Backyard PermitADU and garage conversion rules, city by city

Building an ADU or backyard cottage in Seattle

City code and Washington law · Last reviewed October 9, 2026

Seattle lets you add up to two accessory dwelling units (ADUs) on a lot, attached, detached or stacked. An ADU with up to two bedrooms can be 1,000 square feet, and one with three or more bedrooms can be 1,200 square feet. No parking is required. Permits come from the Seattle Department of Construction and Inspections (SDCI), and a pre-approved backyard cottage plan can get a permit in 2 to 6 weeks in most cases. [1][2][3]

Seattle at a glance
ADUs per lotUp to two (attached, detached or stacked) [1]
Max size1,000 sq ft gross floor area (up to 2 bedrooms); 1,200 sq ft (3+ bedrooms) [1]
Not counted in sizeUp to 250 sq ft of attached garage, underground stories, up to 35 sq ft of bike parking [1]
Rear setback (Neighborhood Residential)5 ft for ADUs; none if the rear lot line is on an alley [1][5]
Side setback (Neighborhood Residential)5 ft average, 3 ft minimum (3 ft on lots under 5,000 sq ft in frequent transit areas) [1][5]
Height (Neighborhood Residential)32 ft base limit, as for houses; lower limits for structures in required setbacks [1]
ParkingNone required for an ADU [1][2]
Lot coverage (Neighborhood Residential)50% maximum; ADUs count toward coverage, density and floor area [5]
Owner occupancyNot required (barred by state law) [10]
Pre-approved DADU plansYes, on ADUniverse; permit in 2 to 6 weeks in most cases [2][3]
Selling an ADU separatelyAllowed as a condo unit; can't be split off by subdivision [1][5]
Short-term rentalAllowed in a legal dwelling unit with a city license; register with RRIO if you don't live there [8]
On this page
  1. At a glance
  2. What you can build in Seattle
  3. Size, height and setbacks
  4. Parking
  5. Fees
  6. Pre-approved backyard cottage plans
  7. Garage and other conversions
  8. Renting it out
  9. Older city guides may be out of date
  10. Permit steps
  11. Pre-approved plans
  12. Who to call
  13. Questions people ask

What you can build in Seattle

Seattle's ADU rules are in Seattle Municipal Code 23.42.022. ADUs are allowed "in all zones where housing uses are allowed." No lot may have more than two ADUs, and they "may be attached, detached, or stacked." An ADU must be on the same lot as the main home.

SDCI calls an ADU inside or attached to the house an attached ADU (AADU), and one in a separate building a detached ADU (DADU), often called a backyard cottage. A lot may be big enough for four or more units in total, but only two of them may be ADUs.

These rules were rewritten to follow Washington's 2023 ADU and middle housing laws. Ordinance 127211 (ADUs) and Ordinance 127219 (interim middle housing rules) took effect June 30, 2025. The City Council then passed Ordinance 127376 in December 2025, which updated the ADU section and replaced the Neighborhood Residential zone code. See our Washington ADU law page for the state rules. [1][2][3][4]

Size, height and setbacks

Size. The gross floor area of an ADU with up to two bedrooms "may not exceed 1,000 square feet." With three or more bedrooms the limit is 1,200 square feet. A 1,500 square foot limit applies only in narrow cases in Lowrise zones. Up to 250 square feet of an attached garage, underground stories, and up to 35 square feet for long-term bike parking don't count.

Height and other standards. Unless the zone says otherwise, ADUs follow the same standards as the main home. In Neighborhood Residential (NR) zones the base height limit is 32 feet. Accessory structures in required setbacks are limited to 12 feet, with exceptions, so ask SDCI how this applies to your design.

Setbacks in NR zones:

NR lots have a 50 percent lot coverage limit, and buildings must be at least 5 feet apart. ADUs count toward the lot's density, floor area and lot coverage. Other zones have different standards; check your zone on ADUniverse or with SDCI. [1][5]

Parking

"No off-street motor vehicle parking is required for an accessory dwelling unit." SDCI also lists parking, street improvements and mandatory housing affordability contributions among the standards that don't apply to ADUs. [1][2]

Fees

SDCI bases permit fees on the size and complexity of the project, the value of the work, extra permits needed, review time, inspections and technology fees. For a permit that needs plan review, you pay 75 percent of the plan review and permit fees when your application is accepted. You pay the rest before the permit is issued. Use the SDCI fee estimator and fee schedule to get an idea of cost.

SDCI reports every ADU permit to King County for its sewage treatment capacity charge. You'll fill out a form acknowledging that charge when you apply. For budgeting, see our ADU cost guide. [2][6]

Pre-approved backyard cottage plans

The Office of Planning and Community Development runs ADUniverse. It has a gallery of pre-approved DADU designs by local designers and architects, a step-by-step guide, and a tool to check whether an ADU is feasible on your lot. The city says that with a pre-approved plan, "In most cases, you can get your permit in just 2-6 weeks."

To use the faster standard-plan process, your project must:

The city says some parts of ADUniverse may be out of date while it updates the site for the new state rules. [2][3][7]

Garage and other conversions

You can convert an existing accessory structure, such as a garage, into a DADU "notwithstanding applicable lot coverage or yard or setback provisions." This applies to structures that existed before July 23, 2023, or were replaced in the same configuration. The SDCI Director may also grant waivers to make the conversion work. The converted building must meet the city's minimum housing standards.

An attached ADU can be larger than 1,000 square feet if the part of the house it's in existed as of July 23, 2023. If you're converting a basement or other space in the house, you need a construction addition/alteration permit. See our garage conversion guide for general steps. [1][2]

Renting it out

Short-term rentals (fewer than 30 nights) are allowed in most dwelling units. If the rental is not in the home you live in, for example a backyard cottage you own, you must register with the city's Rental Registration and Inspection Ordinance (RRIO) program. You also need a business license tax certificate from the city's Finance and Administrative Services department, and every short-term rental must meet the city's regulatory license requirements. Ask SDCI at (206) 615-0808 for code questions, or email str@seattle.gov about licensing.

Tiny houses on wheels are treated like camper trailers, and you can't live in one on a lot in Seattle. A tiny house on a foundation is treated as a DADU. See our backyard tiny house guide. [2][8]

Older city guides may be out of date

SDCI's Tip 116B on backyard cottages is dated August 18, 2023, before the 2025 code changes. It still describes older rules, such as a 3,200 square foot minimum lot size and "SF5000" zone names, that don't appear in the current ADU section of the code. Where they differ, rely on the current code and confirm with SDCI. [1][9]

Permit steps in Seattle

  1. Check your lot

    Find your zone, and check whether an ADU is feasible, with the search tool on ADUniverse. Note any environmentally critical areas or shoreline location, which bring extra rules. [1][3][7]

  2. Get coaching if you need it

    SDCI's Applicant Services Center offers live chat, help tickets and video coaching sessions with permit specialists and land use planners, or you can send SDCI a question. [2][11]

  3. Choose a pre-approved plan or a custom design

    Pick a plan from the pre-approved DADU gallery for a faster permit, or have plans drawn. An attached or detached ADU needs a construction addition/alteration permit; legalizing an existing unit needs a construction permit to establish use. [2][3]

  4. Apply on the Seattle Services Portal

    Log in to the Seattle Services Portal and submit a Building & Land Use Pre-Application. Upload your site plan, plan set and the King County sewer capacity charge form. For a pre-approved plan, follow SDCI's standard plan instructions and enter the Standard Plan number. [7]

  5. Pay fees and answer corrections

    Pay 75 percent of plan review and permit fees when the application is accepted. Respond to any review comments, then pay the remaining fees before the permit is issued. [6]

  6. Get related permits

    Electrical, plumbing and side sewer work may need their own permits. Seattle City Light handles electrical service changes, and Seattle and King County Public Health handles plumbing permits. [2][9][11]

  7. Build and pass inspections

    Schedule inspections through the Seattle Services Portal or call SDCI inspections at (206) 684-8950. When the work is done, request a final inspection. [9][11]

Pre-approved and sample plans

Who to call

Questions people ask

How big can an ADU be in Seattle?

An ADU with up to two bedrooms can have up to 1,000 square feet of gross floor area, and one with three or more bedrooms up to 1,200 square feet. Up to 250 square feet of attached garage, underground stories and up to 35 square feet of bike parking don't count. [1]

Can I build two backyard cottages in Seattle?

Yes. A lot can have up to two ADUs, and they may be attached, detached or stacked. Both count toward the lot's density, floor area and lot coverage limits. [1][5]

How close to the property line can a DADU be in Seattle?

In Neighborhood Residential zones, the rear setback for an ADU is 5 feet, or zero if the rear lot line abuts an alley. Side setbacks are 5 feet average and 3 feet minimum, or 3 feet on lots under 5,000 square feet in frequent transit service areas. [1][5]

How long does an ADU permit take in Seattle?

With a pre-approved DADU plan in an NR zone, the city says you can get a permit in 2 to 6 weeks in most cases. For other projects, SDCI reviews your plans and may send corrections you must answer before the permit is issued. [3][6][7][9]

Do I have to live on the property to have an ADU in Seattle?

No. Washington law bars cities from requiring the owner to live in the ADU or the main house. SDCI lists a form to release an owner occupancy covenant for ADUs. [2][10]

Sources (11)
  1. Ordinance 127376 (Council Bill 120993), signed legislation, amending SMC 23.42.022 and adding new Chapter 23.44, City of Seattle. Accessed October 9, 2026.
  2. Accessory Dwelling Unit, Seattle Department of Construction and Inspections. Accessed October 9, 2026.
  3. Encouraging Backyard Cottages, Seattle Office of Planning and Community Development. Accessed October 9, 2026.
  4. Neighborhood Residential Code, Seattle Department of Construction and Inspections. Accessed October 9, 2026.
  5. Seattle's Neighborhood Residential Zone (zoning summary), Seattle Department of Construction and Inspections. Accessed October 9, 2026.
  6. How Much Will Your Permit Cost?, Seattle Department of Construction and Inspections. Accessed October 9, 2026.
  7. How To Apply for a Detached Accessory Dwelling Unit Using a Pre-Approved Standard Plan, City of Seattle, Seattle Services Portal Help Center. Accessed October 9, 2026.
  8. Short-Term Rentals, Seattle Department of Construction and Inspections. Accessed October 9, 2026.
  9. Tip 116B, Establishing a Detached Accessory Dwelling Unit (DADU), updated August 18, 2023, Seattle Department of Construction and Inspections. Accessed October 9, 2026.
  10. RCW 36.70A.681 Accessory dwelling units — Limitations on local regulation, Washington State Legislature. Accessed October 9, 2026.
  11. Applicant Services Center, Seattle Department of Construction and Inspections. Accessed October 9, 2026.

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