The city’s released its final environmental impact statement (EIS) for accessory dwelling units (ADUs)/backyard cottages last week. Other sites in the urbanist blogosphere analyzed the entire document.
This post focuses on the EIS’s study of parking impacts in particular, since worries about street parking availability are a common anti-density talking point.
So: would the ADU proposal make parking on a side street difficult in Maple Leaf or Magnolia?
In fact, not all that much would change (which is a point worth discussing on its own.) In its analysis of the preferred alternative, the plan that would create the most capacity for ADUs, the City projects that only 300 ADUs would be built. That’s about 10 percent of the 3,007 parcels that would be eligible for ADU construction—which itself is a very small slice of the 138,531 parcels in single family-zoned areas across the city.
Analysis of one of the two proposals that would eliminate the requirement that any ADU have a complementary off-street parking spot says that the City “do[es] not expect increased parking demand resulting from ADU production to exceed existing on-street parking availability under typical conditions.” The preferred alternative would create parking impacts that “would be very similar to, but slightly greater than, those described under Alternative 2 due to slightly higher ADU production.”
The EIS concludes that implementing the ADU plan will make only marginal changes to the single family-zoned parking supply, which is already robust. The EIS’s study of street parking supply, which used data collected from 2016-18, concludes that 56 percent of parking in single family zones is in use on a weekday.
That figure may actually overstate demands on the parking supply. Two of the four study areas are located in areas with high parking demand. The southeast study location is between the northeast edge of Columbia City’s business district and Genesee Park. The southwest study area surrounds the West Seattle Junction.
In short, existing evidence suggests that ADUs will not noticeably change the amount of parking available in Seattle’s single family neighborhoods.