ADUniverse: Evaluating the Feasibility of (Affordable) Accessory Dwelling Units in Seattle

Project leads: Rick Mohler, Associate Professor, Department of Architecture, University of Washington; and Nick Welch, Senior Planner, City of Seattle Office of Planning and Community Development

Data science lead: Joseph Hellerstein

DSSG fellows: Emily A. Finchum-Mason, Yuanhao Niu, Adrian Mikelangelo Tullock, Anagha Uppal

Project Summary: Seattle has the nation’s seventh most expensive housing market and third-largest homeless population despite being only its 18th-largest city. Exacerbating this challenge is the fact that three-quarters of the city’s land zoned for housing is reserved for low-density, detached single- family dwellings that few households can afford.

However, this offers an opportunity to increase Seattle’s affordable housing stock through homeowner-developed accessory dwelling units (ADUs)—small, separate residences within or behind single-family homes. ADUs provide access to high-opportunity neighborhoods, have relatively lower rents, allow older adults to age in place, and provide supplemental income for homeowners. Despite these benefits, fewer than two percent of Seattle’s 135,000 single-family lots currently have an ADU. Regulatory, financial, design, and permitting challenges stymie ADU production. Reducing these barriers can increase production and improve our housing landscape.

This project will advance that goal by providing a citywide feasibility analysis and a prototype web-based interactive tool that estimates parcel-level suitability for a detached ADU given property characteristics, housing submarket, and neighborhood-level socioeconomic conditions. The tool would enable individual homeowners to assess the feasibility of building an ADU, and aid nonprofits and policymakers in exploring ADUs as part of an anti-displacement strategy. In particular, the DSSG team will conduct an analysis for policymakers currently considering the development of a program to provide ADU financing for lower-income homeowners struggling with rising costs in exchange for renting the ADU to a lower-income tenant.

This project will integrate data on factors that influence the physical and financial feasibility of constructing a detached ADU, including property data from the King County Assessor, environmental data from the City of Seattle, real estate market data, and spatial analysis of site conditions.  It will involve geospatial analysis, econometric analysis, model building and interface design using a range of public data sources that must be joined and rendered interoperable.

Types of accessory dwelling units

Project Outcomes: The team successfully developed the prototype application to enable homeowners and other users to determine the parcel-level physical and financial feasibility of developing an ADU. In addition, the team created a dashboard comparing demographic data with ADU production on a neighborhood-by-neighborhood basis that can help policymakers and housing advocates more effectively to evaluate and shape City-sponsored or privately funded programs related to using ADUs an affordable housing strategy.

The property research tool takes homeowners through a step-by-step feasibility analysis, starting with binary prerequisite factors for constructing a DADU, such as zoning, lot size, lot dimensions, and lot coverage,. The tool then evaluates more nuanced secondary factors, such as the amount of tree canopy, steep slope conditions, and potential side sewer conflicts. While it cannot be definitive about all these secondary factors, the tool presents information about key property characteristics, bringing these issues to the homeowner’s attention.

The challenges associated with financing an ADU are another barrier this tool seeks to address. After evaluating physical viability, the user proceeds to an evaluation of financial feasibility that considers the type and size of ADU and provides a preliminary assessment of project costs, with individual costs broken out. The tool then prompts the homeowner to how much they might borrow based on the estimated project cost and, assuming an interest rate for a home equity line of credit, determines a monthly debt payment and estimates additional property taxes. Using data from Zillow, the tool estimates the anticipated monthly rent, allowing the homeowner to complete a preliminary cost/benefit analysis.

In addition, the application is intended to promote the development of ADUs by connecting existing ADU owners with those considering their development.  To this end, the tool allows homeowners to see the number and location of existing nearby ADUs.  The intention is to allow ADU owners to share experience and offer advice to those in the planning stages and to build a city-wide community around ADUs while being sensitive to the desire of many homeowners to maintain privacy.

Multiple opportunities for stakeholder engagement punctuated the process. The team toured an existing ADU and DADU and interviewed a homeowner who had recently completed an ADU. The team presented a preliminary version of the application to the Seattle Planning Commission, which is a partner in the project. Over the course of the project, the Seattle City Council discussed and ultimately passed the country’s most progressive ADU legislation; the team was present at the Council hearing in which the legislation was unanimously passed allowing them to hear public testimony both pro and con and individual Councilmembers’ arguments in favor of the legislation.

Subsequently, Mayor Jenny Durkan signed the legislation into law and issued an Executive Order specifically citing the DSSG project and directing Seattle IT to explore further developing and officially launching the application. At the conclusion of the program, the team met with key staff of Seattle IT to present the application and provide them with code for further development. Seattle IT is currently evaluating the cost of building on the team’s work and launching a public-facing tool and will provide this information to the Mayor for further consideration.

The project received honorable mention in the Research & Innovation category at the American Institute of Architects (AIA) Seattle chapter’s Honor Awards for Washington Architecture in November 2019.

Fellows Yuanhao Niu and Adrian Tullock gave a talk about the project at the Learning & Doing Data for Good (LDDG) conference at the University of Washington in September 2022. 

Learn more by viewing the final presentation slidesproject website, and project blog. View a video of all four final presentations here.