2021 ADU Annual Report

In Section 11 of the  legislation , the Council directed the Seattle Department of Construction and Inspections (SDCI) and Office of Planning and Community Development (OPCD) to report annually for five years on citywide ADU permit activity.

Additionally, when she signed the legislation, Mayor Jenny A Durkan also issued  Executive Order 2019-04 , calling for several actions to encourage more affordable ADUs throughout Seattle. In this order, Mayor Durkan directed SDCI and OPCD to monitor and report on ADU development to assess the benefits and impacts of this housing type.

In September 2021, we published an  annual report  describing our progress on efforts to increase access to ADUs and summarizing ADU production since Seattle’s revised ADU regulations took effect in August 2019. This story map supplements the report with interactive versions of our maps and charts.

ADUs are small, secondary dwellings allowed in residential zones. Seattle allows two types of ADUs: attached ADUs (AADUs) are connected to or within a principal residence, while detached ADUs (DADUs), often called backyard cottages, are located in a separate structure from the main house.

Their relatively small size compared to houses and no added land cost mean ADUs often provide lower-cost housing and expand rental options in neighborhoods where housing is scarce and prices are out of reach to most households. Given Seattle’s  legacy  of segregation and exclusion through redlining, racial covenants, and restrictive zoning, increasing housing choices in residential areas is one important part of expanding access for low-income BIPOC households to all Seattle neighborhoods. However, high construction costs, limited financing options, and racial disparities in wealth, income, and homeownership mean that additional efforts are necessary to help ensure equitable access to the financial and technical resources needed to build or rent an ADU. 

Their owners may benefit from the rental income that an ADU can provide, which in some cases could help a homeowner afford to stay in place. Given the high cost of construction, however, the opportunity to build or rent an ADU is not affordable to all Seattle residents. This section provides some preliminary insights about the types of areas where ADUs have been created and identifies areas for further survey research. 

In 2018, the City completed a  Racial Equity Toolkit (RET)  on policies for ADUs and found removing regulatory barriers could increase housing choices in single-family zones and have a positive impact on affordability and displacement. But it also found that regulatory reform on its own is insufficient to address racial disparities in housing and that, absent other policy interventions, wealthy, primarily white homeowners are most likely to have the capital necessary to create an ADU. The RET highlights the importance of actions beyond regulatory changes to ensure that Seattle’s ADU policy more equitably benefits lower-income households and households of color.

36 permits have been issued or initiated for the 10 pre-approved DADU designs as of August 2021. Permitting for these DADUs averaged 45 calendar days from intake to issuance, with an average of 31 days in SDCI review and 14 days out for corrections. 

Council Bill 119544 and Executive Order 2019-04 also direct us to assess and monitor various characteristics and outcomes of ADUs permitted in Seattle. These include DADU design characteristics like the height, gross floor area, and size of the lot; ADU construction cost and rent; the number of off-street parking spaces and vehicle usage associated with new ADUs; and potential effects of ADU development on green space and tree canopy. 

Much of this information is not available through City permitting data alone. For that reason, OPCD and SDCI plan to conduct a survey of ADU owners and occupants that will provide additional insights into how ADUs are used and their effects in our neighborhoods.

In single-family zones, where most ADUs are permitted, tree requirements apply when an applicant proposes development. Tree planting is required if the lot falls short of certain baseline tree requirements, and exceptional trees must be retained unless doing so would prevent the development from achieving the maximum allowed lot coverage. 

Our  annual report  provides an initial assessment of outcomes since Seattle’s landmark ADU reform in 2019. As requested in  Council Bill 119544  and identified in Mayor Durkan’s  Executive Order 2019-04 , SDCI and OPCD, along with partners and stakeholders, will continue to monitor and evaluate ADU production and its effects to encourage more of this housing type.

For more information, including the latest data on ADU permitting, visit  ADUniverse  at  maps.seattle.gov/ADU .