Editor’s note: The podcast version of the story includes an excerpt from a more extensive examination of Chicago-area wooden porches used as a means of egress. To catch every episode, subscribe to our podcast.

Most older U.S. cities have a signature kind of building. In Brooklyn it’s the brownstone, one standing shoulder-to-shoulder to the next. In Philadelphia, newcomers and visitors are struck by the distinctive row houses.

What about Chicago? Well, it’s a city known for its skyscrapers, for sure. Outside of downtown, though, you won’t find soaring steel and glass. In the neighborhoods, it’s wood, brick and stone. The real workhorse of Chicago’s built environment is the modest, ubiquitous (yet fascinating) two-flat.

You know the building. Two stories, with an apartment unit on each floor, usually with bay windows greeting the street through of a facade of brick or greystone. Most were built between 1900 and 1920.

Two-to-four unit apartment buildings make up 27 percent of Chicago’s housing stock, according to data from the DePaul Institute of Housing Studies. The rest is split evenly between single-family homes, condominiums and buildings with five or more units.

We recently got a question that returns some wonder to this everyday building. Our question asker, who chose to stay anonymous, is particularly interested in why the two-flat became so popular. And she wants to know who calls these buildings home. As she observes in the question she submitted to Curious City, they’re somewhere between suburban houses and big apartment buildings:

Chicago-area two-flats straddle the line between apartments and homes. Who were they originally designed to serve? Has that changed?

The answer to that last part? It’s revealed in a story, one you’d miss if you choose to focus on the city’s skyline or crane your neck to see the top of the Willis (Sears) Tower. It turns out the advent of the humble two-flat mirrors the development of Chicago’s middle class. And in many ways it still does today, but in the wake of the 2008 financial and foreclosure crises, that may be changing.

A Bohemian building boom

Through the late 1800s, European immigrants made up almost half of Chicago’s population. Hundreds of thousands of Polish, German and Czech people settled here, often making their first home in narrow one-story buildings usually made out of wood. Those came to be called worker’s cottages.

As Chicago’s big industries grew — Sears, McCormick Reaper and Western Electric, to name a few — so did the population. Soon it made sense for developers and architects to build up as they built out. Hence two- and three-flat buildings, which offered denser housing, and gave the owners a shot at some extra income from renting out their extra unit.

We found several architects from the era who built two-flats by the dozens on spec, meaning they weren’t designing for a specific client, but acting as “owner-architect” in the parlance of records from the era. Many of them were Bohemian. (Today, the former Bohemia is part of the Czech Republic).

In fact, along with Jen Masengarb of the Chicago Architecture Foundation — whom we partnered with on this voting round and helped us research this story — we found an old article from the Chicago Tribune that shows the connection between the city’s booming Czech population and its sprawling housing market. A headline from Oct. 17, 1903 crows: “BOHEMIANS IN LEAD AS BUILDERS OF HOMES.”

At the convention of the Building Association league of Illinois, Bohemian Frank G. Hajicek boasted of “$12,000,000 in shares in force” held by the “the Bohemians of Chicago.” It was a point of pride for the 28-year-old resident of the South Lawndale neighborhood.

“Never in the history of the world, I believe, have people in a foreign land established themselves in homes so securely and rapidly as have the 200,000 Bohemians who make Chicago their home,” said Hajicek in 1903.

In the heavily Eastern European Southwest Side neighborhoods of Pilsen (named for the Bohemian city of Plzeň), North Lawndale and South Lawndale, many of those homes were two-flats.

With Masengarb’s help, we dug up some documents at the Chicago History Museum, including a 1915 “Book of Plans” that enticed homebuyers to order away for all the materials needed to build a two-flat sized for a typical Chicago city lot.

“Our design No. 144 is a two-family flat designed for a money making proposition,” begins one such ad. “Anyone wanting a comfortable home and at the same time a good income on the investment will do well to consider this proposition.”

Many, it seems, did consider it. A 1910 Tribune article reported $38 million of flat building, “a new high record in this field, exceeding by over $4,000,000 the figures of 1908, which also established a new record.”

A ‘workhorse building’ in a western paradise

Data from the U.S. Census Bureau shows that it often wasn’t young first-generation immigrants buying Chicago two-flats. Instead it was those who immigrated to Chicago as children in the late 19th century, and by the early 20th century had built up enough money to graduate from renting.

“What appears to have happened is that the Czech population was essentially moving further west, out of Pilsen and other sort of areas, Maxwell Street areas, to newer land, I guess you could say,” says Matt Cole of Neighborhood Housing Services of Chicago, which administers the Historic Greystone Initiative. “That’s where the name California [Avenue] comes from — it was like their western paradise.”

Jen Masengarb and I take Cole up on his offer to point out one such western paradise: the part of North Lawndale known as K-Town for its K-named avenues (Kostner, Kildare, Keeler, etc.) near Pulaski and Cermak Roads. In 2010 K-Town was listed on the National Register of Historic Places for its collection of classic Chicago apartment buildings.

“It’s like a microcosm of Chicago architecture,” says Cole, pointing out stately greystones, single-family brick residences and flats in styles ranging from Queen Anne to Prairie to mashups of any and all architectural detailing popular between 1900 and 1930. “The reality is that the two-flat and three-flat are the workhorse building of this period of Chicago.”

During our neighborhood walk, Masengarb points out that for a lot of early 20th century Chicagoans, the two-flat was a vehicle of social mobility.

“This two-flat is that bridge, I think, between that older 1880s, 1870s housing,” she says. “And then the bungalow which was the even bigger dream, and a bigger yard, my own space and nobody living upstairs, clomping around. “

Consider Frank Stuchal. Census data shows in 1888 he immigrated to Chicago from Bohemia as a 13-year-old with his parents and two sisters. The census is taken every 10 years, and every 10 years as his income increased — Stuchal was first employed as a typesetter, then a print shop foreman, and finally business manager for a newspaper — he moved further west along Cermak avenue. In 1900 the 24-year old Stuchal rented an apartment at W. 23rd Street and South Spaulding Avenue with his two sisters. In 1920 he and his wife owned a two-flat, half of which they rented out to a German family. By 1930 he and his wife were raising their son in a bungalow they owned in the southwest suburb of Berwyn.

The 1920 census shows the street lined with two-flats occupied by second generation Czech, German, and Polish immigrants in their 40s and 50s, raising Chicago-born teenagers. Stuchal’s neighbors included butchers, policemen, bookkeepers, bricklayers and librarians.

That two-flat Stuchal owned in 1920 was in K-town, near 21st Place and Keeler Avenue. It was built in 1916, and it’s still there.

Today it’s owned by Arquilla Lawrence, whose parents moved in when she was two years old.

“And I love it,” she says. “It’s been my home all my life, ever since I was two we moved into the neighborhood. I’ve been here my whole life except when I went away to college.”

Like many African-Americans, Lawrence’s father moved to the neighborhood from the South — Oklahoma, in his case — during The Great Migration of blacks to northern cities during the middle of the 20th century. After World War II the neighborhood became the first African-American neighborhood on Chicago’s West Side.

“That’s why it’s so well kept,” says Corey Brooks, who also grew up in K-town. “Because most of [the property owners] migrated from the South. This is where they put their roots in, so they all know each other.”

Brooks introduces us to his wife, Rita, who is on her way to check in on her mom. Both of them moved back to their childhood homes in order to care for their parents. Turns out it’s not just the neighborhood’s property ownership that has lasted all these years.

“This is my childhood sweetheart,” says Rita, pointing to Corey. “He was my first boyfriend! Then he got married to someone else, I got married, I lost my husband, and then two years ago we found each other and got married.”

Before we leave K-Town, Jen Masengarb surveys the mishmash of early 20th century architectural styles on display.

“It’s like a metamorphosis or an evolution. We’re gonna try this over here on this block, and then this is five years later we’re gonna try this … You can just see it evolving in the way that we live and the decisions that we’re making in terms of what our families need, what is stylistically impressive,” she says. “This architecture is us, it’s a reflection of us.”

Losing equity: Is the workhorse getting exhausted?

So the form of two-flats was basically a response to economics and demographics, as well as the size and shape of a Chicago city lot. The buildings no longer house predominantly Czech and other Eastern European immigrants, but today’s tenants share a lot with their neighbors across the decades — many of them used two-flats to build community and a little bit of personal wealth in the form of equity. The two-flat was a bridge to a better life for the families that built Chicago as we know it.

One hundred years later, however, it’s not clear how much longer two-flats will be able to fill that role.

K-town is well kempt, thanks in part to incentives from its historic district status. But two-flats are expensive to maintain. And since the 2008 financial and foreclosure crises, a lot of two-flats in other neighborhoods around Chicago are sitting vacant or being bought by developers who don’t occupy the units.

And sometimes the ownership moved in the other direction. Eric Strickland tells us he bought a K-Town two-flat in the 90s. When he purchased the building on 21st Place, it was divided into three units. Once he’d saved up enough money, Strickland converted the two-flat into a single-family home. He lives there now with his wife and daughter.

During the housing crisis two-to-four unit properties were disproportionately impacted by foreclosure. And Geoff Smith from the DePaul Institute of Housing Studies says two-flats don’t really make economic sense for new development, so they may well be lost to history in lower-income neighborhoods.

“What you see more commonly is a single-family home targeted for owner occupancy, or you see a larger rental building,” Smith says.

He adds that, if older two-flats fall into disrepair, there will likely be no two-unit rentals to replace them. “The concern is that in some of these more distressed areas, where there is a substantial stock of these buildings, there is a risk in some neighborhoods that this kind of housing could be lost,” he says.

That prospect matters. According to data from the DePaul Institute of Housing Studies, today there are more than 76,000 two-unit apartment buildings in Chicago. In some neighborhoods — Brighton Park, New City, and South Lawndale — they still make up more than two-thirds of the housing stock, as well as a substantial proportion of the city’s affordable housing.

Prices for two-to-four unit buildings in distressed areas of Chicago fell roughly 70 percent between the pre-crash peak and current figures. That means many homes in those areas are worth less than they were in 1997, says Smith.

So if the “money making proposition” that two-flats once promised to working families is more elusive these days, what will become of the lower-income neighborhoods where these historic buildings are most prevalent?

“Because of changing population dynamics, the changing nature of the city, in some areas you are going to see demand in decline. You may not see it recover, and there just may not be an economic value to some of these properties,” says Smith. “Hopefully some prescient, some really far forward-seeing investor can come in and say ‘these properties have value for the long-term.’”

Chris Bentley is a freelance journalist and reporter for WBEZ’s Curious City. Follow him at cabentley.com and on Twitter at @Cementley. Jen Masengarb is Director of Interpretation and Research for the Chicago Architecture Foundation and contributed reporting to this story.

Correction: A draft of the text for this story misstated the time period during which the majority of Chicago two-flats were constructed. The correct timeframe is between 1900 and 1920.