Getting Started

This guide is for you, the Seattle homeowner curious about building a cottage in your backyard. We’ve decided to compile practical information so that you have a better understanding of costs, the permit process, construction, and rental management should you want to rent out your backyard cottage. While the regulations can be a bit confusing, having a detached accessory dwelling unit (DADU) can be a great choice. Whether you’re looking to increase your income by renting out your detached accessory dwelling unit, making room for a relative, or just looking to create some extra space, we can help.

What is a DADU and Who Can Build One?

A backyard cottage is a small residential structure sharing the same lot as a house, but self-contained and physically separate from the primary house. In the Seattle Municipal Code (SMC) zoning provision, Backyard Cottages are referred to as “Accessory Dwelling Units, detached.” They are also sometimes  referred to as detached accessory dwelling units, granny flats, mother-in-law apartments, and carriage houses. Backyard Cottages generally include a living room, sleeping area, kitchen, and bathroom and have a lockable entrance door. On some lots, a backyard cottage may be placed in a side yard.

In December, 2009 Seattle City Council adopted legislation to allow the construction of backyard cottages on eligible lots in single-family zones throughout the city. Prior to the December 2009 legislation, backyard cottages were permitted only in Southeast Seattle, where they were authorized by legislation in 2006. You must be a homeowner and your property must be at least 4,000 square feet in area and in a single-family residential zone. Your property cannot be in a Shoreline District.


What Permits Do You Need?

Adding within an existing house. You need a construction addition / alteration permit.

Building a detached unit. You need a construction addition / alteration permit.

Legalizing an existing unit. You need a construction permit to establish use.

You may also need to apply for electrical service changes or new services from Seattle City Light.



40 percent of the total square footage of the residence for ADUs. Not less than 300 square feet or greater than 800 square feet. Owner occupancy required in one of the two units. One off-street parking space required. ADUs not currently allowed in new construction.

Reference: development.bellevuewa.gov/zoning-and-land-use/zoning-requirements/accessory-dwellingunits


If detached: 800 square feet or 33 percent of the size of the principal unit, whichever is smaller. If attached: not more than 40 percent of the principal unit. One off-street parking space. One of the dwelling units must be owner-occupied for at least six months a year. Interestingly, Kent does allow ADUs in both new and existing single-family dwellings.

Reference: kentwa.gov/Home/ShowDocument?id=4092


75 percent of gross area of the principal unit or 800 square feet, whichever is less. Most have design standards requiring consistency with the primary residence. Owner must reside within either dwelling. Requires additional off-street parking stall unless within a quarter mile of a transit stop.


No larger than 75 percent of the primary residence or 800 square feet, whichever is smaller. Owner must occupy one of the two units. Recent policy change to incentivize ADUs in new construction: In a subdivision of 10 or more new residential units, development fees for ADUs are waived by 50 percent and waived in full for every third new ADU.

Reference: edocs.rentonwa.gov/Documents/1/edoc/967512/Accessory%20Dwelling%20Units.pdf


One accessory unit allowed per single-family dwelling. May be within or detached from the principal dwelling unit. The primary unit or the accessory unit must be owner occupied. An attached accessory unit cannot exceed 40 percent of the total area of the principal residence and the ADU. The size of a detached ADU may not exceed 800 square feet. The total area of all detached accessory structures on your property may not exceed 1,200 square feet plus 10 percent of the lot area that exceeds 7,200 square feet. The number of residents of the ADU and the principal dwelling unit combined must not exceed five unrelated individuals. One off-street parking space required, in addition to the two required for the primary residence. Primary entrance to the ADU must be clearly secondary to the main entrance of the principal unit.

Reference: kirklandwa.gov/Assets/Planning/Planning+PDFs/ADU+Application.pdf


ADUs are not to exceed 50 percent of the primary residence or 950 square feet, whichever is less. They must match primary residence. One additional parking space must be provided and owner occupancy is required in one of the two units.


15 percent of total lot in R4.5, R6.5, R8, WR R4-8; 20 percent of total lot in R12-28 and WR6-18 zones; or 80 percent of primary floor area, whichever is less. Legal owner must reside within either dwelling. Requires additional off-street parking stall.


Size is limited to 50 percent of the size of the main dwelling. Owner occupancy required in one of the two units. One off-street parking space in addition to two for the main house.

Reference: shorelinewa.gov/home/showdocument?id=2364


Size must be 50 percent of gross primary floor area or 800 square feet max, whichever is less. Legal owner must reside within either dwelling. One additional off-street parking space, in addition to existing SFR requirement.


City updated outdated ADU regulations this year and removed construction barriers to build with minimal impact in existing residential neighborhoods.


Adopted regulations earlier this year to allow ADUs or DADUs up to 800 square feet in order to increase housing stock.


Has a Residential 2 (R2) zone that allows for the construction of duplex, triplex, quadruplex, attached townhomes of no more than 4 units, and small ADUs and DADUs.


Size to be 50 percent of gross primary floor area or 800 square feet max, whichever is less. Must comply with city design standards. One unit occupied by the land owner. One additional parking space required, for a total of three parking spaces.


The city is in the process of reviewing, amending, and updating its ADU regulations with a targeted completion date and anticipated vote at council in January 2019.


Size to be 60 percent of primary residence or 900 square feet, whichever is less. Design elements must be consistent with main house. Legal owner must reside within either dwelling. One additional parking space must be provided.


Updated their cottage residential regulations after a moratorium on their construction. Cottages must be under 1,500 square feet, clustered in development, and their design must promote a sense of openness and community. All homes must have easy access to a central common area or open space and, where possible, be designed with a covered porch and primary area facing the common area to encourage interaction and community.

Reference: https://mbaks.com/docs/default-source/documents/advocacy/issue-briefs/cottage-housing-ordinances.pdf


Current Backyard Cottage Requirements

If you’re thinking about investing in a backyard cottage, you may want to check with a knowledgeable builder or architect about your eligibility or refer directly to  Seattle Municipal Code   23.44.016.D.8.

Here is a summary of the current regulations:

  • Lot size: Your lot must be 4,000 square feet in a single-family zone. The city is not currently accepting variances for this requirement.
  • Cottage size: Only 800 square feet is allowed for DADUs in single-family zones and 650 square feet for units in a lowrise zone. If your DADU incorporates a garage or storage area, that square footage counts toward your total.
  • Codes: As with any construction project, your backyard cottage must meet current land use and building standards. No surprise here.
  • Parking: Just like for an ADU, you must be able to create one off-street parking space for your new backyard cottage.



  • The rise of the accessory dwelling units


  • Is Seattle ready to take backyard cottages mainstream?


  • The many confusing names for ADUs


  • What ADUs are and why people build them







Additional Resources


Current Status and Environmental Impact Statement Process


From Seattle City Councilmember Mike O’Brien

The Office of Planning and Community Development issued a State Environmental Policy Act (SEPA) Determination of Non-Significance (DNS) in May of 2016 on my proposal. Following the release, the Queen Anne Community Council appealed the DNS to the hearing examiner. This past December we received the response from the Hearing Examiner that reversed the DNS. After thorough examination of the Hearing Examiner’s response, we have decided to pursue a full environmental impact statement (EIS). This process will likely take a year to complete. The full EIS will enable us to look deeply into the potential environmental impacts of the proposed code changes and inform our proposal before we bring it to the Planning, Land Use and Zoning Committee.

There will be multiple opportunities for input during the EIS process. We will keep you informed of these opportunities for public comment and encourage you to engage. When the EIS is complete, we hope to bring legislation to committee by mid-2018.

I believe lowering the barriers to creating backyard cottages and in-law apartments is an important part of addressing affordability across the city, and am looking forward to continuing to pursue this legislation.

If you have further questions, please reach out to Susie Levy – susie.levy@seattle.gov or call our office at 206-684-8800.


Accessory Dwelling Units – What’s Happened and Looking Ahead


I appreciate the ongoing interest and excitement around backyard cottages and basement units. My office continues to hear every day from folks eager to move forward with projects and be a part of addressing our current housing crisis. Please watch my video below to learn what’s happened and next steps as 2018 comes to an end.


Posted: December 10th, 2018 under Development and Sustainability, Housing, and Planning and Land Use


Update on Conversations with ADU Final EIS Appellant

Posted: November 21st, 2018 under Housing, Planning, and Land Use.


As many of my constituents probably know, I have been working for over 3 years on legislation to make it easier for property owners to create accessory dwelling units (backyard cottages and mother-in-law apartments). On October 18, 2018, the Queen Anne Community Council, represented by Marty Kaplan, appealed the adequacy of the Final Environmental Impact Statement with the City of Seattle Hearing Examiner. The timing for City Council consideration of proposed legislation is uncertain while the appeal is ongoing.

On Monday I met with Marty Kaplan to discuss our proposed Backyard Cottage legislation.  I am always interested in meeting with constituents – even constituents who I have disagreements with.  I believe it is critically important to all policy makers to have a good understanding of all perspectives on an issue before we make policy decisions, and this issue is no different.

I want to share with the public both what transpired in that friendly meeting as well as facts on how this process will move forward.

Let me start by explaining what Marty’s challenge to the EIS means and, in that context, what a settlement negotiation would entail.   This appeal process is not where decisions are made about forthcoming legislation, but where the Examiner evaluates the adequacy of our analysis about the impacts of a range of policy alternatives.  Marty and his organization appealed, and a hearing has been set for the week of March 25th, with a decision expected soon after from the Hearing Examiner’s office.  That decision will articulate if more analysis is needed for the EIS, or if the EIS is adequate such that the Council can move forward with legislation.  Assuming a positive outcome from the Examiner appeal, my plan would be to introduce legislation based on the Preferred Alternative defined in the Final EIS as a starting point for our legislative process.

Following a favorable resolution of the Examiner’s appeal process, we will begin the legislative process.  We will have multiple committee meetings and a public hearing over the course of a few months to discuss the legislation.  Once we start debating at City Council, we will begin what is in effect a public negotiation with ALL members of our communities who are interested in this legislation.  We will get public comments at meetings, we will get emails and phone calls, and likely most Councilmembers will take private meetings with different groups advocating for different approaches to the policy.  In addition to our committee process, I plan to hold topical discussions leading up to legislation being introduced, where my colleagues and I can get feedback on some of the more debated aspects of the Preferred Alternative, such as the floor area ratio (FAR) requirements, and my ideas around owner occupancy requirements.  My goal with this and any legislation is to make the process public, transparent, and accessible to everyone interested in having their voice heard, regardless of whether they can afford attorneys.

As far as negotiations for an appeal to the adequacy of our environmental review, the leverage Marty has in a negotiation is that he could drop the appeal, at which point we would be able to proceed to pass legislation on a faster timeline.  Why would Marty choose to drop his appeal?  Presumably only if he got something he wanted, and what he wants is different legislation than what is proposed in the Preferred Alternative of the EIS.

I explained to Marty that while the legislation I plan to introduce was likely to reflect the Preferred Alternative in the EIS, I am open to changes to that legislation as we work through the legislative process.  Furthermore, even if I disagree with certain changes to the legislation, a majority of the Council, not me alone, make the decisions about what changes are acceptable.  I also assured him that whatever likely passes the full Council at the end of the process will almost certainly be different than what I introduce, as I am not aware of any complex piece of land use legislation that doesn’t go through some changes in the legislative process.

If Marty was asking me to cut a special, secret deal with him so that he would drop the lawsuit, I made it clear to him that I am completely opposed to that type of back room dealing.  But I also informed him that he would have plenty of opportunities to present his concerns about my proposed legislation in public forums and that I would remain open to understanding why people have different opinions and consider different approaches we could explore to pursue my objectives of creating more housing opportunities.

In our conversation, I asked Marty what kind of changes he would like to see in the legislation, especially to address his complaint that it is a “one-size fits all” bill.  He proposed having different rules depending on lot size or street width.  I informed him that the underlying legislation already contemplates different heights allowed for backyard cottages based on lot size, but that I would be interested in hearing a different proposal if he had a different framework on lot size or street width to bring forward.  He didn’t have anything specific to share on Monday, but I encouraged him to develop a plan and bring his ideas to me, other Councilmembers, and committee discussions as they get underway in the new year.

Despite what Marty claims in his email blasts, I explained the many doors that remain open throughout the upcoming process to influence the outcome of the legislation.

I want to close with a quick note on the tenor of city politics that Marty is playing on in all of his communications.  I have known Marty for a decade, and I appreciated meeting with him on Monday. We clearly disagree on some policy issues but were still able to carry out a thoughtful and intelligent discussion.  It felt like an example of what is good about Seattle politics.  But I have since learned that Marty has publicly represented our friendly conversation as a divisive fight.  Instead of communicating where we have common ground and where we differ, explaining the opportunities to influence the process and sharing my willingness to remain open to alternative approaches during the legislative process, Marty choose instead to double down on a mean-spirited and polarizing approach, representing the worst of our current tone in politics.  As a community, we must decide if we are going to let divisiveness prevail and be the new way we govern, or re-embrace what I have known my entire life in Seattle: a collaborative approach to policy making.

The legislative process exists for a reason, and just because people seem to be leaning one way at the moment, including me, as we learn more from community members during the legislative process, we often change direction.  I encourage Marty Kaplan, and anyone else, to bring the best arguments forward in that public and transparent forum.


Backyard Cottages Are Key to Building Inclusive, Multi-generational Neighborhoods

October 8th, 2018


I am pleased to share we are one step closer to legislation to lower the barriers to building backyard cottages and basement units in Seattle. The Final Environmental Impact Statement (EIS) has been published.  The EIS evaluates the potential environmental impacts of proposed changes to the City’s Land Use Code intended to remove barriers to the creation of accessory dwelling units (ADUs) in single-family zones (also known as backyard cottages or in-law apartments).

The results of the analysis indicate that all of the action alternatives would increase the production of ADUs and would reduce the number of teardowns of single-family homes citywide compared to the No Action alternative.  Here is a brief description of the EIS and Preferred Alternative.  The Preferred Alternative analyzed in the Final EIS represents the changes I intend to include in legislation, hopefully in early 2019.

Some key elements of my proposal will include:

  • Allowing two ADUs on one lot
  • Removing the off-street parking requirement
  • Allowing Detached ADUs (DADUs) on lots of at least 3,200 square feet
  • Removing the owner-occupancy requirement
  • Requiring one year of continuous ownership to establish a second ADU
  • Allowing DADUs of up to 1,000 square feet, the same size currently allowed for AADUs
  • Increasing DADU height limits by 1-2 feet, with flexibility for green building strategies
  • Providing flexibility for one-story DADUs accessible to people with disabilities or limited mobility, with limitations on tree removal
  • Establishing a new floor area ratio (FAR) standard that limits the maximum size of new single-family homes and encourages ADUs

The Environmental Impact Statement, which commenced in October 2017, represents the capstone on a multi-year effort to explore policy changes that would spur creation of ADUs.

Based on comments we received and the analysis that was conducted, we believe that backyard cottages will allow homeowners to increase the number and variety of housing choices in single-family zones. What’s more, the addition of ADUs will afford homeowners and renters alike a mix of housing types at prices accessible to people at a variety of incomes. This includes family members who want to age ‘in place’,or increase their income through a long-term rental.

Without an appeal to the EIS, we would plan to introduce legislation and take Council action in early 2019.  If the EIS is appealed, Council cannot take action and the timeline becomes more uncertain, depending on the complexity of the appeal and scheduling availability.

I am eager to move forward with legislation and have heard from hundreds of people over the last few years who are ready and waiting for us to move forward.

The Final EIS is available here on our project website and you can also view this piece is a piece published about the EIS in the Seattle Times.

Posted: October 8th, 2018 under Housing, Planning, and Land Use