Seattle DADU Rules 2026: Size, Cost & Permits

Seattle allows accessory dwelling units broadly, but a buildable DADU depends on the property’s current zoning, setbacks, trees, utilities, environmentally critical areas, and existing structures. Under the city’s 2026 code, a lot with a principal dwelling may have up to two accessory dwelling units. An ADU with up to two bedrooms may contain as much as 1,000 square feet; one with three or more bedrooms may contain as much as 1,200 square feet.

Those are maximums, not promises for every backyard. Start with an address-specific feasibility study and the current Seattle Municipal Code before paying for complete plans or relying on an old “five-foot setback” diagram.

Important 2026 update: Seattle adopted Ordinances 127375 and 127376 after earlier 2025 interim ADU changes. Older guides may still state a universal 1,000-square-foot cap or superseded Neighborhood Residential standards. The current code allows up to 1,200 square feet for an ADU with at least three bedrooms and generally applies the underlying zone’s standards to ADUs.

Seattle DADU rules at a glance

Issue	Current general rule
What is a DADU?	A detached accessory dwelling unit on the same lot as a principal dwelling
Where allowed?	ADUs are allowed in zones where housing uses are allowed, subject to the underlying zone and special overlays
Maximum number	Up to two ADUs on a lot with a principal dwelling; they may be attached, detached, or stacked
Maximum size	1,000 sq. ft. for up to two bedrooms; 1,200 sq. ft. for three or more bedrooms, with code-defined exceptions and exclusions
Parking	Seattle’s ADU guidance says new parking is not required, although existing required parking and the site plan can create separate constraints
Owner occupancy	Seattle removed its former general ADU owner-occupancy covenant requirement; rental and short-term-rental rules remain separate
Building permit	A detached unit requires a construction permit and may require separate electrical, utility, drainage, grading, or other approvals

What counts as a DADU?

A detached accessory dwelling unit—often called a backyard cottage, carriage house, or detached ADU—is a legally permitted dwelling in a structure separate from the principal dwelling. It contains the facilities required for independent living. A tiny house on a permanent foundation can be reviewed as a DADU; Seattle says a tiny house on wheels is treated as a camper trailer and cannot be used as a dwelling on a city lot.

An attached ADU is inside or physically attached to another dwelling. A converted detached garage can become a DADU if the project establishes a legal dwelling and meets applicable land-use and building standards. Calling an unpermitted shed a “guest house” does not make it legal sleeping or rental space.

How many ADUs can a Seattle lot have?

Current SMC 23.42.022 says no lot may have more than two accessory dwelling units. They may be attached, detached, or stacked and must share the lot with the principal dwelling unit.

This ADU allowance exists alongside Seattle’s broader middle-housing rules. A four- or six-unit development strategy is not simply “a house plus as many DADUs as fit.” Unit count, floor-area ratio, lot coverage, amenity area, access, trees, and the applicable development type must be evaluated together.

Seattle DADU size limits in 2026

The current general gross-floor-area limits are:

Up to two bedrooms: no more than 1,000 square feet.
Three or more bedrooms: no more than 1,200 square feet.

The code contains specific exclusions and rules for areas such as below-grade space, exterior-access storage, garages, and bicycle parking. Do not subtract those areas casually from a marketing floor plan; have the designer calculate gross floor area using the definitions in the current code.

Setbacks, height, lot coverage, and trees

Seattle’s 2025 and 2026 changes made ADU height and setback treatment more consistent with the underlying zone. That does not create one universal DADU setback for every parcel. The controlling geometry can depend on:

the property’s zone and lot dimensions;
front, side, and rear setbacks under the current development type;
alleys, easements, and required access;
existing buildings and projections;
maximum lot coverage and floor-area rules;
exceptional trees and tree-protection areas;
steep slopes, wetlands, riparian corridors, liquefaction areas, or other environmentally critical areas;
shoreline or historic-district overlays;
fire separation, emergency access, and building-code openings.

The safest early deliverable is a measured site plan showing property lines, structures, grade, utilities, trees, easements, and proposed access. A builder’s aerial screenshot is not a zoning analysis.

Is parking required?

Seattle’s current ADU project page and Tip 116B say additional parking is not required for an accessory dwelling unit. However, a proposal can still be affected by existing legally required parking, alley access, curb cuts, tree protection, or the space needed for construction and utilities. Do not assume “no new parking required” means an existing garage or required stall can always be removed without review.

Is owner occupancy required?

Seattle removed the former general requirement that the property owner occupy the principal dwelling or an ADU. The city’s project resources include a process for releasing old owner-occupancy covenants.

That zoning change should not be confused with other rules. Financing documents, condominium arrangements, long-term rental registration, and short-term-rental licensing can impose separate obligations. Review recorded title documents and the intended rental use before underwriting the project.

Seattle DADU permit process

Research the parcel. Confirm the zoning, overlays, permit history, lot dimensions, sewer and water service, trees, critical areas, and recorded easements.
Test feasibility. Prepare a code summary and scaled concept showing the DADU, access, setbacks, utilities, drainage, and construction staging.
Build a complete budget. Price design, surveys, engineering, permits, utility work, site work, construction, financing, and contingency—not merely the structure.
Submit the pre-application. SDCI directs applicants to begin through the Seattle Services Portal with a Building & Land Use Pre-Application.
Prepare permit documents. Typical submittals include site plans, floor plans, elevations, structural plans, energy calculations, and other site-specific reports.
Complete departmental reviews. The project may involve SDCI, Seattle City Light, Seattle Public Utilities, King County sewer capacity charges, and other agencies.
Obtain trade permits and build. Electrical and other trade work may require separate permits.
Pass inspections. The unit cannot be legally occupied until required inspections and final approval are complete.

The SDCI accessory-dwelling-unit page is the best starting point. Permit intake and review time vary with project completeness, corrections, critical areas, tree removal, and departmental work.

Are pre-approved plans faster?

Seattle’s ADUniverse offers pre-approved structural designs. The city says a qualifying standard-plan application can often receive a permit in two to six weeks. That estimate is for the permit pathway, not the entire project.

You still need a property-specific site plan and review of zoning, foundation, grading, electrical, fire, drainage, trees, utilities, and other conditions. A pre-approved plan can reduce structural review and correction cycles, but it does not prove the design fits the lot or eliminate construction documents and professional fees.

What does a Seattle DADU cost in 2026?

There is no official flat DADU price. Published 2025–2026 estimates from Seattle-area designers and builders span roughly $350 to $600 or more per square foot, depending on whether the number includes design, permits, site work, utilities, financing, and high-cost lot conditions. One local architecture firm uses about $350 per square foot as an early design-and-construction planning figure; a Seattle-area design-builder reports all-in 800–1,000-square-foot projects commonly falling around $375,000–$600,000 or more.

Those figures are planning references, not bids or appraisals. Small units often cost more per square foot because every dwelling still needs a kitchen, bathroom, utility connections, foundation, mechanical systems, and permit work.

Budget categories people miss

surveying, architecture, structural engineering, energy, geotechnical, arborist, or civil work;
permit review, trade permits, King County wastewater capacity charges, and utility fees;
demolition, excavation, shoring, contaminated soil, rock, dewatering, and tight-site access;
water, sewer, electrical, communications, and possible service upgrades;
stormwater management, grading, retaining walls, walkways, fencing, and landscape restoration;
appliances, window coverings, security, mail, waste storage, and tenant-ready finishes;
loan fees, interest during construction, escalation, temporary housing, and insurance;
a design contingency plus a separate construction contingency.

Seattle’s 2026 fee summary uses a 500-square-foot DADU example with approximately $3,453 in plan-review and building-permit fees based on the city’s example project value. That is not the complete permit-and-utility cost for every project. Use SDCI’s current fee estimator and obtain written utility estimates for the actual address.

Can a Seattle DADU generate rental income?

Yes, a legal DADU can generally be used as a long-term rental, but “potential rent” is not profit. Build a property-specific model:

Use recent comparable rents for legal units of similar size, bedrooms, parking, finish, and micro-location.
Subtract vacancy and leasing costs.
Subtract utilities paid by the owner, insurance, property-tax effects, maintenance, replacements, landscaping, and management.
Reserve for major systems and tenant turnover.
Include debt service and construction-period interest.
Stress-test lower rent, longer vacancy, and a cost overrun.

Seattle rental housing generally falls under the Rental Registration and Inspection Ordinance. Landlord-tenant rules, screening requirements, notices, deposits, relocation obligations, and tax treatment can change the economics. Consult the City’s Renting in Seattle materials and qualified tax or legal professionals for the specific plan.

Can a DADU be used as an Airbnb or short-term rental?

A DADU is a separate dwelling unit and requires its own short-term-rental authorization when operated as one. Seattle generally requires a business license tax certificate and short-term-rental operator license. Most operators may operate up to two units, and when operating two, one generally must be the operator’s primary residence. A dedicated non-primary-residence short-term rental must also comply with RRIO registration requirements.

Confirm eligibility before treating nightly revenue as the financing plan. The City’s short-term-rental page controls; platform availability does not establish legality.

How to choose a DADU contractor

Look for documented Seattle DADU permits, a current Washington contractor registration, appropriate insurance, transparent exclusions, and experience coordinating SDCI corrections, utilities, trees, drainage, and constrained access. Compare the same drawings and specifications—not three different concepts labeled “turnkey.”

Use our Seattle DADU contractor selection guide and broader Washington contractor verification checklist before signing a preconstruction or design-build agreement.

Frequently asked questions

How big can a DADU be in Seattle?

Under current general rules, an ADU with up to two bedrooms may be as large as 1,000 square feet; an ADU with three or more bedrooms may be as large as 1,200 square feet. The lot and underlying zone can impose a smaller practical envelope.

Can Seattle lots have two DADUs?

The current code allows up to two ADUs per lot with a principal dwelling, and ADUs may be attached, detached, or stacked. Site constraints and the broader development proposal still control what can actually be permitted.

Does a DADU need a separate address?

SDCI assigns and coordinates addressing during the permit process. Do not invent or begin using a unit address before the city’s addressing step is complete.

How long does a Seattle DADU take?

A qualifying pre-approved-plan permit may be issued in two to six weeks according to the city, but feasibility, design, intake, corrections, utilities, procurement, and construction make the complete project much longer. Build the schedule from actual agency and contractor commitments.

Is a garage conversion cheaper than a new DADU?

It can be, but only when the existing foundation, structure, height, moisture protection, fire separation, and utilities are suitable. Read our garage conversion planning guide before assuming the shell has reusable value.

Official Seattle resources

Rules and official resources checked August 2026. This is planning information, not legal, engineering, tax, appraisal, or permit advice. SDCI and the current code control for a specific address.

For weak soils, overlapping footings, or heavily loaded additions, learn how a mat slab foundation differs from slab-on-grade and spread footings.

Seattle DADU Rules 2026: Size, Permits, Cost and Rental Use