Missing middle housing refers to a lack of medium-density housing in the North American context. The term describes an urban planning phenomenon in Canada, the United States, Australia and more recent developments in industrialized and newly industrializing countries due to zoning regulations favoring social and racial separation and car-dependent suburban sprawl.^[1]

Medium-density housing is characterized by a range of multi-family or clustered housing types that are still compatible in scale and heights with single-family or transitional neighborhoods.^[2] Multi-family housing facilitates walkable neighborhoods and affordable housing, and provides a response to changing demographics.^[3]

Instead of focusing on the number of units in a structure, density can also be increased by building types such as duplexes, rowhouses, and courtyard apartments.

The term "missing middle housing" was introduced by architect Daniel Parolek in 2010.^[4][3][5][6]

Many forms of what is now described as "missing middle" housing were built before the 1940s, including two-flats in Chicago; rowhouses in Brooklyn, Baltimore, Washington, D.C., and Philadelphia; two-family homes or "triple-decker" homes in Boston, Worcester; and bungalow courts in California.^[7] Post-WWII, housing in the United States trended significantly toward single-family with zoning making it difficult to build walkable medium-density housing in many areas and, therefore, reducing the supply of the now "missing" middle.^[8][9]

At the end of the 19th and beginning of the 20th century, Canadian and American cities with few exceptions, most notably New York and Chicago which already had many tall buildings, were not dramatically different in form from their European counterparts. They had a relatively small physical footprint compared to their population size, and buildings were largely 3-7 stories tall surrounded by a relatively modest ring of streetcar suburbs.^[10] Most city dwellers who were in the lower to middle-income brackets lived in dense urban environments within a practical distance of their workplace.

The less well-off typically lived on either the upper floors of multi-unit residential buildings, as most did not have elevators, or in tenements. Merchants frequently lived in a residential unit above their store. Those who were better-off may have lived in a rowhouse or terrace, and starting toward the end of the 19th century, perhaps in a streetcar suburb still relatively close to the city centre.^[10] Overall, the typical arrangement of urban spaces was one where communities were serviced by small-scale owner-operated shops and transport to non-walkable destinations was done by bicycle, bus, streetcar, or train.^[10]

Traditionally those in the highest income brackets had typically lived in large houses outside of, but often near to, the city. They travelled to the city originally by horse carriage and later by automobile.^[10] For most people, the need to live close to their job significantly limited spatial social stratification beyond economic class. This situation collapsed in the wake of explosive expansion of post-war suburban sprawl, which enabled white flight.^[11]

The early to mid-twentieth century implementation of the suburb was thoroughly informed by this social context, and it was not uncommon for policymakers to inappropriately conflate small residential unit size, insanity, and crime with the traditional urban form; while simultaneously idealizing “rural” and upper class style estate living as its cure-all. The new car suburb was an affordable imitation of upper-class housing which became possible at such a vast scale when, after the war, factories could be turned over from producing military vehicles to consumer cars, helping to reduce the nominal price of a private automobile.^[12]

Originally in the US, the legal rule was that "all persons have an equal right in the highway, and that in exercising the right each shall take due care not to injure other users of the way".^[13] Pro-automobile interests advocated for the removal of non-drivers from the road, and particularly targeted pedestrians with the invention and criminalization of “jaywalking.”^[14][15][16] Importantly Federal, State, and Provincial governments undertook massive highway building programmes and also directly subsidized the purchasing of new suburban homes (Levittown being the prototype).^[17][18][19]

These government policies helped to make cars a practical choice and fostered the wholesale adoption of the car by the middle classes by the 1960s and helped to create the conditions for a decline in the quality, availability, and financial viability of public transportation.^[20] Increasingly the prestige and influence of New York and Chicago, with their high land prices and abundant skyscrapers, fostered a sense among many Canadian and Americans that “real cities” have tall buildings, and a "downtown" dominated by them, meanwhile European cities remained relatively medium-rise, dense, and pluricentric.^[21]

With this in mind, it is possible to understand the factors considered most important by policy makers in the mid-twentieth century context and how they pursued policies which would no longer allow for the previously dominant medium-density building types. The resulting policies radically reformed cities into ones that typically have a unicentric urban core which is dominated by tall buildings built to be reliant on office uses with the area often referred to as the central business district (CBD). This new "urban core" of stacked office uses is typically surrounded by swathes of sub-urban and peri-urban landscapes dominated by single-family homes with gardens serviced by the private automobile, car-centric retail destinations, and vast highway networks.^[22]

The loss of flexible middle-density development serviced by affordable and widely used public transportation has resulted in high commute times for commuters, which have remained stubbornly unaffected by further investment in new road capacity due to the nature of induced demand, a practical limit on the space required to move large volumes of people in relatively large vehicles, and greatly increased costs for both the vehicle owner and government due to the inherent inefficiency compared to previous modes of transport.^[23][24][25] Other problems include difficulty for low-income residents to find affordable accommodation within a reasonably affordable and practical commute of their place of employment.^{[citation needed]}

Car-centric cities are less climate-friendly due to impacts relating to inefficient use of resources, volume of paved area contributing to flood risk, and potential loss of natural habitats to human development.^{[citation needed]}

Without middle-density development to support them, cities have lost retailers not operating with substantial economies of scale. “Mom and pop shops” are replaced by big-box stores.^{[citation needed]}

Cities without middle density have also seen the loss of third places, places where people spend time which is neither their private residence nor their place of work.^{[citation needed]} These places are important for recreation, meeting neighbours, for adults to make friends, and for community organization. The loss of these third places and small businesses is due to the need of both to rely on proximity to a large number of people for whom visiting them is easy, can be spontaneous, and would not require a special trip.^{[citation needed]} Some have characterized the replacement of these “third places,” where historically people of all backgrounds in the neighbourhood gathered organically, by relatively fewer spaces where people must choose to drive, as a source of social filtering and potential source of social alienation.^{[citation needed]}

Some have suggested the loss of genuine “third spaces” as a contributing factor to a perceived reduction in a sense of belonging, inter-group social cohesion, and a rise in generalized loneliness.^[26]

According to the environmental geographer Laura Pulido, the historical processes of suburbanization and urban decentralization have contributed to contemporary environmental racism.^[27]

The polarization of Canadian and American cities into ones dominated by low- and high-density development with little in-between, has been due to implementing strict single-use land-use zoning laws at a municipal level which prioritizes these use types while making new medium-density illegal. This, combined with shifts in transportation planning at all levels, had helped to create a development paradigm which takes the private motor vehicle as its default mode of transportation, and only after that, considering other modes like walking, cycling, buses, streetcars, and subways. Public transport, where it still exists, has typically also built within this paradigm of car dependency. For example, GO Transit rail services in the Greater Toronto Area is one of the few commuter-rail services in either Canada or the United States, but is designed for commuters to drive to parking lots with a train platform where the rail service will take passengers to the CBD in the morning and return them to the parking lot in the afternoon, service has been unidirectional and only operated during rush hour.^[28]

Missing middle housing offers a greater choice in housing types that still blend into existing single-family neighborhoods, create more affordable housing options, and help reach sustainability goals.^{[29][30][31][32]} Missing middle housing units are usually smaller units than single-family homes because they share a lot with other homes, which results in lower per-unit land costs and, therefore, lower housing costs.^[33] Missing middle housing types are also one of the cheapest forms of housing to produce because they are typically low-rise, low parking and wood-frame construction, which avoids expensive concrete podiums.^[34][35][36] Because the construction and building materials are comparatively less complicated than larger mid- and high-rise structures, a larger pool of small-scale and local home builders can participate in the creation of this form of housing.^[35][36] To support municipal budgets, the denser and more efficient use of land and infrastructure may be financially productive for municipalities with more people paying taxes per acre for less infrastructure than large lot single-family homes.^[9][37][38]

Increasing missing middle housing options may allow families of different sizes, types, and incomes to access quality housing. Missing middle housing tends to become naturally affordable housing as it ages, and provides a level of density that supports the shops, restaurants, and transit that are associated with walkable neighborhoods.^[39][40][41] Walkable neighborhoods may then support sustainability, health, and affordability goals by reducing reliance on personal vehicles.^[42] This would promote active transportation, reduce sprawl, reduce pollution, and reduce transportation costs by lessening the need for personal vehicles.^{[34][43][44][45]}

Missing middle housing options may allow seniors to downsize without leaving their neighborhood. For example, accessory dwelling units can enable multi-generation households to have privacy while all living on the same property.^{[46][47][48][49]} Missing middle housing may enable a wider range of families to achieve homeownership by offering a wider range of housing options and prices.^[50] Additionally, missing middle housing types such as accessory dwelling units can support mortgages through the rents of those secondary units.^[51] Overall, missing middle housing options can create housing at a wide range of prices for a range of family types.^{[52][53][54][55][29]}

Some property rights advocates believe that widely permitting missing middle housing expands property rights by allowing property owners more choice in how to use their property.^[56][57][58] Some equity advocates feel that permitting more diverse housing choices, such as missing middle housing, may reduce historic and modern inequities that keep less affluent people out of certain amenity-rich neighborhoods.^{[59][9][60][61][62][63][64][65][66]}

An aerial view of the Rosslyn-Ballston corridor in Arlington, Virginia. The high density, mixed use development is concentrated within ¼–½ mile from the Rosslyn, Court House and Clarendon Washington Metro stations (shown in red), with limited density outside that area.

Increasingly from the 1990s onwards, transit-oriented development (TOD) has been put forward by many urban planners as a way to create more medium-density development. The idea is that creating communities of mixed-use development around public transport nodes will help to recreate demand for public transport and help to re-urbanize Canadian and American municipalities. TOD developments in Canada and the United States are typically near a public transport node, made of large plots with between one and a few buildings on them owned by one to a few owners and, typically, tall buildings and or buildings of intermediate size with a mixture of uses permitted within them predominating. Uses often include shops at the ground floor with residential and office uses interspersed throughout the upper floors.

Some critics of the way the TOD concept has been implemented in Canada and the United States point out that the large TODs fail to engage in placemaking and the result is relatively large highly controlled characterless places not unlike the suburbs and strip malls they are meant to replace.^[67] These critics say that the problem is with trying to zone for what planners think a city looks like; rather than creating the transport and legal conditions to allow it to take shape organically.^[68]

However, it is worth noting that urban planning in Japan uses a zoning system and has not lost middle-density housing. Instead of single-use zoning, zones are defined by the "most intense" use permitted. Uses of lesser intensity are permitted in zones where higher intensity uses are permitted but higher intensity uses are not allowed in lower intensity zones.^[69] This results in nested zoning, where the higher intensity zones are inclusive of related lower intensity ones. Zoning districts in Japan are classified into twelve use zones.^[69] Each zone determines a building's shape and permitted uses. A building's shape is controlled by zonal restrictions on allowable building coverage ratios, floor area ratios, height (in absolute terms and in relation to adjacent buildings and roads), and minimum residential unit size. These controls are intended to allow adequate light and ventilation between buildings and on roads, and to ensure a decent quality of housing. In this system, rather than trying to plan for how and where to create dedicated districts of medium-density housing, planners are left to focus on creating the conditions necessary to encourage land owners to intensify the use of their plots, and ensuring new areas of medium density that arise have the amenities they need to be successful.

When discussing medium-density housing, it is important to explore the differences between the approach this type of zoning uses with respect to single-detached housing compared to that traditionally used in the United States and Canada. In the United States and Canada, single-detached homes typically require large setbacks for off-street car parking and yards/gardens; while single detached homes in Japan are in many cases similarly large single detached houses but on small plots taking up virtually the entirety of the plot fronting directly on to the street with no requirement for off-street car parking; instead assuming a reliance on public transport rather than cars to meet daily needs. Roads in these areas are slow and drivers are aware they must legally share responsibility for mutual safety with all the other types of road users equally.

This type of single detached house can achieve medium housing density while fostering a sense of community, municipal fiscal viability, and good residential amenity. This is achieved while maintaining privacy and access to sunlight by regulating the direction of windows, the use of very small setbacks, much higher maximum building coverage ratios, higher floor area ratios, and other considerations discussed in the previous paragraph. It is also worth noting that Japanese houses offer, on average, larger living spaces than that of many wealthy European countries which have not lost their medium-density housing.^[71][72] This approach to not require car parking provision or private yards/gardens in areas with high degrees of good connectivity is seen as desirable because: access to common outdoor green space is seen as sufficient for these needs or, at the very least, an acceptable tradeoff for the convenience of improved connectivity; the provision of sprawling lower-value land-uses like private car parking and residential garden spaces in such locations is viewed as a poor return on investment by developers eager to maximize living space and plot utilization; urban planners who are eager to avoid the imprudent use of limited public funds with respect to the large nominal and operational costs of public transportation, water, power, roads, etc. which usually increase over distances, while access costs for users do not; and urban planners seeking to avoid wasteful and shortsighted opportunity costs.

This approach to zoning gives the landowner more flexibility in using the land while still precluding harmful or inappropriate development and maintaining the benefit of remaining predictable and easy to understand. The result is that when demand changes, like with new public transport investment, land-owners are able to, on an individual basis, redevelop their land to meet demand in a manner that can be reactive to local demand and distributes risk for the local community; for example the failure of a medium-sized building to find tenants may have a relatively small impact, whereas a large one failing to do so may hamper development of other types in the same community. This type of zoning may also help to foster a more organic and local character to communities, especially over time.

A form-based code (FBC) is a means of regulating land development to achieve a specific urban form. Form-Based Codes foster predictable built results and a high-quality public realm by using physical form (rather than separation of uses) as the organizing principle, with less focus on land use, through municipal regulations. An FBC is a regulation, not a mere guideline, adopted into city, town, or county law and offers an alternative to conventional zoning regulation.^[73]

Missing-middle housing comes in a variety of building types and densities but may be characterized by location in a walkable context, lower perceived density, small building footprints, smaller homes, reasonably low amounts of parking, simple construction, and focus on community.^{[74][75][76][77]} Forms of missing middle housing may include side-by-side duplexes, stacked duplexes, bungalow courts, accessory dwelling units (carriage houses, basement apartments, etc.), fourplexes, multiplexes, townhomes, courtyard apartments, and live/work units.^[78][79]

These building types typically have a residential unit density in the range of 16 to 30 units per acre but are often perceived as being less dense because they are smaller in scale.^[80] Because of its scale, missing middle housing may mix into single-family neighborhoods, act as an end-grain of a single-family housing block, act as a transition between higher density housing and single-family housing, or act as a transition from a mixed-use area to a single-family area.^[81][82] The resulting density may support broader community desires, including walkable retail, amenities, public transportation, and increased "feet on the street".^[83]

Many local governments do not allow the zoning necessary to build MMH. Owning a studio, 1 bedroom, or 2 bedroom condominium that is 600–1,000 ft² in a multi-unit complex with a reasonable HOA monthly fee and a 1.5 detached garage isn't allowed in many areas because of zoning ordinances. Many 5-over-1 complexes were built starting in the 2010s but primarily for leasing and not owning.^[84]

The resurgence of missing middle housing is due to many factors including resurgent market demand for this type of housing, demand for housing in amenity-rich walkable neighborhoods, the necessity of housing affordability, environmental efforts to support walkability, transit-oriented developments, and changing demographic trends.^[85][86] In 2014, the American Association for Retired Persons (AARP) released a report showing that more and more, Americans want to "age in place" and need easy access to services and amenities available in walkable, urban, transit-oriented communities.^[87]

Millennials have been shown to drive less and seek housing choices in walkable neighborhoods close to transit.^[88] The number of automobile miles traveled increased each year between 1946 and 2004. In 2014, Americans drove less than 2004, and no more per person than in 1996. The decline was driving is most striking among young people aged 16 to 34, who drove 23% fewer miles on average in 2009 than their age group did in 2001.^[89]

Research suggests that millennials prefer amenity-rich, transit rich, and walkable neighborhoods.^[90][91] In 2015, Small Housing B.C. stated that "The structure of the traditional North American suburb has failed to live up to the expectations of many who settled in suburban neighborhoods, and new ways are being sought to re-engineer suburban living and re-build those settlement patterns."^[92]

Several American states have adopted or proposed legislation aimed at increasing the stock of missing middle housing. Most notably, Oregon adopted House Bill 2001 in 2019.^[93][94] The bill requires Oregon's medium-sized cities to allow duplexes on each lot or parcel zoned for residential use that allows for the development of single-family homes.^[93] Oregon's large cities, with a population over 25,000, and cities in the Portland Metro region, must allow duplexes, triplexes, quadplexes, cottage clusters, and townhouses in residential areas.^[93] The Bill set aside funds for planning assistance to local governments to help draft local codes and allows municipalities to set reasonable design and infrastructure standards.^[93]

In Massachusetts, H.5250 was adopted to require municipalities near the MTBA to reasonably allow duplex or multi-family housing near transit stations.^[95][96] The Bill also created financial incentives for communities to zone for "smart growth" and made it easier for municipalities to adopt zoning ordinances or zoning amendments.^[97][98][99] In 2019, Washington State adopted E2SHB 1923 encouraging all cities under the Growth Management Act (GMA) to increase residential capacity by supporting many forms of missing middle housing.^[100][101]

The State of Washington provided grant funds to help support code changes, housing action plans, and sub-area plans to support missing middle housing types.^{[102][101][103]} In 2022 Maine adopted bills LD2003 and LD201 that implement several affordable housing strategies including allowing accessory dwelling units and duplexes on residential lots statewide and permitting fourplexes in certain "growth areas".^{[104][105][106]}

The states of Vermont,^[107][108] New Hampshire,^[109][110] and California^[111][112] have adopted a number of bills that promote accessory dwelling units and reduce regulatory barriers to accessory dwelling unit construction. State-level action has also occurred in Australia where, citing an effort to promote more 'missing middle' development, New South Wales launched the Low Rise Housing Diversity Code and Design Guides for Low Rise Housing Diversity.^[113][114] The State of Connecticut House and Senate approved legislation to reduce some zoning restrictions on missing middle housing types such as accessory dwelling units.^{[115][116][117][118][119]}

Other states have considered but not adopted similar legislation to support missing middle housing types. Illinois considered HB4869 which would have required municipalities to permit and reasonably regulate accessory dwelling units.^[120] Virginia considered HB 152 which would have required municipalities to allow, and reasonably regulate, missing middle housing types (duplexes, cottages, etc.) on all lots currently zoned for single-family housing.^[121] Maryland considered HB1406 "Planning for Modest Homes Act of 2020" which would have required census tracts that are affluent, transit-adjacent, and/or near a large number of jobs, to allow missing middle housing types.^[122][123]

Nebraska considered LB794 would mandate every city with more than 5,000 people to allow missing middle housing in areas previously zoned exclusively for single-family detached residential.^[124][125] Montana considered HB 134 which would have allowed duplex, triplex, and fourplex housing in certain municipalities.^[126][127] North Carolina considered House Bill 401 and Senate Bill 349, which would have allowed middle housing in any neighborhood zoned for detached, single-family homes.

Many municipalities are updating their land-use and zoning regulations to better support missing middle housing.^[128][129] Changes to land use regulations to support missing middle housing may also include changes such as form-based-codes, transit-oriented development, and other updates.^[130]

In the United States, Portland, Oregon, has a number of historic missing middle housing types located throughout the city, most of which are duplexes, that were built before the 1920s before the city's first zoning plan was approved. Zoning for single-family homes was expanded in the 1950s and the building of duplexes or triplexes largely became illegal in Portland. In the 2010s Portland began updating its zoning regulations to permit Missing Middle Housing types.^{[131][132][133]} Missing Middle zoning updates have spread through the Pacific Northwest and now include Seattle,^{[134][135][136]} Walla Walla,^[137] Lake Stevens,^[138][139] Orting,^[140][138] Wenatchee,^[138][141] Eugene,^[142][143] Olympia,^[144][145] Spokane,^[146][147] and Bellingham,^[148] Tacoma,^[149][150] and Tigard^[151] among others.^[152][153]

Zoning updates to support missing middle housing are not just found in the Pacific Northwest. Notably, In Minnesota, the Minneapolis 2040 plan called for up-zoning the city to allow more missing middle housing types throughout the city.^{[154][155][156][157][158][159]} The new zoning in Minneapolis does not prohibit the construction of single-family homes, but no neighborhoods in the city are zoned exclusively for single-family zoning.^[157] The city also eliminated mandatory parking minimums from its zoning regulations allowing builders and business owners to choose the amount of parking they provide based on the market and their unique needs.^{[160][161][162]}

In California, Sacramento voted to permit up to four housing units on all residential lots and reduce parking requirements in order to help the city alleviate its housing crisis and to achieve equity goals.^[163] The City of Berkeley, California has voted unanimously to zone for several missing middle housing types city-wide by 2022 citing equity and housing affordability as goals.^{[164][165][166]}

Bryan, Texas implemented a pattern-zoning policy in which the City provides several pre-designed and pre-approved plans for missing middle housing types (with significantly reduced permitting procedures) in the "midtown" portion of the city.^[167][168] The goal of the program is to reduce housing costs caused by design fees and lengthy permitting procedures, reduce burdens on city staff, achieve public input and support for housing designs in advance, and ensure quality housing designs.^[168][169] Norfolk, VA also has a missing middle pattern book with free designs for missing middle housing types including duplexes and quadplexes.^{[170][171][172]}

Many local governments across the United States have chosen to zone for missing middle housing types in significant portions of their zoning districts including Grand Rapids Michigan,^[173][174] Durham, North Carolina,^[175][176] Kirkland Washington's cottage housing zoning,^[177] Montgomery County, Maryland's numerous housing studies,^[178] Bloomington, Indiana,^[179][180] and Dekalb County, Georgia.^{[181][182][183]} Indianapolis, Indiana chose to permit missing middle housing types (in addition to higher density housing types) along bus rapid transit corridors.^[184][185] Indianapolis also included missing middle housing types in its residential infill guidelines.^[186]

Other cities are making long-term plans to increase the supply of missing middle housing. Charlotte, NC added language in their comprehensive plan to allow duplexes and triplexes across the city.^[187][188] Citing missing middle housing as a component of a larger affordable housing strategy, Raleigh, NC voted to permit several missing middle housing types in most residential zones.^{[189][190][191][192]}

While some communities have not adopted regulations to widely permit the full range of missing middle housing types, they have made changes to permit accessory dwelling units. Diverse examples include large cities such as Los Angeles, CA,^[193] the City of Chicago, IL,^[194] and smaller cities such as Lexington, KY,^[195] and Santa Cruz, CA.^[196]

Outside of the United States, cities in both Australia and Canada have adopted missing middle housing reforms. Notable examples in Canada include Edmonton, Alberta's missing middle zoning reforms,^[197] and Vancouver British Columbia's secondary unit zoning.^{[198][199][200]} Montréal, Québec is notable for its distinct architecture and urban planning that has historically included significant amounts of missing middle housing.^[201][202] Due to its unique history, many neighborhoods in Montreal include low-rise attached duplexes, triplexes, and apartments often with exterior stair-entry, minimal front setbacks, and with small backyards. This creates a significant level of density without high-rises.^{[201][202][203]} In Australia, The 30-Year Plan for Greater Adelaide includes a focus on missing middle housing^[204][205] as does Moreland's Medium Density Housing Review.^[206]

• Affordable housing
• Bicycle-friendly
• Duplex
• Form-based code
• Green building
• New Urbanism
• Stacked triplex
• Starter home
• Streetcar suburb
• Sustainable city
• Traditional neighborhood development
• Urban sprawl
• Zoning codes