Denny-Blaine, Madrona, and Leschi are among Seattle’s most coveted neighborhoods. Laced with lush parks and beautiful houses commanding magnificent views of Lake Washington and the Cascades, they are closer to downtown than any other lakefront neighborhoods. Yet for all their desirability, in the more than four decades since 1970—as Seattle’s population has increased by more than 130,000—the total population of these neighborhoods has decreased by more than 800 people, or 13 percent.

Approximately one-third of Seattle’s land area lost population since 1970.

Like other large swaths of Seattle where community rules—namely zoning—only allow detached, single-family houses, they have fewer inhabitants now than they had a generation ago. Approximately one-third of Seattle’s land area lost population since 1970, because it is zoned for single-family houses but families have been shrinking. Since 1970, the average number of people living in each Seattle home in the city declined from 2.5 to 2.1. At the same time, Seattle’s single-family zoning (which covers over half of the city’s land) tightly restricts the number of dwellings, preventing most homeowners from making full use of their homes even as households grow smaller. Throughout Cascadia and North America, a similar trend is likely in any large city that sets aside a lot of land for single-family houses.

In booming cities such as Seattle, diminishing population in any neighborhood is a lost opportunity for equity and sustainability. It’s a sign that inflexible rules are exacerbating the housing shortage, driving up prices and displacement. By stifling gentle increases in the number of households that share each lot, these community zoning decisions create exclusive neighborhoods where lower-income people cannot find affordable options—indeed, where more affordable options are downright illegal! They also degrade walkability and transit, and push homebuilding into sprawling, car-dependent suburbs.

The good news is that Seattle leaders who care about affordability and equitable access to the promise and opportunity of all the city’s neighborhoods can turn this around: with small adjustments in rules, single-family neighborhoods can refill to their 1970 populations and more. These places in the city that are close to jobs, transit, parks, recreation, and excellent schools have room to comfortably accommodate more people.

Where population growth is running in reverse

Seattle’s population grew from 530,831 in 1970 to an estimated 662,400 in 2015, an increase of 25 percent. The map below illustrates how Seattle’s population changed by census tract using the most current available data, the American Community Survey 5-year average from 2011 to 2015. The 1970 data are from Brown University’s Longitudinal Tract Data Base that adjusts historic census data to the 2010 census tract boundaries.

Each polygon in the map is one of Seattle’s 134 census tracts, and the number on each tract indicates the population difference between 1970 and 2015. Negative numbers indicate a loss of population. Each tract is a shade of gray that fades between white, where the most population was lost, to black, where the most population was gained. Thirty-four tracts declined in population by a total of 17,136 people, and 100 tracts increased in population by a total of 140,972 people. The census tract that overlaps Denny-Blaine, Madrona, and Leschi is the white one marked “-814” that is due east of downtown on Lake Washington.

The second map reveals the relationship between population change and zoning. The areas shaded red delineate where Seattle’s zoning allows multi-family housing. The pale yellow areas identify industrial and institutional areas where private housing is typically not allowed at all. In general, the areas with the most population gain (darkest) have more multi-family zoning in their vicinity. Conversely, the areas that lost the most population (lightest) tend to be isolated from multi-family zoning, in many cases located in expensive neighborhoods near the water, such as Laurelhurst, Broadview, Magnolia, Fauntleroy, and Seward Park.

Seattle’s zoning varies at a much finer spatial scale than do the census tracts, so quantitative analysis of the relationship between zoning and population change is impossible at the tract level. Finer-grained population data are available by census block. Typically, census blocks are each one city block, and a typical tract is divided into 50 or 60 blocks. Unfortunately, analysis at the block level is complicated by changes in their boundaries over time. Also, block data are only available from the decennial census, and not from the annual American Community Survey. Sightline identified 7,502 Seattle census blocks that match between 1970 and 2010, leaving 1,295 blocks unmatched. This 85 percent match is a sufficient sample to reliably represent citywide trends.

The census blocks are not granular enough to align perfectly with zoning boundaries, so I associated each block with a zone based on the block’s centroid (the central point of the block polygon). Sixty-five percent of the matched blocks lie in areas currently zoned single-family. Of these, 63 percent lost population between 1970 and 2010. Total population in the single-family blocks dropped by 15,809 people, or about 5 percent. Looked at another way, 80 percent of the blocks that lost population are located in single-family zones. The net loss of population in single-family zones was more than offset by a net population gain of 61,892 people in blocks not located in single-family zones, an increase of 42 percent.

Where did all the people go?

Excluding public parks and rights-of-way, 54 percent of Seattle’s land is reserved for single-family houses, while multi-family homes are only allowed on 18 percent. Over the past few decades virtually all of Seattle’s population growth has been confined to that 18 percent of the city’s land—the areas shaded red in the second map above.

Single-family zoning limits population density because it means the number of homes can never increase by much. Over the past several decades single-family zones saw a trickle of houses built on vacant or subdivided lots. In more recent years, single-family homeowners constructed about 1,500 granny flats—or accessory dwelling units (ADUs). But both of these additions are small relative the total 134,000 single-family houses in the city. Indeed, as the analysis above shows, not only did population not rise in many single-family areas of the city, it actually declined.

This decline likely flowed from Seattle’s shrinking average household size, which fell from 2.5 to 2.1 over the four decades from 1970 to 2010. All else being equal, that drop would reduce population density by 16 percent, which is three times the 5 percent loss estimated for the census blocks in single-family zones. The difference may be explained by larger households seeking detached houses, which have more bedrooms, rather than apartments. Consequently, household size in single-family homes may have declined less than in multi-family dwellings.

Two other possible minor sources of population loss in single-family zones are the replacement of worn-out, grandfathered, small-scale multi-family buildings with new single-family houses, and the conversion of houses with multiple units into standard single-unit houses (many of Seattle’s older duplexes and triplexes were single-family homes to begin with). Under current city rules, Seattle’s grandfathered multi-family buildings in single-family zones, currently about 10,200 homes, can only legally be replaced by single-family houses.

What’s sacrificed when single-family neighborhoods are unable to adapt to changing household size?

Based on the census block analysis above, roughly a third of the city’s land—much of it in the city’s choicest neighborhoods—is now supporting fewer people than it was 40 years ago. That’s the 63 percent of single-family census blocks that lost population multiplied by the 54 percent of city land (not counting parks and rights-of-way) that is zoned exclusively for single-family homes.

Meanwhile, on just 18 percent of Seattle’s land—the land where the city has deemed multi-family housing legal—population grew by a net 42 percent. In other words, Seattle’s zoning leads to shrinking population over large portions of the city while it forces all the growth, whether newcomers or long-time residents who cannot afford to buy detached houses, into higher-density housing in the space that’s left. That’s a problem—for several reasons.

When there’s not enough to go around, people compete for what is available, prices go up and those at the lower end of the economic ladder lose out.

First, to tame housing prices, Seattle needs to make a lot more room for all the people who want to live in the city. Contrary to myth, Seattle does not have plenty of land zoned for new housing—and spiraling prices are the evidence. When there’s not enough to go around, people compete for what is available, prices go up, and those at the lower end of the economic ladder lose out. Yet Seattle’s single-family zones aren’t even treading water and in fact are making the shortage of capacity worse.

Overall, a neighborhood with fewer people than history has shown it can comfortably hold is a squandered opportunity to create a more equitable and sustainable city. Among the ill effects:

Fewer people on the lower side of the income spectrum get to enjoy the benefits of the “ high opportunity neighborhoods ” located in Seattle’s family-friendly single-family zones—places that often have the best public schools and parks along with good access to transit and jobs.

Public investments go underutilized, including utilities , parks, and neighborhood assets like community centers and libraries.

Private urban spaces, from bedrooms to yards, also go underutilized, sitting empty, often heated or maintained regardless. ( Sightline estimated in 2012 that 27 percent of the city’s bedrooms are empty on a typical night, and last year the Seattle Times estimated that King County has nearly 200,000 empty bedrooms .)

Land where it’s legal to build modestly sized “missing middle” homes such as townhouses remains scarce, limiting the supply of lower-priced options for first-time buyers, families with children, and downsizing baby boomers.

New housing is relegated to larger buildings located in more intensely developed neighborhoods near commercial areas and high-traffic roadways that expose residents to more noise, air pollution, and risk of injury from motor vehicles.

Serving the city with transit—which operates more efficiently as population density increases—remains more costly.

Walkability suffers because fewer parts of the city have enough people to support neighborhood businesses.

Gentle density and single-family neighborhoods

Seattle’s rigid form of single-family zoning enforces an imbalance in where the city can grow—an imbalance amplified by the demographic shift toward smaller household sizes. It’s a policy choice that makes neighborhoods more and more exclusive, elite, and even emptier! Seattle policymakers can correct that harmful imbalance by adjusting rules to allow homeowners in single-family zones to adapt more flexibly to smaller families: by allowing “gentle density.”

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Gentle density means adding more dwellings without making a neighborhood’s buildings grow (or grow much). It means welcoming more families into the existing fabric of neighborhoods. Indeed, such changes are exactly what Seattle’s Housing Affordability and Livability Agenda (HALA) recommends:

The City should allow more variety of housing scaled to fit within traditional single-family areas to increase the economic and demographic diversity of those who are able to live in these family oriented neighborhoods.

Specifically, HALA calls for holding steady the maximum size of buildings in all single-family zones, but allowing these homes to be shared or divided among more families as small duplexes, triplexes, and stacked flats. It also calls for liberalizing ADU regulations so that more homeowners can install modest in-law apartments and backyard cottages in the most expensive and sought-after parts of the city. And it proposes that a small fraction of single-family zones that are a short walk from transit and community services should be rezoned to allow multi-family homes.

Adding just one in-law apartment per census block in Seattle’s single-family zones would yield almost 5,000 additional homes citywide.

Some opponents of these changes contend that public infrastructure is insufficient to support more housing in single-family zones. But such concerns deflate in light of the evidence that nearly two-thirds of these areas once housed more people than they do today. After all, when more populated in decades past, these neighborhoods were well provided with all the essential services such as schools, parks, and utilities. (Most of the infrastructure grumbles don’t hold up to scrutiny anyway, as this article by University of Washington professor Rick Mohler illustrates.) Seattle’s single-family zones have both the room and the public services to accommodate more residents. The steps proposed by HALA lay out the path for opening the door.

The math of gentle density is impressive. Adding just one in-law apartment per census block (roughly, per city block—not block face, but entire block) in Seattle’s single-family zones would yield almost 5,000 additional homes citywide, which at the city’s average household size would house more than 10,000 people. Doubling that modest pace by adding one in-law apartment and one backyard cottage per block—or converting one big, old house into three stories of flats—would double those numbers and turn a four-decade population decline into a small gain in population. And, of course, in many Seattle neighborhoods, most homeowners on most blocks could easily accommodate—and many already want—either a backyard cottage or an in-law apartment. In Vancouver, BC, one-third of single-family homeowners now have one or the other, and many have both; in Seattle, the comparable figure is one percent.

Denny-Blaine, Madrona, and Leschi are the kinds of neighborhoods many Cascadians dream of living in: livable, beautiful, and convenient. Many of those who already live there, or in Seattle’s other single-family areas, will happily welcome more neighbors to refill their neighborhoods to their prior populations or more, if policymakers will simply legalize the flexibility recommended by HALA.

And the resulting gentle density will be a model for the rest of Cascadia and beyond.

Thanks to Glenn Pittenger, whose preliminary analysis of the same question inspired this article and who generously shared his data, and to Matt Stevenson of Core GIS, who handled the gnarly block-level spatial analysis for this article.

NOTES

The census data show a large population drop (-3,395 people) in census tract 53.02 that encompasses the University of Washington campus, and a large gain (+5,607 people) in census tract 53.01 that abuts it to the west. These two extreme changes are an artifact of how the Brown University Longitudinal Tract Data Base split the population in the 1970 tract 53 into tracts 53.01 and 53.02 that were created out of tract 53 for the 2010 census. It’s unclear why the Brown University researchers assumed such a large share of the 1970 population in tract 53 was located in tract 53.02, but the results indicate that the assumption was inaccurate.