In his 1937 essay “What is a City?,” Lewis Mumford described an evolutionary process through which the “badly organized mass city” would evolve into a new type of “poly-nucleated” city, “adequately spaced and bounded”: 

“Twenty such cities, in a region whose environment and whose resources were adequately planned, would have all the benefits of a metropolis that held a million people, without its ponderous disabilities: its capital frozen into unprofitable utilities, and its land values congealed at levels that stand in the way of effective adaptation to new needs.”

For Mumford, such cities, designed with strong public participation, would become the nuclei of new poly-nucleated metropolitan regions that result in:

“A more comprehensive life for the region, for this geographic area can, only now, for the first time be treated as an instantaneous whole for all the functions of social existence. Instead of trusting to the mere massing of populations to produce the necessary social concentration and social drama, we must now seek these results through deliberate local nucleation and a finer regional articulation.”

Unfortunately, since Mumford wrote these words, we have not achieved poly-nucleated cities or regions. Nor have we advanced a theory of urban evolution. Urban theorists have described cities, used basic pattern recognition to detect relationships among the potential components of urban evolution, or offered narrow prescriptions to fix one urban challenge while generating inevitable unintended consequences that pose new challenges. This is because we have never developed a real science of cities. 

For more than a century, planners, sociologists, historians, and economists have theorized about cities and their evolution by categorizing them, as noted by Laura Bliss in a well-documented 2014 CityLab article about the likelihood of an emerging evolutionary theory of cities. They generated multiple typologies of cities, from functional classifications to rudimentary taxonomies (see Harris, 1943, Functional Classification of Cities in the United StatesAtlas of Urban ExpansionAtlas of Cities). But they based these classifications on arbitrarily chosen categories and did little to inform our understanding of how cities became what they are or to presage what they might become. 

Even Jane Jacobs, in a foreword to her 1961 book, The Death and Life of Great American Cities, called for the development of an ecology of cities—a scientific exploration of the forces that shape cities—but provided only narrative accounts of what defined great cities, mostly with regard to design, as part of her ongoing assault on the orthodox planning profession. In some of her later work, Jacobs set out principles to define great cities, based mostly on form, but she never provided a framework to improve the science of urban theory.

Modern urban theory is plagued by several shortcomings. It is not analytic. It fails to provide a framework for generating hypotheses and the empirical analysis to test those theories. And the research, in general, focuses on big iconic cities, rather than a representative global selection of urban settlements that captures the differences between big and small cities, primary and secondary cities, industrial and commercial cities. Importantly, the research provides little guidance regarding how we might intervene to improve our future cities to support sustainable human habitation on the planet. 

The New Urban Agenda—to be announced in October at the third UN-Habitat conference, in Quito, Ecuador—will present consensual global objectives for sustainable urbanization. These objectives provide guidance for United Nations member states as they prepare for the gargantuan task of welcoming 2.5 billion new urbanites to the world’s cities over the next thirty years—culminating the 250-year process through which human settlement moved from almost entirely rural and agrarian to predominantly urban contexts. But before we attempt to implement the New Urban Agenda, we must confront the serious limitations in our understanding of cities and urban evolution. A new “science of cities” would buttress our efforts to get this last stage of urbanization right.

I do not intend to present a new science of cities in this message. Instead, I will suggest a way to frame one that borrows from evolutionary theory. The evolution of species is driven by four main forces, and it seems reasonable that corollary forces help to shape the evolution of cities. These forces are: natural selection, gene flow, mutation, and random drift. And they play out in predictable ways that shape cities—where city growth replaces reproductive success as an indicator of evolutionary success.

Natural selection is a process of impulse and response. It relates to how a city responds to changing external factors (impulses) that support or inhibit success. Impulses can be economic, environmental, or political, but they are, importantly, outside the control of the city. Economic restructuring, for example, might select against cities that depend on manufacturing, have inflexibly trained workforces, or extract or produce single commodities that face changes in demand in global markets. Climate change and sea-level rise will inhibit the success of coastal cities or those exposed to severe weather events. Political impulses might include regime changes, social uprisings, or war. Or they might be something as seemingly minor as a change in allocation formulae for national revenues. Every impulse will benefit some cities and harm others. A city’s ability to respond to different impulses might be a measure of its resilience, which is directly influenced by the three other evolutionary forces. 

Migration (gene flow) helps to diversify the economic, social, and age structures of cities through the exchange of people, resources, and technologies. Presumably, the in-migration of people, capital, and new technology improves a city’s ability to respond to external impulses. Out-migration, in general, would reduce this ability. 

Mutation, for cities, is an unpredictable change in technology or practice occurring within a city. It might be shorthanded as innovation or disruption. 

Random drift involves longer-term changes in cities that result from cultural or behavioral shifts. These might include decisions to maintain or preserve long-term assets, real or cultural. Drift describes the unpredictable ways that cities might change their character. 

As noted, I do not want to lay out a new theory of urban evolution here. I merely want to recommend this direction in order to invigorate our thinking around urban change more rigorously and systematically. A significant amount of work has already gone into quantifying elements of this framework. Risk theorists and insurers have quantified many of the external impulses that challenge cities. Demographers and population theorists have studied human migration, and macroeconomists have studied capital flows. A lot of attention has been paid to innovation and disruption in the last couple of decades. Random drift is a little less studied. But, as Bliss points out, big data and new technologies might help us to detect longer-terms drift. In any case, a larger framework that weaves these disparate areas of work together would advance our understanding of urban evolution. 

On a cautionary note, while an evolutionary theory of cities would be a signal advancement of urban theory, it is useful to remember that, unlike evolution, which is a mostly passive process—species enduring the external forces that act on them—cities, in theory at least, are driven by more purposive behavior: planning. But planners need better tools to drive their practices and to test their approaches. If we are to successfully implement the New Urban Agenda, a toolkit based on evolutionary science would be hugely helpful. As Mumford concluded in his 1937 essay:

“To embody these new possibilities in city life, which come to us not merely through better technical organization but through acuter sociological understanding, and to dramatize the activities themselves in appropriate individual and urban structures, forms the task of the coming generation.”

We at the Lincoln Institute of Land Policy stand ready to support coming generations in comprehensive and scientific analysis of urban evolution and the important role that effective land policies can play in driving it. Our urban future depends on it.

Recent news stories routinely feature “hot market” U.S. cities with astronomical housing prices that end up displacing residents with moderate or low incomes. In Portland, Oregon, Mayor Charlie Hales declares a state of emergency, directing a budget cut from the city’s general fund to create more affordable homes. San Francisco’s epic housing battles pit longtime residents against tech workers. In Seattle, 40 people, 35 jobs, but only 12 housing units arrive daily. In Denver, Mayor Michael Hancock pledges $150 million for affordable housing in the next decade. Boston Mayor Martin J. Walsh plans to build 53,000 units by 2030, while neighboring Cambridge adds density in infill areas and near transit. And in Boulder, Colorado, public officials consider a host of housing options in an approach they call “gentle infill.” 

“Hot markets exist for many reasons, but in Portland, Seattle, San Francisco, Boulder, and other cities, housing issues are clearly a result of strong economic development,” says Peter Pollock, FAICP, manager of Western programs for the Lincoln Institute of Land Policy. In these cities, a jobs-housing imbalance leads to inadequate housing options. The “gentle” or “sensitive” infill approach is about “trying to find ways to make infill compatible with surroundings to achieve urban design goals and enable production of more housing,” he says. The term also “puts a positive spin on something that may not be universally accepted”—namely, density—“and suggests that we can do a better job.”

While half of all households nationwide are spending more than 30 percent of their income on housing, many residents in hot market cities are spending more than 50 percent and being forced to leave. Housing activists, such as those at the recent first national YIMBY (“Yes in my backyard”) gathering (see sidebar), are challenging city planners and elected officials to create more diverse infill options to house people, stem displacement, make better transit connections, and create more environmentally sustainable communities.

How Did We Get Here?

Desirable cities are growing rapidly because they’re attracting millennials and cultural creatives for job opportunities and lifestyle amenities, and the newcomers have gravitated in numbers that far exceed places to live. The tech industry, with its influxes of well-paid workers, is often blamed for driving up housing costs and causing displacement. But other factors are also in play. Many cities built little if any housing during the Great Recession. Mortgage credit is tighter. Construction costs are escalating. New housing is priced at market rates that drive up the cost for existing homes. Zoning that favors single-family detached houses or luxury apartments has led to expensive housing monocultures. What’s being viewed as a crisis in many cities is the loss of housing not just for lower-income residents but also for workforce and middle-income residents—teachers, nurses, firefighters, small business owners, young professionals, young families, and others who typically provide a foundation for communities.

Restoring the “Missing Middle”

The good news is that cities across the United States are already working on solutions. Communities are overturning policies that prohibit housing or place tight restrictions on where and how it can be built, to allow for more diverse and affordable places to live. Many urban planners and public officials are focused on developing housing types that restore the “missing middle,” to shelter moderate and middle-income households. 

The missing middle, a concept that grew out of new urbanism, includes row houses, duplexes, apartment courts, and other small to midsize housing designed at a scale and density compatible with single-family residential neighborhoods. Since the 1940s, this type of development has been limited by regulatory constraints, the shift to car-dependent development, and incentives for single-family home ownership. Three- or four-story buildings at densities of 16 to 35 dwelling units per acre used to be a standard part of the mix in urban neighborhoods. Many urban planners say this scale and density of housing is needed again to offer diversity, affordability, and walkable access to services and transit. Cities are using a variety of additional approaches to inject more moderately priced housing into residential neighborhoods, from shrinking or subdividing lots to adding accessory dwelling units (ADUs) to expanding legal occupancy in homes. Some of these gentle infill approaches are showing great potential or in fact adding needed units on a faster track. 

How does gentle infill work? It depends on the city, as demonstrated by the following examples from Portland, Oregon; Boulder, Colorado; and Cambridge, Massachusetts.

Portland, Oregon: More Housing is Better

Portland typically ranks atop lists of “best places” to live but has recently slipped a few notches because of its housing prices, which ballooned 13 percent in 2015. According to a recent study released by Metro, the regional government organization, Portland area rents increased 63 percent since 2006, while the average income of renters rose only 39 percent. The population grew by 12,000 in 2015, to more than 632,000 residents in 250,000-plus households. 

Since 1973, Portland has been living with statewide urban planning that mandates an urban growth boundary to protect farmland and forests from urban sprawl and to ensure efficient use of land, public facilities, and services within the urban boundary. This city has an ambitious agenda to meet its growth projections with several big planning efforts: a new zoning map and the 2035 Comprehensive Plan, its first update in 30 years, adopted by city council in June 2016; a new land use code with regulations that affect a range of growth from multifamily and mixed-use development to transportation corridors and parking; and Central City 2035, a long-range development plan for the city center and its districts. 

The city is relying on policy changes in view of the 142,000 additional jobs, 135,000 extra households, and 260,000 more people that it will need to accommodate by 2035, according to Metro. About 30 percent of new housing will be built in the city center, 50 percent in mixed-use centers and corridors, and 20 percent in Portland’s single-family residential zones, which comprise about 45 percent of the city’s 133 square miles of land. The city has about 12,000 buildable lots, assuming that some current lots can be subdivided to provide more sites.

Since 2010, an estimated 20,000 new residential units have been built or are in the pipeline, and tax increment financing in designated urban renewal areas has invested $107 million in new and preserved affordable housing. In March, the state legislature lifted a 17-year ban on inclusionary zoning, which will allow the city to require builders to set aside units for new workforce housing. The city is focused on funding strategies to provide more affordable homes for households below 80 percent of the area median income (AMI). To increase the number of middle-income units for people earning more than 80 percent of AMI, the city is relying on policy changes, rather than funding strategies.

By the end of 2016, a stakeholder advisory committee for the Residential Infill Project (RIPSAC) will provide advice regarding the size and scale of houses, small-lot development, and alternative housing types. One proposal under consideration is to allow more internal conversions of large historic houses into multiple units, an approach that would provide more housing while avoiding teardowns and preserving the historic fabric of neighborhoods. Building on the legacy of small homes that exist from a century ago, Portland is looking to add little houses on undersized, pre-platted lots. And the city is considering whether to allow the development of more tall “skinny” homes of up to 1,750 square feet on 2,500 square-foot lots, half the square footage of land required under R-5 single-family zoning. 

“Five or ten years ago, people would ask, ‘Why is this house being built on a narrow lot?’” says RIP project manager Morgan Tracy. “Now it’s not so surprising. They’re really becoming popular because they’re at a lower price point for buyers.”

Policy changes regarding accessory dwelling units have helped generate new moderately priced housing and have drawn the attention of public officials from other cities in search of solutions to their own housing crises. ADU construction has exploded since 2010, when the city waived development fees covering sewer, water, and other infrastructure connections, reducing construction costs by $8,000 to $11,000 per unit. The waiver inspired a surge in construction: almost 200 ADUs were permitted in 2013—six times the yearly average from 2000 to 2009. In 2015, the city granted 350 new ADU permits, for a current total of more than 1,500 units. Tracy says ADUs “are a well-accepted means of producing more housing because they’re better integrated into a site and don’t necessitate a home being demolished.”

Any single-family house in the main zoning districts can have an ADU, and a proposal would allow up to two units—an interior apartment plus a separate carriage house or granny flat. The city does not limit the number of ADUs within a neighborhood or require off-street parking. It has also streamlined some ADU standards to allow for improved designs with slightly greater height and setbacks. RIPSAC is considering proposals to allow any house to have two ADUs, both interior and detached, triplexes on corner lots where duplexes are now allowed, and duplexes on interior lots, with a detached ADU. Allowing duplexes on interior lots and triplexes on corners “doesn’t mean everyone will take advantage” of the policy changes, says Tracy, noting that only 3 percent of corners now have duplexes. But “if every property owner took advantage of additional unit potential, we would double the number of housing units in each neighborhood.” 

The next phase of infill housing policy considerations will address how medium-density housing types might fit into small infill and multi-dwelling sites. The city has already been moving in that direction: Portland’s Infill Design Toolkit guide focuses on integrating rowhouses, triplexes and fourplexes, courtyard housing, and low-rise multifamily buildings into neighborhoods. 

“What may be shocking and alarming for some people becomes more acceptable as you see it more,” says Tracy. “We’re seeing that with duplexes and triplexes in single-family neighborhoods. The last time we built them was in the 1930s and ’40s. We’re trying to promote a wider diversity of housing forms, and some folks are supportive because they understand the need to be able to house more people on available land.”

Boulder: More Housing Is Better, But There Are Down Sides

Boulder is studying what other cities are doing to encourage gentle infill, and a recent trip to Portland by city officials, staff, and business leaders offered perspective on what could work at home. Like Portland, Boulder has determined to halve carbon emissions by 2030, provide more infill housing in the developed city core, protect open space, and encourage public transportation use. But with one-sixth of Portland’s population and different challenges and opportunities, Boulder seeks its own consensus on what gentle infill means. 

Located 25 miles northwest of Denver in the foothills of the Rockies, Boulder also ranks high on the lists of healthy, livable, and entrepreneurial places. The natural beauty and high quality of life in this 25.8-square-mile city of 105,000 have attracted start-ups and established tech firms such as Google and Twitter. The influx has fed a digitally paced lifestyle and “1 percent” housing market in which the median single-family detached house costs over $1 million. 

In the past two years, housing prices overall have risen 31 percent. Factors beyond the tech industry have limited affordability for many years (disclosure: for 23 years, I’ve lived, worked, and raised two kids in a formerly modest Boulder neighborhood that has been largely rebuilt with higher-end homes). The University of Colorado-Boulder, a key economic driver with 38,000 faculty, staff, and students, generates significant housing demand. A jobs-housing imbalance translates to an estimated 60,000 cars arriving and departing daily, despite regional and local bus service. 

State law prohibits rent control, and the state’s “condominium construction defects legislation” has squelched that type of construction for middle-income housing. Boulder is also home to many independently wealthy “trustafarians” and speculative buyers who purchase homes with cash from selling property in other high-end markets. Some are second or third residences; others are reserved for short-term rentals like airbnb. In June 2015, city council voted to restrict short-term vacation rentals, saying they impacted affordability and reduced the number of long-term housing opportunities. 

Development limitations include few residential lots, a 45,000-acre ring of protected open space around the city, and a height limit, to preserve mountain views, capped at between 35 and 55 vertical feet, depending on planned development intensity and location near transit. The city is within sight of a theoretical build-out; a forecast of 6,760 additional units by 2040 is being considered for the current update of the Boulder Valley Comprehensive Plan. A 2015 housing survey conducted for the plan indicated that most residents were willing to increase density and building height to allow for more housing, at least in some parts of the city.

Since 1989, while the percentage of lower-income households has held steady, middle-income households have declined from 43 percent to 37 percent of the populace. The segment disappearing at the fastest rate is households earning between $65,000 and $150,000 as well as families with children. City council, the planning board, and local newspaper op-ed pages field lively debates over the “Aspenization” of Boulder and infill housing options that could slow or reverse the city’s momentum toward greater exclusivity and less diversity. 

Boulder has been working on affordability and inclusivity for some time. Its inclusionary zoning ordinance has produced 3,300 affordable housing units since 2000. Developers of projects with five or more units are required to construct 20 percent as permanently affordable, build off-site, donate land, or make a cash-in-lieu payment to the city’s affordable housing fund. The city’s goal is 10 percent permanently affordable housing; some 7.3 percent of the city’s housing stock now qualifies. 

Part of the affordable program is aimed at middle-income housing: the city has a goal of creating 450 permanently affordable units for households earning 80 to 120 percent of AMI. Since 2000, 107 units for middle-income households have been built in new mixed-income neighborhoods on land annexed in north Boulder. Many are in the Holiday neighborhood, a mixed-use model of 42 percent affordable units integrated within a total of 333 townhomes, row houses, flats, live-work studios, and cohousing. Recently built middle-income units are located in the Northfield Commons neighborhood, where half of the 43 percent of affordable units in duplexes, fourplexes, sixplexes, and townhomes are reserved for middle-income households.​​

YIMBYs Unite in Boulder

On a hot sunny weekend in June, the first-ever YIMBY (“Yes in my back yard”) “unconference,” as the democratically run gathering was called, drew 150-plus young and old urbanists to Boulder from 25 cities, including New York; San Francisco; Sitka, Alaska; and Brisbane, Australia.

“YIMBYTown” drew urban planners, architects, elected officials, and advocates for affordable housing, transportation, public health, the environment, and social justice. It was sponsored by the San Francisco based Open Philanthropy Project and the Boulder Area Realtor® Association and hosted by Better Boulder, a local advocacy group that last November spearheaded a successful campaign to defeat two ballot initiatives intended to limit growth in the city. (Disclosure: The author is a Better Boulder member-volunteer.) Presentations and discussions focused on housing, zoning, gentrification, coalition building, and NIMBY challenges, including titles such as “How F-cked is San Francisco—Lessons From the Worst Housing Market in the Country” and “Reframing the Sacredness of Single-family Zoning.”

The gathering was bookended by references to the social and economic implications of rising housing costs and displacement. In the opening plenary, Sonja Trauss, founder of the San Francisco Bay Area Renters’ Federation (SFBARF), says a key goal of the movement is to “repopulate cities” as “an integrative process to counter the segregation of the suburbs.” In closing remarks, Sara Maxana of Seattle for Everyone noted a growing body of evidence that “exclusionary zoning causes housing shortages in high-demand cities and leads to exclusion by class. It induces segregation by wealth and reduces access to opportunity, good jobs, schools, healthcare, and open space.”

“It’s very expensive to subsidize people making $70,000 to $130,000 per year,” says Aaron Brockett, a city council member and former planning board member, referencing a middle-income housing study prepared for the city that defined Boulder’s middle market as 80 to 150 percent of AMI. He advocates for “market solutions like smaller units as a trade-off in those areas that have amenities and services such as mixed-use areas where people can walk to transit and redeveloping areas.”​

In preparing a comprehensive housing strategy, Boulder is exploring ideas for middle-income infill housing in transit corridors, commercial strips, business parks, and industrial areas that could be rezoned and redeveloped, and in walkable mixed-use neighborhood centers in residential areas. “The 15-minute neighborhood is the Holy Grail for a lot of communities, but it takes a lot of work,” says Jay Sugnet, project manager for Housing Boulder. “Are they in single-family neighborhoods or at the edge of service-industrial areas? Where are you willing to locate those, and what’s appropriate? You also need a concentration of people to support retail. Boulder has lots of commercial corridors, but they need a sufficient number of people to support all of them.”

The city also plans to adjust the ADU ordinance to achieve more middle-income affordability in neighborhoods of mostly single-family detached houses, which comprise about 41 percent of the city’s 46,000-unit housing stock. An ADU ordinance in effect since 1981 has permitted only 186 ADUs and 42 OAUs (owner’s accessory units) because of requirements regarding off-street parking, minimum lot size, and limits on ADU density. “We’d like ADUs for diversity of housing in neighborhoods,” says David Driskell, executive director of planning, housing, and sustainability. “Physically we could put in quite a few here, but, politically, there will be quite a lot of discussion about parking and traffic impacts.”

City council is considering “creative adjustments” to existing housing that could have less impact on the footprint and “character” of residential areas, such as loosening code restrictions on the number of unrelated people who can share a home. In most residential zones, no more than three unrelated people can share a house, even if it has six bedrooms and multiple bathrooms. A ballot measure petition launched recently by University of Colorado graduate students asks Boulder voters to overturn the occupancy limit and adopt a “one person = one bedroom” policy. Allowing higher occupancy is controversial, because, although it would provide more places for students and others to live legally, it could further drive up housing costs for families, as monthly rent in group houses, particularly close to the university, often costs as much as $1,000 per bedroom.

The city is also discussing a revision of its 20-year-old cooperative housing ordinance. No co-op projects have been permitted because the ordinance was “essentially a path to No,” says Driskell. Three affordable rental co-ops were established under other measures. City council is considering a more welcoming ordinance that supporters say would benefit the city by offering a sustainable and community-oriented lifestyle for single residents, young families, seniors, and people who work lower-wage jobs. 

“We tend to be a regulatory city, and we have really embraced deliberative planning,” says Susan Richstone, deputy director of planning, housing, and sustainability. “It hasn’t always been easy, but we’re having the discussions and making changes in planning and zoning levels within a regulatory framework. It’s in our DNA.”

“Density is a bogeyman here, and people are up in arms,” says Bryan Bowen, an architect and planner who is a member of the Boulder Planning Board and the city’s Middle Income Working Group.  Residents are anxious about both modest homes being scrapped and replaced with 5,000 square-foot $1.5 million new homes and the possibility of greater density with more large edgy-looking multifamily apartment buildings. “That’s probably why gentle infill feels good, though it has an interpretive quality. It’s a question of what people find to be compatible and palatable.” There’s no consensus yet about which infill approach will work best, Bowen says. “But frankly, in moderation, some application of all of them might be needed.”

Accessory Dwelling Units (ADUs): A Preferred Infill Housing Approach

Demographic changes such as aging populations, shrinking household size, college-loan-strapped millennials, and cultural preferences are leading many cities to allow home owners to build ADUs, also known as in-law apartments, granny flats, and carriage houses. Advocates say ADUs—built in the interior of a home, rebuilt from a garage, or newly built as a separate cottage—offer affordable options for elderly parents, adult kids, and caregivers. They’re also a source of rental income that can help residents stay in their homes. As older home owners wish to downsize and age in place, some are choosing to live in the ADU and rent out their main house. 

Typically ranging from 200 square feet to more than 1,000 square feet, ADUs are part of a long tradition of modest apartments and multigenerational houses that were common before the era of single-family suburban homes. Many housing advocates are keen on ADUs as a way to add units quickly, with home owners financing the infill of existing neighborhoods, compared to the lengthy and costly process of land acquisition and development of larger-scale multifamily projects by municipalities, nonprofit affordable housing organizations, and private developers. At Denver’s Bridging the Gap housing summit in May, a session on small-scale affordability posed a potential scenario for the city: 70 neighborhoods multiplied by 300 ADUs per neighborhood would equal 21,000 moderately priced housing units.​

At the recent YIMBY conference in Boulder, Susan Somers of AURA (formerly Austinites for Urban Rail Action) in Austin, Texas, described a coalition effort to become “an ADU city” and achieve much greater housing density in the mostly single-family detached city. They accomplished their mission; in November 2015, the Austin City Council passed a resolution relaxing ADU regulations and allowing them on smaller lots. AURA hopes to help home owners entitle 500 new ADUs annually. The units provide “affordable housing and a source of income to allow folks to stay in their homes,” says Somers. In gentrifying East Austin, “this is how families stay together.”

Cambridge: Bridging the Income Gap

Cambridge, located across the Charles River and three miles west of Boston, has the most expensive housing in Massachusetts and bears keen pressure to produce more missing-middle options. The population has increased more than 10 percent since 2000, to 110,000 residents within a compact 6.5 square miles, and is projected to grow by 6,200 homes before 2030, according to the Metropolitan Area Planning Council (MAPC), the regional planning agency for Metro Boston. The city has 117,000 jobs and more than 52,000 housing units, about half of them located in mixed-use commercial areas. The average listed single-family home price in 2015 exceeded $1.2 million. Median monthly rent for a one-bedroom apartment is $2,300.

“Cambridge has become a bifurcated place of very high income and very low income,” says Andre Leroux, executive director of the Massachusetts Smart Growth Alliance. “It’s hard for middle-class people to live there.” Cambridge has the infrastructure to support much greater density and to add significantly more residential development and huge residential towers, “but it doesn’t want to be downtown Boston.”​

The city is in the first year of a three-year comprehensive plan process, its first since 2000 (the state does not require municipalities to develop comprehensive plans). Affordable housing for low, moderate, and middle incomes—a resounding theme through the public process—is the number-one priority, says Iram Farooq, assistant city manager for community development.

“For a lot of working people, there are fewer affordable options in the city,” says Farooq. The greatest population decline has occurred among residents earning between 50 and 80 percent of AMI, she says. Middle-income households earning between 80 and 120 percent of the area’s AMI are also leaving the city for housing options elsewhere in the urban region. She notes that a city program that offered low-interest financing to home buyers earning up to 120 percent of AMI experienced little demand. 

“Just creating the program doesn’t mean people are going to use it. With the same financial commitment, they are able to go three miles down the road and find a nicer or bigger house for the same money. Being able to hold onto the middle is more challenging than at other income levels.”

The city is using regulatory strategies to fund more affordable housing. An incentive zoning ordinance enacted in 1988 required linkage payments to offset the effects of commercial development on the housing market. In 2015, the city updated the ordinance, increasing the rate for developers from $4.58 to $12 per square foot and broadening the requirement to include any nonresidential development, including healthcare and university facilities, labs, and office space. The city is also considering new zoning for infill sites and an expansion of its inclusionary housing ordinance, which now requires 11.5 percent affordability in new projects, to 20 percent affordable units for moderate, middle-income, and low-income households.

Cambridge has been building infill housing, mostly in projects ranging from 50 to 300 units, on larger sites. East Cambridge, for example, has seen the development of thousands of housing units in the past decade, along with millions of square feet of office space and restaurants, on land that was formerly industrial. The city is requiring residential units with all new development; 40 percent of a new commercial project in East Cambridge’s Kendall Square will be dedicated to housing. Some of this new development is subsidized for the middle class. But few parcels exist in residential areas, land costs are high, and residents are pushing back.

For years, housing advocates have been urging the city to add more infill housing and increase density in Central Square, the historic municipal center of the city. Located on Massachusetts Avenue, Central Square has a subway station and a bus-transfer station where eight bus routes converge. The area has some three- and four-story buildings as well as one- and two-story buildings that could be redeveloped for dense mixed-use housing next to transit. The square historically had taller, denser buildings before some third and fourth stories were removed to reduce taxes during the Depression. In 2012, however, some neighbors tried to persuade the city to downzone Central Square. 

“Downzoning is not appropriate in a crisis in which we’re so restricted in our ability to build housing,” says Jesse Kanshoun-Benanav, an urban planner and affordable housing developer who started the civic group A Better Cambridge in response to the downzoning effort, to promote increased density for infill housing opportunities. The city council tabled the downzoning effort and since then has been allowing zoning changes in Central Square and providing incentives such as additional height and density in exchange for the development of more affordable housing.

At the eastern end of Central Square, Twining Properties is developing Mass + Main, a multiparcel mixed-use project with a 195-foot tower and 270 apartments, 20 percent of which will be affordable for low, moderate, and middle-income residents. The project required a zoning variance, notes Farooq. “We’re now hearing political desire to rezone the rest of Central Square. People don’t seem to be as opposed to density as height, so we’ll have to explore what that means in terms of urban form.”

Townhouses, duplexes, and triple deckers are the norm in Cambridge, and only 7.5 percent are single-family detached homes. New rules passed in May that allow the conversion of basements into accessory dwelling units in single- and two-family homes throughout the city could enable 1,000 legal ADUs. The ADUs don’t need a zoning variance, and off-street parking is not required. The square footage of the new units won’t count as gross floor area (ADUs previously were prohibited in most cases due to the existing floor-area ratio and requirements for lot area per dwelling unit). Supporters say the rules won favor because they allow for more efficient use of large homes and won’t alter the look of the neighborhood. 

“It’s important that there are people in the city who are willing to accept trade-offs,” says Farooq, noting that the YIMBY movement has “great political capital” to counter NIMBY pushback against infill housing. “There is a community desire to see more housing, and many young people, including a lot of renters, recognize that it’s important to increase the supply and not have steep increases in rent, to make housing more manageable and accessible.”

Regional Approaches

Leroux from the Massachusetts Smart Growth Alliance and others across the nation say that housing needs should be addressed as a regional issue, and cities and towns should work together to allow urban infill housing and approaches like ADUs under state zoning laws. In June, the Massachusetts Senate passed a bill that would reform 1970s-era zoning laws to permit ADUs and multifamily housing districts in every community. A coalition including the Alliance; the Senate President; mayors; and advocates for the environment, public health, affordable housing, and transportation supported the bill, which is poised to become state law next legislative session. A legal and policy strategy, it includes a fair-housing clause that prohibits communities from making discriminatory land-use decisions, which Leroux and others say increase segregation in many metropolitan areas, as low-income residents, including people of color, get pushed out of redeveloping urban neighborhoods.

Suburban communities also need to do their fair share, he says. Many suburbs are still zoning and building for the auto-oriented market, with “a lot of modest homes being torn down and replaced with McMansions,” he says. “We think there’s a grand bargain to be made between cities and towns and the real estate development community to unshackle development near walkable places, infrastructure, and transportation while curbing sprawl and protecting natural areas.” To allow for more diverse housing growth, he says, the Alliance and others are promoting “as-of-right,” or permitted zoning uses, in walkable areas, commercial centers, villages, town centers, and urban squares, because “that’s where the market is and where we need to let the market do its job.”

Kathleen McCormick, principal of Fountainhead Communications, LLC, lives and works in Boulder, Colorado, and writes frequently about sustainable, healthy, and resilient communities.

Photograph: Meghan Paddock Farrell