Environmental Impact Study Builds the Case for Accessory Dwelling Units in Seattle

Proponents of a new Accessory Dwelling Unit rule in Seattle say the tactics of opponents backfired with this Environmental Impact Study.

2 minute read

May 24, 2018, 12:00 PM PDT

By James Brasuell @CasualBrasuell


Accessory Dwelling Unit

radcliffe dacanay / Flickr

Dan Bertolet reports from Seattle, where the city recently released a highly anticipated Environmental Impact Study (EIS) on a proposed rule change that would allow for more construction of Accessory Dwelling Units in Seattle's residential neighborhoods.

According to Bertolet, the study was provoked by an appeal by anti-development activists led by Marty Kaplan. Bertolet provides a scathing critique of Kaplan's obstructionist positions in an earlier post from April 2017.

Now the environmental study responds to the appeal, and Bertolet says it reveals the appeal as "bunk": "baseless claims eviscerated by analysis and evidence." As an example, see here Bertolet's analysis of concerns about displacement:

The appeal’s most grievous complaint was that making it easier to build accessory dwelling units (ADUs) would lead to displacement in lower-income communities of color. In other words, poor people would lose homes to rich speculators. 

The appeal’s most grievous complaint was that making it easier to build accessory dwelling units (ADUs) would lead to displacement in lower-income communities of color. In other words, poor people would lose homes to rich speculators. As the appeal’s de-facto leader, Marty Kaplan, warned: “There would be a feeding frenzy for anybody with a truck and a nail bag to go buy homes and convert them into three rental units and displace the population.”

Math begs to differ. The EIS finds that relaxing ADU rules would lead to fewer teardowns of existing single-family houses—which would decrease the likelihood of renter displacement—and that teardowns are less likely in lower-priced neighborhoods to begin with. It also demonstrates that in Seattle the value of selling a house, with or without ADUs, eclipses the value derived from renting. So much for any rental conversion “feeding frenzy.”

Bertolet also analyzes the study's findings regarding parking, before pointing a finger at the enabler for the study's delay: the Washington State Environmental Policy Act (SEPA). A lot more analysis of the study follows. If you're looking for more, there's also another article along the same lines written earlier in the week by Erica Barnett.

Thursday, May 24, 2018 in Sightline Institute

portrait of professional woman

I love the variety of courses, many practical, and all richly illustrated. They have inspired many ideas that I've applied in practice, and in my own teaching. Mary G., Urban Planner

I love the variety of courses, many practical, and all richly illustrated. They have inspired many ideas that I've applied in practice, and in my own teaching.

Mary G., Urban Planner

Get top-rated, practical training

Logo for Planetizen Federal Action Tracker with black and white image of U.S. Capitol with water ripple overlay.

Planetizen Federal Action Tracker

A weekly monitor of how Trump’s orders and actions are impacting planners and planning in America.

April 30, 2025 - Diana Ionescu

Close-up on Canadian flag with Canada Parliament building blurred in background.

Canada vs. Kamala: Whose Liberal Housing Platform Comes Out on Top?

As Canada votes for a new Prime Minister, what can America learn from the leading liberal candidate of its neighbor to the north?

April 28, 2025 - Benjamin Schneider

Hot air balloons rise over Downtown Boise with the State Capitol building visible amidst the high rises.

The Five Most-Changed American Cities

A ranking of population change, home values, and jobs highlights the nation’s most dynamic and most stagnant regions.

April 23, 2025 - GoodMigrations

People biking along beach path with moored ship in San Diego, California.

San Diego Adopts First Mobility Master Plan

The plan provides a comprehensive framework for making San Diego’s transportation network more multimodal, accessible, and sustainable.

7 hours ago - SD News

Sleeping in Public

Housing, Supportive Service Providers Brace for Federal Cuts

Organizations that provide housing assistance are tightening their purse strings and making plans for maintaining operations if federal funding dries up.

May 2 - KSL

Conductor walks down platform next to Amtrak train at station in San Jose, California.

Op-Ed: Why an Effective Passenger Rail Network Needs Government Involvement

An outdated rail network that privileges freight won’t be fixed by privatizing Amtrak.

May 2 - Streetsblog USA

Urban Design for Planners 1: Software Tools

This six-course series explores essential urban design concepts using open source software and equips planners with the tools they need to participate fully in the urban design process.

Planning for Universal Design

Learn the tools for implementing Universal Design in planning regulations.

Senior Manager Operations, Urban Planning

New York City School Construction Authority

Building Inspector

Village of Glen Ellyn

Manager of Model Development

Central Transportation Planning Staff/Boston Region MPO