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

Red 1972 Ford Pinto with black racing stripes on display with man sitting in driver's seat.

Analysis: Cybertruck Fatality Rate Far Exceeds That of Ford Pinto

The Tesla Cybertruck was recalled seven times last year.

July 2, 2025 - Mother Jones

Close-up of park ranger in green jacket and khaki hat looking out at Bryce Canyon National Park red rock formations.

National Parks Layoffs Will Cause Communities to Lose Billions

Thousands of essential park workers were laid off this week, just before the busy spring break season.

February 18, 2025 - National Parks Traveler

Paved walking path next to canal in The Woodlands, Texas with office buildings in background.

Retro-silient?: America’s First “Eco-burb,” The Woodlands Turns 50

A master-planned community north of Houston offers lessons on green infrastructure and resilient design, but falls short of its founder’s lofty affordability and walkability goals.

February 19, 2025 - Greg Flisram

Screenshot of shade map of Buffalo, New York with legend.

Test News Post 1

This is a summary

0 seconds ago - 2TheAdvocate.com

Red 1972 Ford Pinto with black racing stripes on display with man sitting in driver's seat.

Analysis: Cybertruck Fatality Rate Far Exceeds That of Ford Pinto

The Tesla Cybertruck was recalled seven times last year.

18 minutes ago - Mother Jones

test alt text

Test News Headline 46

Test for the image on the front page.

March 5 - Cleantech blog