Study: Portland's Accessory Dwelling Units Reduce Car Impacts

Since Portland began subsidizing accessory dwelling units (ADU) by waiving development fees, construction of ADUs have increased to a pace of more than 100 a year. A new study finds additional benefits in the low impact of ADUs on neighborhoods.

1 minute read

July 17, 2014, 2:00 PM PDT

By James Brasuell @CasualBrasuell


Accessory Dwelling Unit

radcliffe dacanay / Flickr

Michael Anderson reports the findings of a new study by the State of Oregon Department of Environmental Quality: "When it comes to cars, accessory units in backyards and basements are nearly as low-impact as big apartment buildings next to bus lines."

The study, titled Accessory dwelling units in Portland, Oregon: evaluation and interpretation of a survey of ADU owners [PDF], found that "[the[ average rental unit in Portland brings 1.31 cars on site, according to the U.S. Census. For transit-oriented apartment buildings, that falls to 0.83 cars — and for accessory dwelling units, it’s 0.93 cars."

"In other words, transit-oriented apartments have been bringing 37 percent fewer cars into the city than the typical rental unit, and accessory dwellings (usually defined in Portland as being less than 800 square feet with a private entrance, bathroom and kitchen, among other requirements) bring in 29 percent fewer cars."

Thursday, July 10, 2014 in Bike Portland

"Planetizen federal action tracker" logo superimposed over black and white image of US Capitol dome.

Planetizen Federal Action Tracker

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

October 28, 2026 - Zachary Whalen

A car drives through a puddle, making a large splash. Rain pours down.

California plans to bury El Niño’s downpour

If the state plays its cards right, it may be able to store a billion acre-feet of El Niño’s water in aquifers.

October 28, 2026 - Grist

Lake Powell with much of the water evaporated.

Desperate measures: Advocates weigh draining Lake Powell to fill Colorado River

Draining the reservoir could reduce evaporation losses by a couple hundred thousand acre-feet.

October 30, 2026 - Yale Environment 360

Nilu Jenks wears a long gray coat and speaks with one man and two women outside.

Housing is the top priority of Seattle’s sole new councilmember

Jenks campaigned on building 1,000 new units of affordable housing.

November 8 - Reason

People sit on wooden benches on a bus in Miami

New tax cuts may slash $1.5 billion in funding for Miami transit, public parks

Amendment 3 will save Miami-Dade homeowners $2,700 in 2028.

November 8 - Miami New Times

Two men install solar panels on the roof of a house.

Arizona, Wisconsin, Pennsylvania hope to finally advance climate goals post-midterms

The three states have spent years under Republican or divided control.

November 8 - Grist

FREE Course: Walkable City 1: Why Walkability?

After describing his path towards focusing on walkability as the essence of good planning, Jeff Speck marches through his five principal reasons for making more walkable places.

Reinventing Malls: Planning Alchemy—Turning Gray Fields Into Gold

The course focuses on the opportunities and imperatives that shape reinvention of mall sites.