There’s a slick chart making the rounds right now that neatly summarizes 400 years of the architecture of single-family homes in America.

Made by the infographic designers Pop Chart Lab, the poster presents front-door views of typical American homes from the colonial days up to the present.

There’s a simple Dutch Colonial from the 1600s, the colonnades of Greek Revivals from the mid-1800s, a broad-porched Craftsman from the early 1900s, and an archetypal McMansion from the not-so-distant past—or, depending where you look, the present.

Seeing the history of the single-family house in this way makes it easy to think of all the changes in style and size and structure as an evolution.

The architecture can be thought of as a response to local conditions and the changing fashions of the time, the size and ornamentation as responses to the demands of the market and the fluctuations of the economy. Of course, this evolution continues today.

But the evolution of the single-family home is not linear.

At the same time the median size of new single-family houses is on the rise, cities across the country are testing new ways of providing smaller homes, loosening rules on building new accessory units, and even rethinking the whole concept of single-family neighborhoods.

A growing number of cities are bending strict zoning rules to accommodate smaller homes.

So-called “tiny houses” are catching on in cities like Washington, D.C., Charlotte, and Portland, where people are trying to dodge high home and land prices by going small.

Often measuring in at less than 200 square feet, these tiny homes can seem cramped, but are often designed to be incredibly efficient. Construction costs are typically in the tens of thousands rather than the hundreds of thousands, so they make for cheap housing, especially in expensive markets.

City ordinances and zoning codes don’t often allow such tiny homes to be built legally, but these rules are starting to change.

In Portland, for example, tiny homes are allowed, but only as “accessory” units on properties with existing primary residences, and there’s even a small community in northwest Texas called Spur that has proclaimed itself “the nation’s first ‘tiny’ house friendly town.”

With city ordinances in place, the town of 1,200 is hoping to lure aspiring tiny house builders to come and build legally.

According to its website, “Spur is welcoming a new generation of pioneers with an open mindset.”

A multi-family version of the tiny home concept is under construction in New York City, where modular units starting at just 260 square feet will be stacked atop one another to create a 55-unit apartment building geared toward singles and space savers.

But smaller houses don’t have to be such tight fits.

In Atlanta, City Council member Kwanza Hall is trying to kick-start the tiny house movement in the South.

He’s proposing a rewrite of a section of the city code that prohibits the construction of single-family homes that are smaller than 750 square feet, arguing that smaller homes would make homeownership a more viable option for a wider spectrum of the city’s residents.

“Affordable and accessible housing speaks to the need of many Atlantans,” he told Atlanta Magazine.

Other cities are finding new ways of sneaking these types of smaller homes into existing neighborhoods.

Vancouver has been particularly proactive in developing “laneway houses”—secondary units or “granny flats” added into backyard space on single-family properties.

These units essentially turn single-family lots into double-family lots, and sneak new housing into a heavily built-out city.

About 350 building permits have been issued in each of the past three years for laneway houses, which city officials are hailing as a valuable new source of affordable housing.

It’s an idea other cities are looking at, especially those seeing skyrocketing demand for limited housing supplies.

San Francisco, for example, is rethinking accessory dwelling units as a way to add more housing stock despite geographical constraints.

Urbanism from Within, a recent exhibition at the urban planning research and advocacy group SPUR, proposed a handful of architectural plans that could be used as the model for integrating new density into the city’s tightly-packed single-family settings.

Cities like Los Angeles, where upwards of 85 percent of residential land is zoned for single-family housing, are also thinking about new ways to integrate accessory units.

Seattle Mayor Ed Murray recently proposed what may be the most sweeping change to the single-family home in the U.S. through a proposal to replace single-family zoning with a “lower density residential zone” that could include backyard granny flats, duplexes, and triplexes.

“An adequate, affordable supply of housing is the lifeblood of vibrant, urban centers,” city officials wrote in a draft letter introducing the proposal, which highlighted income, ethnic and race-based inequities in the city.

Some residents were quick to oppose the proposal. “I am offended. This is just appalling,” Ellen Monrad, a community council chair, told Seattle Weekly. “The strongest neighborhoods in this city are single-family neighborhoods. People want a little bit of space, and then for them to imply that you are racist if you live in a single-family neighborhood. This may be a wake up call for this city.”

Another community council member called it “an invasion to create more space for developers.”

Though others tried to frame the proposal as being beneficial to the city by helping to make housing more affordable, the backlash proved too much for Murray, who backed away from the plan last week, citing “sensationalized reporting” after a Seattle Times columnist published a leaked draft of the proposal.

Though Seattle’s plan isn’t moving ahead, it has been a high-profile example of a city reconsidering what its housing stock can and should be.

The single family home is by no means disappearing—620,000 single-family homes were built in 2014, compared with 264,000 units of multi-family housing. But as cities grow and change, the single-family home will continue to evolve.