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.
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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."

Full Story: Backyard homes are almost as car-lite as apartments on transit lines, study finds


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