If you’ve browsed apartments in a city like Portland, OR with lots of new construction you’ve likely noticed plenty of trends: “gentrifier gray” flooring, dark cabinets, stainless steel appliances. One of the most consistent throughlines for these units is the open layout: a combined kitchen-living room-dining room space that’s usually the focal point of the apartment. In contrast, older Portland apartments are much more likely to have a separate dining and living room, and sometimes a den as well, in addition to the bedrooms in the unit.

Kate Wagner, writing for Bloomberg CityLab, describes the origin of the open concept as follows:

The interior-wall-free open concept became popular starting in the 1970s, evolving from the cedar contemporary homes known for their tall ceilings and windows, and from styled ranches whose steeper rooflines allowed for newly in-vogue cathedral ceilings. Overall, the open concept was a reaction against years of small, low-ceilinged living, which felt restricting and stuffy to a new generation of homebuyers.

This is a good explanation of the aesthetic and cultural origins of the open concept, but I have another theory that I think helps explain not just where the concept originated but why it got so popular when it did. I call it “the Zillow-ification of housing”.

Zillow is a site for searching for housing, either for rent or for sale. If I wanted to look for apartments in Northwest Portland under $2,000 a month with at least two bedrooms, I can easily put that in as a query on their site:

As I understand it, this is a pretty typical way to use the site: there’s quite a few options to search by, but here I’m using the one that the search interface makes most readily available. Using other search options also means you’re dependent on landlords tagging their properties consistently; it’s not that unusual to see units with A/C but without the A/C tag on their Zillow posting, or similar limitations. It’s not even possible to look for a unit with a separate living and dining room, at least for rentals.

These features aren’t unique to Zillow; pretty much every other online listing site has them. I just use Zillow the most, which is why this post isn’t called “The Apartments.com-ification of housing”.

This is a big change from pre-internet apartment seeking, when prospective tenants would have been looking on average at fewer homes and relying more on in-person viewings to make decisions. Now, tenants have access to hundreds or thousands of listings at once and need easy and consistent metrics to filter down their search. Bedrooms and bathrooms fit the bill perfectly because they’re well defined in building codes and are more or less the same even across very different apartments and houses.

This creates an incentive for landlords and developers: where’s the money in using precious square footage to create discrete rooms in a unit when you’re not rewarded for it at all in search results on many of the major apartment lookup platforms?

One implication of this is that the open concept trend should follow the popularity of Zillow as a platform. Is that something we can see in any available data? Google Ngram says yes:

Zillow was founded in 2006 and we can see the trend really take off around 2010. That matches up not only with Zillow but with the rise of other online tools for searching for housing, like Craigslist or Apartments.com.

This is obviously not perfect evidence for the “Zillowification” theory. Good data about the number of open concept listings over time would’ve been a stronger indicator, particularly paired with data about how popular online rental listing sites were over time. But it’s a start; I think it’s interesting to think about how the internet shapes incentives not just for housing consumers but for housing producers and sellers, and I’m interested to see if anyone eventually researches this theory more.