the underlying zone and other applicable codes.
Lot / Zoning Eligibility Rules
ADUs are allowed as a housing use in Seattle zones where housing uses are allowed. They must be on the same lot as the principal dwelling unit and are generally subject to the standards that apply to principal dwelling units unless the underlying zone provides otherwise. ADUs in the Shoreline District must comply with Chapter 23.60A, and other overlays or site constraints can add requirements.
Size, Height & Placement
Setback Rules
Setbacks depend on the underlying zone and lot configuration. In Neighborhood Residential zones, an ADU generally has a 5-foot rear setback, while no rear setback is required where the rear lot line abuts an alley. Side setbacks are generally 5 feet average with a 3-foot minimum; a 3-foot side setback applies to certain lots under 5,000 square feet in frequent-transit service areas. Other zones, building-separation rules, environmentally critical areas, shorelines, and site-specific conditions can change the applicable location standards.
Lot Coverage / FAR Rules
ADUs count toward applicable density, floor-area, and lot-coverage limits. In Neighborhood Residential zones, maximum lot coverage is generally 50 percent, with up to 60 percent for qualifying stacked-unit development. Environmentally critical areas can reduce the lot area used for development calculations. Applicable floor-area limits and exceptions vary by zone and development configuration.
Attached ADU Rules
An attached ADU may be created within or attached to a principal dwelling unit and must meet applicable zoning, building, life-safety, and energy requirements. Current code allows stacked ADU configurations. A qualifying attached ADU created within portions of a principal structure that existed by the code's specified date may use an existing-structure exception to the otherwise applicable ADU floor-area limit.
Detached ADU Rules
A detached ADU is a separate dwelling structure on the same lot as the principal dwelling unit. Seattle permits up to two ADUs on a lot, including configurations with two detached ADUs, subject to the overall unit limit and applicable height, setback, lot-coverage, separation, structural, utility, drainage, and life-safety rules. A new DADU is assigned a separate Seattle address and building ID during permit review.
Conversion ADU Rules
Existing living space or an existing accessory structure may be converted or legalized as an ADU through the applicable permit process. Current code provides special flexibility for certain accessory structures that existed before the specified code date, including potential relief from otherwise applicable setback or lot-coverage standards while still requiring minimum housing and safety standards. An existing unpermitted dwelling unit is legalized through the Construction Permit — Establishing Use path.
Design, Occupancy & Parking
Design Standards
ADUs must comply with the objective zoning, building or residential, energy, structural, and site-development standards applicable to the project. Requirements may also arise from historic or shoreline regulations, environmentally critical areas, tree protection, drainage, and other site-specific rules; those requirements are not waived merely because the project is an ADU.
Owner Occupancy Rules
Seattle does not require the property owner to occupy either the principal dwelling unit or an ADU. Some properties may still have older recorded owner-occupancy covenants from prior regulations; SDCI provides a process to release obsolete ADU owner-occupancy covenants.
Parking Rules
Seattle does not require an off-street motor-vehicle parking space solely because an ADU is created. Any parking voluntarily provided must comply with applicable parking location, access, and other zoning or building requirements.
Rental / Short-Term Rental Rules
An ADU may be rented without an ADU owner-occupancy requirement. Long-term rental housing is subject to applicable Seattle rental-registration and inspection rules. An attached or detached ADU used as a short-term rental is treated as a separate dwelling unit for Seattle's short-term-rental licensing rules, and dedicated non-primary-residence short-term rentals are also subject to applicable RRIO requirements.
Separate Sale Rules
Seattle's current code does not prohibit sale or conveyance of an ADU as a condominium unit merely because it was originally constructed as an ADU. However, the ADU may not simply be separated from the principal dwelling through ordinary subdivision under the ADU rules; a legally compliant condominium or other ownership structure is required for separate conveyance.
Address, Utilities & Trade Permits
Address Rules
A new detached ADU receives a separate regular street address and building ID from SDCI as part of permit review for emergency response, utilities, and mail. Addressing for attached or converted ADUs depends on the project configuration and SDCI addressing review; an Address Assignment Request is available when a new or changed address is needed.
Utility Connection Rules
Utility requirements depend on the ADU configuration and existing services. Seattle Public Utilities generally allows one domestic water service per parcel, including parcels with a DADU, and a private submeter may be used where appropriate. Development or new utility service may require a Water, Drainage and Wastewater Availability Certificate, and new or altered sewer connections can require a side-sewer permit. Electrical service changes or new service may require Seattle City Light review. SDCI reports ADU permits to King County for applicable sewage-treatment capacity charges.
Trade Permit Rules
Separate trade permits may be required depending on project scope. Electrical, mechanical, refrigeration, plumbing, gas, backflow, side-sewer, fire-sprinkler, or other specialty work can require permits or approvals from SDCI, Public Health — Seattle & King County, Seattle Public Utilities, Seattle City Light, or the applicable administering agency. The construction permit does not replace required trade permits.
Fire, Energy & Special Requirements
Fire / Life Safety Rules
An ADU must comply with the applicable current Seattle Residential, Building, Existing Building, and Fire Code provisions. Depending on the building and permit path, requirements can include smoke and carbon-monoxide alarms, compliant egress and emergency escape openings, fire-resistance or separation provisions, and automatic fire sprinklers where the applicable code configuration requires them. ADU status alone does not create one universal sprinkler rule for every configuration.
Energy / Solar Rules
New ADUs and qualifying additions or alterations must comply with the applicable 2021 Seattle Energy Code. Requirements can address the building envelope and air leakage, heating and cooling, ventilation, water heating, lighting, controls, metering, and renewable-energy provisions. Residential-code solar-ready provisions apply where triggered by the applicable new-construction code path; additions and alterations follow the energy-code provisions applicable to their scope.
Special Area / Overlay Rules
Additional requirements can apply to properties in or near Environmentally Critical Areas, the Shoreline District, historic districts, and other regulated areas. Ground disturbance or development can trigger a Pre-Application Site Visit, geotechnical review, drainage review, tree protection, or other site-specific requirements. Shoreline District ADUs must comply with Chapter 23.60A. Seattle's 2026 Stormwater Code and Manual apply to covered applications completed on or after July 1, 2026.
Development / Impact Fee Rules
Seattle exempts ADUs from off-street parking requirements, street-improvement requirements, and Mandatory Housing Affordability contributions that would otherwise arise solely because of the ADU. SDCI reports ADU permits to King County for applicable sewage-treatment capacity charges. Since January 1, 2026, Seattle Public Utilities system-development charges can also apply when a project requires qualifying new or enlarged water or wastewater service or increases hard surface for drainage, subject to the utility charge rules.
Preapproved Plans
Preapproved Plan Rules
Seattle's ADUniverse program offers pre-approved designs for detached ADUs. Using a pre-approved design makes permitting more predictable, but the project still requires site-specific review for zoning and property conditions and may require foundation, utility, drainage, tree, environmentally critical area, or other review. Seattle states that, in most cases, permits using the pre-approved DADU process can be obtained in about 2–6 weeks.
Permit & Review Path
Permit / Review Path
Adding an ADU within an existing house or building a detached ADU uses the Construction Permit — Addition or Alteration path. Legalizing an existing unpermitted dwelling unit uses the Construction Permit — Establishing Use path. A detached ADU using an ADUniverse standard plan follows the pre-approved-plan process with site-specific review. Environmentally critical areas, shoreline conditions, drainage, utility changes, structural conditions, or other site-specific work can add related reviews or permits.
Inspections
Inspection Stages
Inspection stages depend on project scope. New detached ADU work can require site or preconstruction inspection where applicable, foundation inspection before concrete placement, subfloor or structural inspection, framing inspection before concealment, insulation inspection, required electrical/plumbing/mechanical inspections, and a final building inspection after related permit work is complete. Conversions may require fewer construction stages but still require the inspections necessary to establish the ADU legally.
What You Need Before Applying
Requirements
An ADU must be legally established through Seattle's permit process and comply with the current zoning, residential or building, mechanical, electrical, energy, environmentally critical area, shoreline, drainage, and utility rules that apply to the site and project. Confirm the allowed unit configuration, size, height, setbacks, density and lot coverage, site constraints, and utility needs before applying.
Exemptions
Seattle does not provide a general permit exemption for establishing an ADU. Creating an attached ADU, building a detached ADU, or legalizing an existing unpermitted dwelling unit requires the applicable construction permit path. A tiny house on wheels is treated as a camper trailer rather than a DADU and cannot be used as a dwelling on a Seattle lot under the ADU process.
Required Documents
Typical application materials include a site plan, complete legal description, floor plans, elevations where applicable, structural information for new structures or additions, and energy-code information. Site-specific projects may also require drainage, geotechnical, tree, environmentally critical area, shoreline, or utility documents. Applicants creating an ADU must acknowledge applicable King County sewage-treatment capacity charges.