In the past ten (arguably fewer) years, the politics of housing in America have witnessed a remarkable shift in the Overton Window. Single-family zoning codes—by which I mean rules that prohibit any type of building in a neighborhood other than a detached home on its own lot—are no longer sacrosanct. A wave of moves to universally allow such building types as triplexes (the city of Minneapolis), fourplexes (the state of Oregon), and accessory dwelling units (many cities, but Seattle is a recent example) have brought high-profile national attention to the movement to diversify the built form of America's vast swaths of residential neighborhoods, and allow incremental development in them universally. The latest volley is a paper in none other than the Journal of the American Planning Assocation by three high-profile UCLA scholars, titled, unsubtly, "It's Time to End Single-Family Zoning."

Rewinding to 2009 makes it clear how remarkable this shift is. A decade ago, there was plenty of talk about the costs of the suburban development pattern—including environmental and quality of life concerns but also, in the wake of the recent housing market collapse, the economic and financial sustainability of the suburban growth model. (Strong Towns, then a nascent blog, was a leader in asking the latter set of questions.)

What there wasn't in 2009 was any real movement to touch zoning codes or other building regulations in America's enormous swaths of single-family neighborhoods. Virtually nobody was using the term "YIMBY" in 2009. Nobody was using the term "Missing Middle Housing," which was coined in 2010 by Dan Parolek to describe the range of residential building types from duplexes up to small-scale apartment buildings that fit easily within the fabric of a neighborhood. The idea of allowing for more dense construction in existing cities (rather than continuing to build on their outskirts) was well established, but the discussion was largely limited to mid- and high-rise transit-oriented development (TOD) or large-scale, New Urbanist infill projects like Orlando's Baldwin Park.

So, now that the terrain has shifted, the idea of allowing missing-middle construction in single-family neighborhoods has serious traction, if not mainstream acceptance just yet. And the debate in many urbanist circles has moved on to, "Is it enough?"

Urbanists tend to be motivated by a sense of one or more overlapping crises with the prevailing suburban experiment: housing unaffordability, environmental unsustainability, financial insolvency, or all of the above. Is broadly legalizing the next increment of development—say, triplexes or 4-plexes anywhere you want to put one—enough to meaningfully address these crises? Or is it woefully insufficient, and should our focus be on getting larger buildings built?

Quantifying the Potential of the Missing Middle

Zillow's data wizards have some answers to that question. A new report is aimed at helping Americans conceptualize just how much a wave of missing-middle development would affect the housing supply in our large metro areas. With presumed apologies to Jonathan Swift, the report is titled A Modest Proposal: How Even Minimal Densification Could Yield Millions of New Homes. Lead author Issi Romem does the math, providing a number of anecdotes like these ones:

Across 17 metro areas analyzed, allowing 10% of single-family lots to house two units instead of one could yield almost 3.3 million additional housing units to the existing housing stock. That’s on top of the roughly 10 million already expected to be built through 2040 under status quo assumptions. If the San Francisco Bay Area, for example, continues to add housing largely as it has over the past two decades – in single-family homes on the periphery and in distinct clusters of large multifamily buildings – it could grow its housing stock by an estimated 11.8 percent over the next two decades. But if just 1 in 10 lots currently home to a single-family residence were redeveloped or otherwise allowed to accommodate two homes, the area’s housing stock could grow by 18.4 percent over the same period – or more than 200,000 homes beyond the status quo.

The important number there is 10%. Imagine a block of ten homes. Now imagine one of them—just one—replaced with a duplex. This is not a radical transformation of neighborhood character or even really a significant one, and yet it yields a startling number of new homes in the aggregate.

The key here is just how many single-family detached homes there are out there. These houses are 60% of the U.S. housing stock, and occupy over 80% of the land in most cities. Even in fast-growing, large metros, population growth is rarely more than 2% per year, so it really wouldn't take that much building to keep up with demand.

It's not realistic, of course, to imagine that duplexes or triplexes will end up spread evenly across the American landscape, one house in every ten, or five, or whatever. There will naturally be places with a lot of pressure for redevelopment and growth, and places with little or no such pressure.

What Zillow's numerical experiment (complete with interactive tool you can play around with) reveals is that missing-middle construction is more than up to the task, on a simple numerical level, of addressing the housing shortfall in America's big cities. And we can do it without expanding our cities' boundaries into a single new cornfield, or building a single new suburban highway.

It is also up to the task of addressing other big urban problems. The main environmental objection to the suburban development pattern is its car dependence. And car dependence is also the primary driver of financial insolvency. Accommodating all our cars—with staggering amounts of parking, turn lanes, landscaped buffers, stormwater retention ponds to compensate for all that asphalt—is by far the top reason our cities have vast wastelands of unproductive “non-places” generating a pitiful return on investment.

We've been clear before that there is no threshold density for a strong town—it's the wrong question to ask—but is there a threshold density that makes it viable for a place to be car-optional (i.e. you can walk to many destinations, and convenient mass transit is supportable)? If there is, it's not exactly Hong Kong. A lot of evidence suggests the magic number is often in the ballpark of 10,000 people per square mile. This kind of density can be achieved in surprising ways. It is visually illustrated by several familiar American cities. Here's Chicago: