Especially in Oregon and the rest of the Pacific Northwest.

They're naturally inexpensive. They're often islands of physical accessibility. They're supremely green without even trying - in part because they're an essential ingredient for a truly walkable neighborhood. A lot of them are even sorta cute.

They're four-story apartment buildings, and they may be the most underrated building block of a healthy city or town, especially in the Pacific Northwest.

Four-story apartment buildings hit a "sweet spot" of low costs and high benefits, according to Nathan Teske, executive director of Hillsboro-based affordable housing developer Bienestar.

In the transect of housing types, a spectrum that runs from farmhouses to skyscrapers, this keystone of the Cascadia region's potential future housing growth sits plop in the middle, nestled between the townhouse and the "5-over-1." Here in the Northwest, four-story apartment buildings are almost always made of wood; under US building codes, they're the tallest structures a team of workers can easily build without using more expensive, more complicated, and more energy-intensive concrete.

Add all this to the shortage of construction labor and to overlapping state and federal prevailing wage rules that kick in with the fifth story, and you've got a good case that four is an even more magical number in our corner of the continent than elsewhere.

"This is timber country, so folks [here] know how to build with it,"

said Meaghan Bullard, managing principal at Portland-based Jones Architecture, which has designed market-rate and below-market housing in Astoria, Pacific City, Portland, and Seattle.

As Oregon in particular looks for ways to accelerate housing production across the state without letting prices rise further, it may be coming to grips with the fact that removing the many regulatory barriers to new apartment buildings should be a bigger part of the solution than it has been so far. And if Oregon wants to start building more apartments, it should start thinking a lot about the number four.

In my next article, I'll explore how four-story apartment buildings have generally become illegal to build in Oregon, especially in the areas richest with jobs, infrastructure, and services where people actually want to live. For now, though, let's take a short walk through the reasons why we should care about these four-story structures: four-story apartments are affordable to live in, inexpensive to build, and physically accessible; plus, they save energy costs and fit anywhere.

1. Four-story buildings are inexpensive to live in

Unfortunately, the American Housing Survey doesn't offer recent price data for the state of Oregon. However, it did recently conduct a survey for the half of the state's population that falls within the Portland metro area, and here's what it found:

Some of the savings compared to a detached home simply reflect the fact that most apartments are smaller than most oneplexes. But the benefit of having homes of various sizes on the market should be obvious: it lets people choose to save money by living smaller if they want to or if that's what they can afford. Put another way: making apartments legal in more places lets people prioritize amenities or price, if that's what they want, rather than just size.

Then there's the smaller price difference between smaller buildings and bigger ones - look at the rightmost bar on that chart. One reason it's a little taller than the middle three is about location; the whole point of a high-rise is to fit many homes in a premium location. Some of it reflects a building's age; because modern zoning leaves a so-called "missing middle" between oneplexes and skyscrapers, small apartment buildings tend to be older. But again, the chart above shows why midsize apartment buildings should be allowed to exist. If they aren't, people get stuck with the more expensive extremes.

But why four stories, not just three? In Oregon, four-story buildings are a ...