Access to affordable housing in Seattle has become a major concern as the city’s economy and population have increased. Lack of housing stock make it increasingly difficult for people that work in the city to live here, resulting in widespread traffic congestion and resulting pollution. Providing sustainable solutions to increase housing stock has become a central focus of the Seattle policy agenda.

The City of Seattle has developed a portfolio of solutions to address the housing shortage, starting with the 2015 Housing Affordability and Liveability Agenda (HALA) and the more recent Neighborhoods for All initiative in 2018. Both agendas offer a wide variety of policy interventions to address the diverse housing related issues plaguing the city, from lack of affordable units to outright housing insecurity. Given Seattle’s unique land area concerns, the prospect of increasing available housing is a central challenge and deciding how to accomplish this goal in a politically palatable way. HALA, for example, was critically received by citizens, stemming from its focus on upzoning areas that had historically been single family zones.

One important measure has been to encourage Seattle residents to maximize their own land usage by building accessory dwelling units on their property. By lowering regulatory and administrative barriers, such as restrictions on lot size, coverage, and owner occupancy, the City hopes to make the process simpler and less resource intensive for citizens. In July of 2019, the Seattle City Council voted unanimously to reduce restrictions on homeowners and explore expanding ADUs as a means of providing affordable housing in some Seattle’s most desirable and accessible neighborhoods.

Seattle’s most recent push for ADUs follows in the footsteps of other major Pacific Northwest cities like Vancouver, B.C. and Portland, who have not only passed cornerstone legislation on ADUs, but also expanded the availability of educational resources on the logistics of building them. The efforts in Vancouver and Portland have been extremely successful in building momentum around ADUs.

Although building accessory dwelling units has been legal in Seattle since 1995, only around 2% of Seattle homeowners on eligible lots have elected to build them. Many homeowners express the desire to create their own ADU, either in the service of housing aging parents or for the income potential through renting the unit, but have run into obstacles including a lack of easily accessible information about the physical and financial considerations of building an ADU. Seeing an opportunity, the City of Seattle began to hold community forums on the South End in 2010 and were able to provide educational tools on what ADUs are and how to go about building them. These campaigns were successful in encouraging a greater number of homeowners to adopt ADUs, but there is still significant work to be done in expanding ADU uptake across the city and the solution is in providing information and access to resources.

Our work fills this gap by providing an easy-to-use tool for Seattle citizens to determine whether an ADU is right for them. Drawing inspiration from existing proprietary tools, as well as novel, open source data, our web application will provide homeowners with a score that takes into account both the physical feasibility of the build given their lot characteristics as well as the financial feasibility given their desired usage of the unit.


We hope to answer three questions through our work:

1) Where have ADUs been created historically?

We do this by exploring characteristics of sites where AADUs and DADUs have been permitted to date (using City of Seattle permit data). This has to do with parcel-level physical characteristics (single-family zoned, size and width of the lot; size of existing house; access to street corners and alleys; year built; and placement on tree canopy, environmentally critical areas and steep topography), neighborhood-level demographic characteristics (median income, race), and median rent and home values.

2) What factors affect ADU feasibility, and how can we use this information to help a property owner understand his or her development potential and decide to proceed?

We know that if the home is outside of a single-family zone, they would be informed, through our online tool, that they would not be eligible for the building of an ADU. Similarly, if their lot was less than 3200 square feet, they could not build a DADU. We will also provide warnings if their properties are placed on environmentally critical areas, steep slopes, have significant tree cover, invasive side sewers, unwieldy lot configurations or existing structures. We would then also help them see the financial viability of construction and loans and support in the permitting process and provide other resources.

3) Where should the City focus its programmatic strategies?

The city will be able to use this historical model and tool to target certain areas most likely and potentially most able to build ADUs and encourage and incentivize them to build ADUs. They might finance ADU construction for low-income homeowners, or finance ADU renting for low-income renters, or connect homeowners interested in housing disadvantaged renters with potential qualified renters. This would be a part of the city’s larger anti-displacement strategy to enable homeowners to stay in their homes and renters to live in previously-difficult-to-access neighborhoods.

We are filling the needs of users of all levels.


(1) Homeowners interested in building an ADU: We help these homeowners to understand if an ADU would be the right and the feasible decision for them. We understand that homeowners might be interested in building ADUs for different purposes. Some are building ADUs for the rental income while others might be more interested in housing family members. Based on these choices by the users, we to present different information relevant for them. For example, we help to project rental income if the owner chooses the former option.

(2) City planning committee: We provide information on the broader neighborhood level. City planners can use this app to identify neighborhoods they should market to or could provide financial and educational support to in order to increase the number of ADUs in the city.

(3) AARP (American Association of Retired Persons): AARP loves the flexibility of ADUs. It expands housing options for elders, for instance enabling more people to stay at home through retirement.


The tool we seek to develop is mostly used for informational purposes. One ethical question we considered was the issue of data privacy. We have tax payer information associated with addresses. Also, with the app in place, we expect to be able to display information regarding parcels of land based on any given address within the city. The question was raised of how much information this may divulge on an owner’s private property.

First, there is no plan or incentive to have the tax payer’s name enter the application space. Also, the information that will display is publically available through various other means.

Also, we have access to city-wide building permit data that reveals construction/destruction intentions, timelines for application and project completion, project value, and the types of structures built. While we may potentially use some of this data to generate our models, none of the specific information will be made accessible or visible through the application or in our other deliverables.

Lastly, given that we are entering a project around an active political issue, we have taken the time to understand various stakeholders relevant to the topic. This includes the motivations and experiences of our project leads, the pro- and counter-narratives on both sides of the issue, and other homeowners actively engaged in the process of building ADUs. This has afforded the team an awareness of the pros and cons of this solution, as well as the means with which to form our own opinions on the matter.