This article is part three in a mini-series about the climate benefits of ADUs. If you are eager for more, check out part 1 and part 2.

My son has chubby feet. There’s (almost) no getting around them, literally: most shoes simply do not fit. Luckily, a certain chain kids clothing store actually carries a line of shoes that fit his chunky trotters. And even more luckily, the store had a location in a shopping center near my home. It was less than a mile from my front door. Walkable on a sunny day when I had the time. Otherwise an easy drop in when I was in the shopping center for some other errand.

So I was a bit sad when, a few weeks ago, I noticed the store had closed. Now the closest place to find his shoes is 12 miles away. There’s no walking 12 miles with two toddlers in tow.

I can’t say why the store closed. But I can say that most of the area within easy walking distance of my shopping center is covered by single-family homes, automatically limiting the potential customer base for all the shops in that area. That might make it hard for any shop to stay open. The result is that, next time my son needs shoes (which, at the rate his feet grow, will be next week), I have a long drive ahead of me.

Neighborhoods comprised predominantly of single-family, detached homes all but force residents into heavy car reliance. During the early 20th century, city planners weren’t thinking about the climate when they devised the homebuilding rules for Cascadia’s cities. They cemented sprawling urban form into the region’s fabric, designing cities with vast swaths of single-family zoning that often went hand in hand with personal car ownership.

Single-family zoning covers many Cascadian city landscapes. Just look at zoning maps from Bellevue, Bellingham, Eugene, Portland, Salem, Seattle, and Spokane. This zoning type covers about half of Seattle. No surprise that cars are the largest source of greenhouse gas emissions in Seattle, and one of the top contributors across the United States.

Adding accessory dwelling units (ADUs) to existing single-family neighborhoods could be a step toward re-thinking this outdated zoning model and reducing household car reliance in the region’s single-family neighborhoods. These small homes—which include backyard cottages, basement apartments, and mother-in-law suites—integrate into existing single-family neighborhoods, causing minimal change to the established urban form. But the additional households they make room for help these communities wean off their gas-guzzling vehicles, supporting better transit and local businesses.

This year, Seattleites have an opportunity to update their city’s climate-killing layout by relaxing local  ADU rules. Here are three ways loosening restrictions on ADU development could help reduce household driving in the Emerald City, moving the city towards its climate commitments and creating a blueprint for other Cascadian cities to follow.

#1. Support/Improve public transit

Public transit is expensive to build and operate. Though fares alone usually don’t cover the whole cost, they help offset expenses. Fares come from ridership, and one key driver of ridership on public transit is residential density, typically measured in terms of housing and employment units per gross acre within a quarter to a half mile of transit stops.

On the whole, residential areas in Seattle’s single-family zones have an average density of slightly less than five housing units per gross acre, according to city acreage and housing unit estimates. Of course, there is variation across the city’s neighborhoods: for example, some, mostly older, pre-WWII, single-family neighborhoods have smaller lot sizes and therefore housing densities closer to seven housing units per gross acre. Others, because of topography and other factors, have fewer homes per acre and pull the city-wide average down.

(Some readers may be surprised by the low average gross housing unit density of residential areas of Seattle’s single-family zones—4.68 housing units per gross acre. This calculation includes housing parcel acres and rights-of-way in the city’s single family zones, and excludes all other non-residential land uses in single-family zones, including parks, cemeteries, and city-owned land. Note that Seattle’s single-family zones have an average net housing density of between six and seven units per acre. Net density calculations not only exclude all non-residential land, including parks, schools, and other publicly-owned lands, but also exclude rights-of-way from the total acreage. However, transit-supportive density benchmarks are typically concerned with the total area within walking distance of transit lines, and therefore rely on gross, rather than net, density. See the methods note at the end of this article for more information regarding gross versus net housing density calculations for Seattle.)

By way of perspective, planners with the Puget Sound Regional Council (PSRC) suggest that local bus lines, those which connect residential zones with denser urban areas and transit hubs, should have a minimum residential density of seven or eight dwelling units per gross acre in their service corridors. The service corridor for a local bus line is the land within a quarter mile of the route—the typical distance a person is willing to walk to catch a local bus. King County Metro uses a slightly different rule of thumb for estimating transit supportive densities in residential areas: 15 housing units per acre in a bus route’s quarter-mile corridor can support service every 15 minutes, 7 housing units per acre supports service every 30 minutes, and 4 housing units per acre supports hourly service. (These benchmarks are for residential areas only. Areas with places of employment have different benchmarks.)

Despite benefitting from access to public transit, many of Seattle’s single-family neighborhoods struggle to meet these benchmarks.

Of course, single-family neighborhoods are only a piece of a much broader transit web. Transit lines connect these neighborhoods to other areas of the city with more abundant housing and employment opportunities, and this interconnectivity helps keep the transit system humming. For example, Seattle’s multi-family zones average close to 17 homes per gross acre, and many feature a multitude of employment opportunities.

ADUs could modestly help support existing, and potentially new, bus service throughout the city. If Seattle reached Vancouver, BC’s level of ADU coverage, in which 35 percent of the single-family homes have at least one ADU, it would add approximately 42,000 more homes to its single-family zones. This would boost average density in residential areas of single-family zones  to nearly nine housing units per gross acre. The additional residents would help make local bus service in single-family zones slightly more cost-effective, and perhaps even slightly more frequent, giving more households the option to use public transit for daily commutes or other errands, instead of turning the keys in the ignition of a personal car.    

#2. Build car share membership

Free floating car sharing services like Car2Go, Modo, and Zipcar also help reduce personal car travel. Convenient access to shared cars can offset the perceived downside of relying completely on transit, tipping the scales for many households toward going car free. And when a household goes car free, even if it has access to shared car, it substantially reduces car travel, instead relying more on public and other alternative transit options and shaving off some travel miles all together.

In Seattle, 7 percent of car share service members have shed a private vehicle because of shared vehicle access. The impact may be even larger: a study of car share users across North America found that 17 percent of members sold at least one personal vehicle after joining a sharing service. On average, car share members reduce their vehicle travel by more than 1,700 miles per year after joining a sharing service, with members who shed personal vehicles making the most dramatic decreases.  

Numerous studies show that car shares are more viable in denser areas. That makes sense—more households mean more potential customers and higher demand, resulting in a higher utilization rate of the cars. For example, the densest of Cascadia’s three largest cities, Vancouver (which not coincidentally also has the most ADUs), is also home to more than half of the shared cars in the region.

Seattle’s level of car sharing service lags behind that of other US cities of similar population. Seattle has fewer than 15 carshare vehicles per 10,000 residents. Meanwhile, Washington, DC, which has a population slightly smaller than Seattle’s, has about 30 carshare vehicles per 10,000 residents. (Other Cascadian cities vary: Vancouver, BC, has about 40 vehicles per 10,000 residents, and Portland just shy of 13.)

ADUs can help boost car-sharing service to neighborhoods with extensive, less-accessible, single-family zones by adding new households—that is, potential customers—to these areas.  And the convenience of the additional service may encourage new and established residents alike to take advantage of car sharing services, and shave off driving miles from the city’s total.

At Seattle’s current rate of car sharing service membership, welcoming some 42,000 ADUs to Seattle’s single-family neighborhoods would add between 8,500 and 10,000 car share members to the city’s lists. Between 600 and 700 of these individuals would likely choose to get rid of their personal vehicles. And because of the expanded service for these new members, some additional current Seattle residents would likely choose to join a car share as well, further boosting membership numbers, and shaving Seattle’s vehicle emissions.

#3. Support neighborhood businesses

Just over one-quarter of the average US household’s annual road travel miles are for work commutes or business-related travel. The other three quarters are split among daily errands and other non-work related travel—trips to the grocery store, library, and pool, to a café to meet a friend, a park for a playdate, or garden center for some spring time color. The more of these trips Cascadians can make without the use of a car, the better. ADUs could help build cities in which more errands are within walking and biking distance of more Cascadians’ doors.

A sustainable customer base for a given business differs depending on the business type, clientele, and other variables. One Puget Sound region-specific study reported that it takes approximately 2,000 households to support a small neighborhood business cluster containing a convenience grocery or drug store, and perhaps one or two other small shops. A slightly larger commercial area—covering about 30,000 square feet of commercial space and including a mid-sized grocery store and a few other destinations—requires closer to 3,300 households. At least half of those households should be located within a quarter mile radius—a comfortable  walking distance—of the stores. This suggests that small business clusters thrive best in neighborhoods with at least eight households per gross acre, while a grocery store needs a surrounding neighborhood with about 13 households per gross acre, though there are of course exceptions to this rule of thumb.

As I stated, residential areas of Seattle’s single-family zones average just over four and a half dwelling units per gross acre, making it difficult for small businesses to thrive in areas surrounded exclusively by single-family zoning. By nudging the inventory needle up, ADUs can help support new and existing local businesses in walking distance of more residents’ doors.

ADUs give single-family homeowners a chance to support more climate-friendly travel

Seattle will need to employ a suite of tools to cut car travel and meet its carbon-reduction pledges. ADUs can be one of these tools. These unassuming homes are not only energy-efficient, but they also support more sustainable neighborhoods, bolstering transit and improving walkability.

Bellingham and Portland both recently reaffirmed their commitments to climate-conscious development by loosening their restrictions on ADU construction. Seattleites will have the opportunity to follow suit later this year when their city council votes on a package of updates to local ADU regulations. It’s time to update these codes, and the city’s land use patterns, by taking advantage of the housing accessibility accessory dwelling units can bring to the city’s outdated zoning scheme.

Thanks to a number of Seattle city planners who provided feedback on some of the information presented in this article, including Michael Hubner and Jason Kelly.

Notes on methods:

Seattle residential density per gross acre

Seattle’s land use zoning data shows that the city has to 45,775 gross acres of residential land (for these calculations, residential lands includes all zones except master planned communities, industrial zones, and major institutions). I used 2016 American Community Survey five year estimates to estimate the number of Seattle housing units at 322,795, giving a city-wide housing density of 7.1 homes per gross acre of residential land.

At any one time a subset of Seattle’s housing units are vacant. The ACS estimates this number at 18,638. Calculating residential density for only occupied housing units yields a density of 6.6 occupied housing units, or households, per gross acre. For the purposes of this article, I’ve chosen to base housing unit density estimates on the most conservative numbers possible, and so have chosen to calculate densities based on total housing units, rather than only occupied units.

Seattle residential density in single-family zones per gross acre

City land use data show that single-family zones cover 34,414 gross acres. These zones contain about 133,135 homes based on the city’s estimate of 124,397 single-family homes in single-family zones, plus the total number of multi-family homes and ADUs in the city’s single-family zones from Sightline’s previous mapping research (the city has also published additional data on multi-family units in single-family zones). Taken together, I find that Seattle’s single-family zones have an overall gross density of 3.87 dwelling units per gross acre.

A somewhat more conservative estimate of gross housing density in Seattle’s single-family zones would consider only areas of single-family zones used for residential purposes. This would exclude parks, cemeteries, utilities, major institutions, and other city-owned, non-residential land that sits within the city’s single-family zones.  

City land use data show that the city’s single-family zones contain 21,224 residential parcel acres. Similarly, in direct personal communication with me, City of Seattle planners estimated the number of residential parcel acres in the city’s single-family zones at 19,750 acres. Single-family zones contain an additional 8,643 acres in rights-of-way. Taken together, and using the more conservative estimate from Seattle city planners, Seattle’s single-family zones contain 28,393 gross residential acres.

Using the above estimate of 133,135 homes in the city’s single-family zones indicates the residential areas of Seattle’s single-family zones have a gross housing unit density of 4.68 homes per gross acre.

  • [Some rights-of-way in single-family zones run through the non-residential areas of these zones. I do not have access to data showing this breakdown. However, as the vast majority of single-family zone rights-of-way are in residential areas, the much smaller portion of roads in non-residential areas should not impact the density estimate significantly.] 

    Seattle residential density in single-family zones per net acre

    Most transportation planners rely on density measures by gross acreage, but sometimes refer to density per net acre for specific purposes. Though none of the benchmarks I cite in this article rely on density per net acre, it’s important to note the difference, especially when comparing data in this article to that in other related articles. Measures of net acreage exclude parks, rights-of-way, and all other publicly owned lands from the total acreage. There are approximately 19,750 net residential acres in Seattle’s single-family zones (see previous section). Using my above estimate that these zones contain 133,135 housing units yields a household density of 6.74 homes per net acre.

    Seattle residential density in multi-family zones per gross acre

    To calculate household density in Seattle’s multi-family zones I subtracted the number of homes in single-family zones (133,135) from the Census’ estimate of total households across Seattle (322,795) to yield 189,660 housing units in multi-family zones. According to Seattle zoning data, the city’s multi-family zones cover 11,404 gross acres, yielding an average density of 16.63 housing units per gross acre.

    How many ADUs would Seattle add if it reached Vancouver, BC’s ADU coverage rate?

    To calculate the number of ADUs Seattle would get if 35 percent of single-family home owners built one, I started with the city’s estimate of the number of single-family homes in single-family zones: 124,397. If thirty-five percent of these added an ADU that would mean the city would have 43,539 ADUs. The city already has approximately 1,456 permitted ADUs, resulting in 42,083 new ADU units, boosting density to 5.1 housing units per gross acres in single-family zones. Using the more conservative estimate of only residential gross acres in single-family zones, these additional ADUs would bring household density in those areas to 8.87 units per gross acre.

    Current Seattle Municipal Code permits ADUs on any parcel with a single-family home (SMC 23.44.041), and does not specify that they must be located in single-family zones. According to Census 2016 ACS estimates, Seattle has a total of 139,473 total single-detached homes. Achieving Vancouver, BC’s 35 percent rate of ADU coverage would mean 48,816 of these would have an ADU, including the 1,456 existing ADUs, plus 47,360 new units. For the purposes of this article, which focuses on climate impacts of ADUs in Seattle’s single-family zones, I constrained my calculations to the addition of ADUs to single-family zones only.

    Seattle also likely has many unpermitted accessory dwelling units. If the city implemented an ADU clemency program, similar to Vancouver, BC’s, it would need fewer new ADUs to meet Vancouver’s level of 35 percent ADU coverage.

    Impact on Seattle car share membership from additional ADUs

    To calculate the number of car share service customers 42,083 ADUs would add to Seattle, I used the Seattle Department of Transportation’s current estimate of car share membership in the city (65,000 members), or 9.7 percent of total Seattle residents. Assuming that each ADU had an average Seattle household size of 2.1 residents and that ADU dwellers would join car shares at at least the same rate as the rest of the city. This resulted in 8,588 new car share members.

    Using the city’s estimates that 7 percent of Seattle car share members get rid of personal cars at least in part because of their access to the car share, this suggests that 601 of these ADU resident car share members would shed their personal cars.  

    Seattle’s average household size in one-unit structures is 2.5 people, slightly higher than the citywide average. Using this population per ADU suggests 42,083 ADUs would house 105,067 people, 10,191 of whom would likely join a car share, and 713 of whom would likely shed a personal vehicle.

    A survey of Portland ADU dwellers found that they are slightly less likely to own cars than renters of other home types (see Appendix B, page 13). It’s difficult to predict how this lower rate of car ownership would impact an ADU-dwelling household’s decisions if they join a car sharing service, so I haven’t factored this in to the calculations above.

    Average density to support local businesses

    A quarter mile radius around a point, or shop, covers 125 gross acres. To meet minimum customer base for a convenience store would require about 1,000 households within that area, or eight households per gross acre. Density requirements for more average size grocery stores require approximately 2,000 households in the customer base, with approximately 1,650 within the quarter-mile radius of the store, or 13.2 households per gross acre in the surrounding quarter-mile area.

    My above comparison of Seattle’s four housing units per gross acre in single-family zones uses housing units as a proxy for households, or occupied housing units. Though some portion of homes in Seattle’s single-family zones are vacant, that portion is not big enough to substantially influence the estimate of four households per gross acre in these zones.