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

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

Historic homes in St. Augustine, Florida.

Florida Considers Legalizing ADUs

Current state law allows — but doesn’t require — cities to permit accessory dwelling units in single-family residential neighborhoods.

March 18, 2025 - Newsweek

Aerial view of suburban housing near Las Vegas, Nevada.

HUD Announces Plan to Build Housing on Public Lands

The agency will identify federally owned parcels appropriate for housing development and streamline the regulatory process to lease or transfer land to housing authorities and nonprofit developers.

March 17, 2025 - The Wall Street Journal

Canadian flag in foreground with blurred Canadian Parliament building in background in Ottawa, Ontario, Canada.

Has President Trump Met His Match?

Doug Ford, the no-nonsense premier of Canada's most populous province, Ontario, is taking on Trump where it hurts — making American energy more expensive.

March 11, 2025 - Toronto Star

White bike symbol painted on green bike lane.

OKC Approves 7.2 Miles of New Bike Lanes

The city council is implementing its BikeWalkOKC plan, which recommends new bike lanes on key east-west corridors.

March 21 - Oklahoma City Free Press

Aerial view of downtown Houston, Texas skyline with low-rise housing in foreground.

Preserving Houston’s ‘Naturally Occurring Affordable Housing’

Unsubsidized, low-cost rental housing is a significant source of affordable housing for Houston households, but the supply is declining as units fall into disrepair or are redeveloped into more expensive units.

March 21 - Urban Edge

Small tree in bloom with pink flowers in front of home in Toronto, Canada.

The Most Popular Tree on Google?

Meet Rodney: the Toronto tree getting rave reviews.

March 21 - Toronto Star