Most of Seattle's extraordinary population growth has been concentrated in a handful of neighborhoods, including South Lake Union, Pike/Pine and Lower Queen Anne.

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Seattle’s recent population boom may be even more astonishing than you thought.

Last week, I reported on new census data showing that Seattle is the fastest-growing big city of the decade. Since 2010, we’ve added 114,000 residents for a population increase of about 19 percent.

Those are some eye-popping numbers. But they’re even more remarkable when you consider that much of the city has barely changed at all. Most of Seattle’s extraordinary growth has been concentrated in a handful of neighborhoods.

I looked at changes in population from 2010 to 2017 for each of Seattle’s 132 census tracts, using data from the Washington Office of Financial Management, and the numbers show how uneven the distribution of growth in Seattle has been: Just 13 percent of the city’s land area has absorbed more than half of the total population increase.

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South Lake Union represents the extreme. If you’ve lived in Seattle for a while you know that this bustling area, home to Amazon’s massive headquarters and a ton of development, was a relatively desolate stretch of downtown not so long ago. Remarkably, the two census tracts that cover this one neighborhood have taken in nearly 10 percent of Seattle’s total population growth since 2010 — around 11,000 new people. That has more than doubled the number of residents here.

The Pike/Pine section of Capitol Hill, which is lined with upscale apartment buildings, has also experienced tremendous growth. The neighborhood has transformed into Seattle’s dining and nightlife center since the start of the decade, and many of the city’s young tech workers call it home. The population here has increased by more than 70 percent since 2010. That’s about the same rate of growth as Lower Queen Anne, another neighborhood that has seen a lot of development this decade.

Other pockets of population increases exceeding 30 percent can be found across a number of neighborhoods that include areas zoned for apartment buildings and other types of multifamily housing: Downtown, Ballard, Fremont/Wallingford, Westlake, the University District, Roosevelt, Green Lake, West Seattle Junction, the Central District  and Brighton in Rainier Valley.

(There’s one section of downtown, in the south end of First Hill, where population increased by only 4 percent. That area includes Yesler Terrace, the public-housing project that is being redeveloped. More than one-third of the families living there were displaced by construction, which explains the slow growth rate.)

Nowhere in Seattle lost population this decade.  But in terms of density, many areas have come out of this period of unprecedented population growth looking pretty much the same as before it started.

Nearly half of the city’s census tracts had single-digit growth rates since 2010.  In some of these residential parts of the city — central Beacon Hill, Fauntleroy in West Seattle, Laurelhurst and other well-heeled parts of North Seattle — the population increased by only about 5 percent.

Why so little growth?

It’s certainly not because nobody wants to live there. But these neighborhoods have been intentionally zoned to prevent much growth.

In 1994, Seattle implemented its “urban village” strategy, which has concentrated density in existing urban areas of the city. Those villages may soon be upzoned, allowing even more density. Meanwhile, the plan let most of the city’s single-family-home neighborhoods off the hook for taking on growth. That’s a large chunk of Seattle, close to half of all developable land mass in the city.

When you can’t build anything but single-family homes, population can only grow so much. Some new homes have been built on vacant lots, but those are increasingly rare in Seattle. A number of older houses have been torn down and replaced by two homes, if the lot is large enough to subdivide. There are also demographic shifts that can increase population a bit — for example, an older, empty-nest homeowner might sell to a young family with children.

The city has also allowed homeowners to build attached dwellings (“mother-in-law” apartments) since 1994, as approved by state law. And since 2010, homeowners citywide have been permitted to construct backyard cottages on their properties. These small homes can help absorb some of the city’s growth, and also provide a more affordable rental-housing option in neighborhoods that are otherwise prohibitively expensive for most people.

About 75,000 single-family-home lots are eligible for these cottages, but only a smattering of homeowners have built them, most likely because of regulatory barriers. The city has tried to lower those barriers, but neighborhood resistance has stalled the process. Last month, Seattle Mayor Jenny Durkan ordered the city to fast-track preapproved designs for backyard cottages.

It’s not hard to see the logic behind Seattle’s “urban village” strategy. It makes sense to concentrate density near transit and services. But when that plan was implemented nearly 25 years ago, nobody could have guessed that Seattle would become the fastest-growing big city in the country, and that home values would hit a median of $820,000.

New realities raise the question: In the face of unprecedented population growth and a housing affordability crisis, is it right for the housing options in so much of the city to remain almost frozen in time?