Virtually all of Seattle's new housing has been crammed into the few parts of town where dense housing like apartment towers are legal. The rest of Seattle is made up of pricey single-family homes.

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Today’s housing column focuses on a topic that sounds boring but is pivotal to understanding the local housing market: zoning. Specifically, how much of Seattle’s housing is traditional, suburban-style single-family homes, and how that has affected our growth and housing prices.

In Seattle, 69 percent of residential plots of land are occupied by single-family houses, according to Attom Data Solutions, which has an extensive nationwide property database and ran the numbers for me. That’s about average when looking at the 50 biggest cities in the country. But compared to other peer cities with expensive housing, Seattle generally devotes a lot more of its housing to single-family homes:

Other ways of looking how the city’s land is used result in roughly the same 2-to-1 ratio, or greater, between single-family homes and denser forms of housing.

According to the city’s comprehensive plan update in 2015, single-family homes make up about 19,000 acres of current land use, while the two types of areas that allow denser housing – multi-family and commercial/mixed-use areas – combine for about 6,200 acres.

Looking at it yet another way: Single-family housing makes up 49 percent of all developable land mass in the city, while 8 percent are multi-family buildings and another 8 percent are commercial and mixed-use structures. (The rest? Another 11 percent are major institutions and utilities; 9 percent are parks and open space; 9 percent is vacant and 5 percent is industrial.)

OK, why does this matter? Because zoning is the whole game when it comes to housing. Let’s take things back to basics for just a second.

About 90 percent of developed land in cities is devoted to housing. There are two main types:

Single-family housing: These are the detached houses, often with a yard, driveway and garage, where most Americans live.

Dense housing: This is any housing that’s attached to another unit, often in taller buildings: apartments, condos, town homes, row houses, etc.

Suburbs and rural areas are generally dominated by single-family houses, while cities have a mix of both types. Typically, cities set zoning rules that dictate what type of housing can be built in various parts of town, which is why you won’t see a 40-story tower next to a bungalow.

In Seattle, tall, dense buildings are generally allowed in the greater downtown region and in outlying areas near transit (think downtown Ballard or the West Seattle Junction). The majority of land – everything in yellow in this map –is zoned exclusively for single-family homes (the actual land use may vary, based on older established uses such as apartments, row houses, and so on):

What does this mean for our crazy growth?

Under the current laws, expanded housing is essentially banned in those yellow areas – which, again, make up more than two-thirds of all residential properties – because only single-family homes are allowed there, and there’s no room left to build more of them (although newer, bigger single-family houses still sometimes replace torn-down, older houses, and homeowners in some areas are able to add cottages to their backyards). And yet Seattle is growing incredibly fast – adding 100,000 people in a decade – so where are they all going?

Most of them are flooding into the few parts of the city where dense, tall housing is allowed: According to a City of Seattle data portal for growth by neighborhood (found here), last year 88 percent of the city’s new housing was crowded into dense neighborhoods that make up just 18 percent of the city’s residential land area (the pink zones here):

Most of the rest of the new housing went to areas that allow medium-density apartments and mixed-use buildings, also outside of single-family zones.

This has been going on for a long time. Going back to 1995, according to that same city database, just 8 percent of the city’s new housing has gone to the single-family zones that hold two-thirds of the city’s residential properties.

So in reality, most of Seattle is not growing – only certain parts are. Single-family neighborhoods like Magnolia, Laurelhurst and Arbor Heights are absorbing virtually no growth while places with lots of density – like South Lake Union, Belltown and the University District – are growing like crazy.

Consider this disparity: Size-wise, the area that stretches from North Capitol Hill to Madison Park takes up about the same amount of land as the neighboring area from South Capitol Hill to First Hill. Yet in the last two decades, Madison Park and North Capitol Hill – single-family housing districts – have added a paltry 17 housings units per year. First Hill and South Capitol Hill, which have a mix of dense housing, added 450 housing units a year.

What does this mean for housing affordability?

Single-family homes offer people a chance at traditional, white-picket fence homeownership. For decades, middle-class people and even some lower-income earners were able to buy those houses in Seattle while the city still managed to absorb slow growth. The zoning laws that outline today’s housing stock were mostly enacted during a time when single-family homeownership was affordable and Seattle had far fewer people and jobs.

Now, if the city were to change that and wipe out suburban-style houses in single-family neighborhoods to build dense housing, then the single-family houses that do remain would get even more expensive because of the lower supply. But practically speaking, that’s irrelevant to most people because they can’t afford those houses now, anyway.

Seattle’s median single-family house now costs $820,000, and at the current pace will hit $1 million in less than two years. Homebuyers now need to have about $145,000 saved up in cash today for the median down payment on a house in the city, according to Attom. On top of that, households must earn about $140,000 a year to afford  mortgage payments – nearly double the city’s typical income.

So what if we went the other way and added more density to single-family zones? The city is slowly doing that, but so far it hasn’t done much for home affordability.

Even the local Sightline Institute, which strongly promotes density, found that when an older single-family house here is bulldozed and replaced with multiple connected homes (like in the slider below), each new town home is typically more expensive than the old house. That’s because modern housing costs more to build and fetches higher prices than the modest bungalows built a century ago.

On average, it takes at least seven new homes on one property before each one is cheaper than the older, single house it replaced, Sightline found. In places like Ballard where these sorts of connected town homes are now allowed to replace single-family homes, the height limit of three stories and the lot size means most of the projects wind up including far fewer than seven new homes. Essentially Seattle has managed to get the bad parts of a luxury housing boom without reaping any of the benefits of a housing-supply boom.

To have a real impact on affordability, Seattle would need to build a lot more of these smaller homes in taller buildings – a complete overhaul to neighborhoods that existing homeowners (the majority of city voters) are likely to oppose. This is why the city has only tiptoed around relatively small changes to single-family neighborhoods (a few years ago, a proposal to add much more density to single-family zones was so poorly received by homeowners it was dropped in a matter of weeks).

It’s easy to see why: Homeowners bought their house expecting quiet streets the way they are today, while any changes to that could block their views, increase traffic and noise, and a host of other issues associated with added housing. Even now, all the upzones the city has proposed and is considering enacting in the coming months would affect just 6 percent of the city’s single-family zones – the current plan would generally just allow taller buildings in areas where some density is already allowed.

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Rents

The flip side, however, is the rental market: Renters make up roughly half of the city’s residents, even though just a fraction of the city’s land is devoted to apartments.

Recent changes by the city to allow for taller buildings (in those pink “urban villages” in the second map above) have helped spark an apartment boom that is adding more rentals to the city this decade than in the previous 50 years combined. In the face of that added supply, rent increases have started to slow way down in the last six months – even dropping a bit from last summer’s high – following years of steep increases.

In theory, Seattle could add a lot more apartments, and cool the rental market even more, if it allowed apartments in all those neighborhoods where only single-family homes are allowed. One of the reasons those new apartments are so expensive – rents in Seattle’s new buildings are 40 percent pricier than older ones  – is because of their location: With apartments allowed in so few lots, that land becomes expensive, and pushes developers to make the most of their land by putting up high-rises that are particularly expensive to build. All those costs get passed onto renters. But even that plan has its limits; typically, when rents start plummeting, developers would be more likely to stop building (and banks would be more likely to halt their construction financing) until the market heats up again.

This story has been updated to include more information on the percent of total land mass in the city that is devoted to various types of housing and other land use. The first map has also been updated to remove parks from areas zoned for housing.

Have a question for a future column? Send me an email at mrosenberg@seattletimes.com, or shoot me a note on Twitter.