Missing Middle Housing: Duplexes to Townhomes

Key Takeaways:

• Missing middle housing is a range of house-scale, multi-unit building types, such as duplexes, triplexes, fourplexes, townhomes, and courtyard apartments, that fall between detached single-family homes and mid-rise apartment buildings.
• These building types were common in American neighborhoods before World War II, but postwar zoning codes largely made them illegal to build across most residential land.
• Because they add homes on ordinary residential lots and use existing infrastructure, missing middle types can create more attainable options for renters and first-time homebuyers.
• States and cities including Oregon, California, Washington, and Minneapolis have reformed their zoning codes to allow missing middle housing where only single-family homes were once permitted.
• Ivory Innovations supports this work through the Ivory Prize, the Fieldworks implementation program, and resources such as The Little Book of Low-Cost, High-Impact Housing Solutions.

Missing middle housing is not a new idea. It is a return to building types that once made up a large share of American neighborhoods. As communities across the country look for ways to add housing supply without dramatic changes to neighborhood form, these modest multi-unit homes have become one of the most discussed tools in housing policy.

What Is Missing Middle Housing?

Missing middle housing refers to a group of residential building types that contain multiple units but are similar in size and scale to a detached single-family home. The category generally includes duplexes, triplexes, fourplexes, townhomes, courtyard apartments, bungalow courts, and small multiplexes. What unites these types is not a specific unit count but their form: they are typically two to three stories tall, fit on standard residential lots, and blend into neighborhoods of single-family houses.

Why the Term "Missing Middle" Was Coined

The term was introduced in 2010 by architect Daniel Parolek of Opticos Design, who later developed it into a broader framework for missing middle housing. Parolek used the phrase to describe two related gaps. The first is a physical gap in the housing spectrum, since most new residential construction in the United States has been either detached single-family homes or large apartment complexes. The second is a gap in affordability, since the households that these in-between housing types once served, including young families, older adults looking to downsize, and moderate-income workers, are often priced out of both ends of the market.

Where Missing Middle Housing Fits Between Single-Family and Multifamily

On one end of the housing spectrum is the detached single-family home, which occupies its own lot and houses one household. On the other end are mid-rise and high-rise apartment buildings, which can contain dozens or hundreds of units and typically require commercial or high-density residential zoning. Missing middle housing sits between these two poles. A fourplex, for example, may look nearly identical to a large single-family house from the street, but it provides homes for four households instead of one. This combination of modest scale and added capacity is what makes these building types appealing to planners and policymakers seeking to increase supply without large-scale redevelopment.

A Brief History of Missing Middle Housing in America

For much of the late 19th and early 20th centuries, missing middle building types were a normal part of American neighborhoods. Streetcar suburbs in cities such as Chicago, Boston, Philadelphia, and Los Angeles were filled with two-flats, triple-deckers, rowhouses, and bungalow courts, often built on the same blocks as single-family homes. These buildings housed working- and middle-class families close to jobs, shops, and transit.

How Postwar Zoning Phased Out Duplexes and Triplexes

The decline of missing middle housing is closely tied to the rise of exclusionary zoning. After the Supreme Court upheld municipalities' authority to separate land uses and restrict multifamily housing in the 1926 Euclid v. Ambler decision, communities across the country adopted zoning codes that reserved large portions of residential land exclusively for detached single-family homes. Following World War II, federal mortgage underwriting standards and the rapid growth of automobile-oriented suburbs reinforced this pattern. Minimum lot sizes, off-street parking requirements, and density limits made it difficult or impossible to build duplexes, triplexes, and fourplexes in most neighborhoods. As a result, many existing examples of these building types became legally "nonconforming," meaning they could remain but could not be replicated. For more background on how these regulations shaped the built environment, see Ivory Innovations' overview of zoning.

The extent of single-family zoning today remains striking. In Connecticut, for example, 2023 Ivory Prize Winner the National Zoning Atlas found that over 91 percent of the state's land is zoned to allow single-family housing by right, compared to just two percent for multifamily housing.

The Recent Revival of Interest in Missing Middle Types

Over the past decade, rising housing costs and a growing recognition of the national housing shortage have renewed interest in missing middle housing. Planners, researchers, builders, and advocates increasingly view these building types as a practical way to add homes to existing neighborhoods incrementally. Rather than concentrating new development in a small number of high-density districts, missing middle housing allows supply to grow gradually across a much larger share of residential land. This shift has been reflected in a wave of state and local zoning reforms, several of which are discussed later in this article.

The Main Types of Missing Middle Housing

Missing middle housing is best understood as a family of building types rather than a single product. Each type offers different tradeoffs in terms of density, ownership structure, lot requirements, and design.

Duplexes and Triplexes

A duplex contains two dwelling units, arranged either side by side or stacked one above the other, while a triplex contains three. These are among the simplest and most familiar missing middle types. Because they can often be built on a standard single-family lot, duplexes and triplexes are frequently the first building types allowed under zoning reforms. They also lend themselves to owner occupancy, since an owner may live in one unit and rent the others, using rental income to help offset mortgage costs.

Fourplexes and Multiplexes

A fourplex contains four units, often arranged as two units per floor across two stories, and can be designed to resemble a large single-family house. Fourplexes carry particular significance in residential finance because properties with up to four units generally qualify for the same types of residential mortgages as single-family homes, including loans insured by the Federal Housing Administration (FHA) when the buyer occupies one of the units. Small multiplexes, which can range from five units up to roughly a dozen, extend this model further but typically require larger lots and may be financed and regulated as commercial properties.

Townhomes and Rowhomes

Townhomes and rowhomes are attached single-family dwellings that share side walls with neighboring units but each have their own front entrance, and often their own small yard. Unlike duplexes or fourplexes, townhomes are frequently sold individually, making them an important entry point to homeownership. Their narrow footprints allow more homes per acre than detached houses while preserving many of the features buyers associate with single-family living.

Courtyard Apartments and Bungalow Courts

Courtyard apartments arrange multiple units around a shared open space, while bungalow courts, sometimes called cottage courts or cottage clusters, consist of small detached homes grouped around a common green. Bungalow courts became especially popular in Southern California in the early 20th century. These formats create a sense of community through shared outdoor space and can deliver moderate density while maintaining a residential character that fits alongside single-family homes.

Accessory Dwelling Units as a Missing Middle Tool

Accessory dwelling units (ADUs), which include backyard cottages, garage conversions, and basement apartments, are often discussed alongside missing middle housing. While an ADU is technically a secondary unit on a single-family lot rather than a standalone building type, ADU reforms operate on the same principle: allowing more homes on land that was previously limited to one household. For many homeowners, adding an ADU is the most accessible way to participate in gentle density.

Why Missing Middle Housing Matters for Affordability

Missing middle housing is not a guarantee of affordability on its own, but it can meaningfully expand the range of housing options available at moderate price points.

Closing the Gap Between Renting and Single-Family Ownership

In many markets, the price of a detached single-family home has risen well beyond what moderate-income households can afford, while apartment rents continue to consume a growing share of household income. Missing middle housing can help bridge this gap. Townhomes and condominium units within small multi-unit buildings typically cost less than detached homes in the same neighborhood because they use less land per unit. Owner-occupied duplexes, triplexes, and fourplexes provide another pathway, allowing buyers to use rental income from additional units to make ownership financially feasible. For a broader look at these dynamics, see Ivory Innovations' overview of homeownership.

Adding Density Without Changing Neighborhood Character

One of the most common objections to new housing is the concern that it will change the character of an established neighborhood. Missing middle housing addresses this concern directly. Because these buildings are designed at the scale of a house, they can increase the number of homes on a block while maintaining familiar building heights, setbacks, and architectural forms. This approach is often described as "gentle density" and can make new housing more politically viable in communities that would resist larger apartment buildings.

Making Efficient Use of Existing Infrastructure

Missing middle housing is typically built as infill development within existing neighborhoods, where roads, water and sewer lines, schools, and in many cases transit service are already in place. Building on existing infrastructure reduces the public and private costs associated with extending services to new greenfield developments on the urban edge. Infill development and smart density are two of the six strategies featured in The Little Book of Low-Cost, High-Impact Housing Solutions, Ivory Innovations' guide for mayors, planners, and housing advocates.

Barriers to Building Missing Middle Housing

Despite broad interest in missing middle housing, several persistent barriers continue to limit how much of it gets built.

Single-Family Zoning and Land Use Restrictions

The most significant barrier remains zoning. In many jurisdictions, building a duplex or fourplex on a residential lot is simply not permitted. Even where multi-unit housing is technically allowed, related requirements such as minimum lot sizes, maximum lot coverage, floor area ratios, and setback rules can make these projects physically or financially unworkable. Off-street parking minimums present a particular challenge for small sites, where dedicating space to parking can reduce the number of units a lot can accommodate.

Financing Challenges for Small-Scale Developers

Missing middle projects are often too small to attract interest from large developers and institutional lenders but too complex for many individual homeowners to undertake. Small-scale builders frequently struggle to secure construction financing on favorable terms, and projects above four units may fall into commercial lending categories with stricter underwriting requirements. In addition, because missing middle housing has been rare in many markets for decades, appraisers and lenders may lack comparable sales data, making it more difficult to value these projects.

Neighborhood Opposition and Permitting Delays

Even where zoning allows missing middle housing, projects that require discretionary review can face lengthy public hearings and opposition from nearby residents. For small developers, delays are especially costly, since carrying costs for land and financing accumulate while a project waits for approval. Ivory Innovations has examined this issue in The Cost of Time in Development, which highlights how approval timelines directly affect the final cost of housing.

Policy Solutions Expanding Missing Middle Housing

A growing number of state and local governments have adopted reforms designed to make missing middle housing easier to build.

Statewide Upzoning and "Missing Middle" Legislation

The City of Minneapolis became one of the first major US cities to broadly end single-family-only zoning through its Minneapolis 2040 Plan, which allows additional units in areas that previously permitted only single-family homes. In 2019, the State of Oregon passed HB 2001, which requires cities with populations over 10,000 to allow duplexes on all residential lots where single-family homes are allowed. Cities over 25,000 and Portland Metro jurisdictions must also allow triplexes, fourplexes, townhomes, and cottage clusters in those areas. California followed in 2021 with SB 9, which allows homeowners in most single-family zones to build a second unit or split their lot, and Washington adopted HB 1110 in 2023, requiring many cities to permit middle housing types on residential land.

Reduced Parking Requirements Near Transit

Parking reform has become a closely related strategy. By reducing or eliminating minimum parking requirements, particularly near transit, cities free up land that can instead be used for homes. A national movement against parking mandates has led a growing number of municipalities to roll back these requirements, which is especially meaningful for missing middle projects on small lots.

Streamlined Permitting for Small-Scale Developments

Some jurisdictions are reducing approval timelines by allowing missing middle housing "by right," meaning projects that meet objective standards can be approved administratively without discretionary review. Pre-approved building plans offer another path. The LA County Pre-Approved Plans Portal, a 2026 Ivory Prize Finalist, allows homeowners rebuilding after the 2025 Los Angeles fires to select from plans that have already been reviewed, significantly reducing permitting friction. Streamlining development approvals and simplifying design standards are both featured as proven strategies in The Little Book.

Missing Middle Housing Innovators: Ivory Prize Portfolio

Ivory Innovations' annual Ivory Prize for Housing Affordability recognizes pioneering solutions from private-sector companies, nonprofits, and government agencies in the housing sphere. Each year, the Prize recognizes winners in each of three areas of focus: Construction and Design, Policy and Regulatory Reform, and Finance. Over the years since the Prize's inception, a number of organizations in the Ivory Innovations portfolio have advanced the missing middle housing model. These organizations are as follows:

Construction and Design Solutions Expanding Missing Middle Supply

Symbium - Symbium is a computational law platform that mechanizes the rules and regulations of planning codes to help homeowners, design professionals, and planners quickly determine whether an ADU is allowed on a property, what the development standards are, and the processes needed to build these units.

DeepBlocks - DeepBlocks uses technology to reshape development and entitlement decisions through real-time analysis of financial and market data in combination with building regulations, helping identify where additional housing can feasibly be built.

Ivory Innovations also puts these ideas into practice directly. Through Fieldworks, Ivory Innovations integrates market-ready construction innovations into its own new-construction affordable and workforce rental developments in the Salt Lake City metro area, which typically range from six to 100 units.

Policy and Regulatory Reform Advancing Missing Middle Housing

Oregon: HB 2001 - The State of Oregon expanded housing options through HB 2001, which requires cities to allow duplexes, and in larger jurisdictions triplexes, fourplexes, townhomes, and cottage clusters, wherever single-family homes are allowed.

City of Minneapolis - Through the Minneapolis 2040 Plan, the city is upzoning nearly the entire city, allowing more units in areas that previously contained only single-family homes while promoting transit-oriented development and inclusionary zoning.

National Zoning Atlas - The National Zoning Atlas is a collaborative of researchers digitizing, demystifying, and democratizing roughly 30,000 US zoning codes through a user-friendly online resource, illuminating where missing middle housing is and is not allowed.

Buncombe County - North Carolina's Buncombe County supports private and nonprofit developers in providing housing for low- to moderate-income families through zoning and density bonuses for developments that include affordable or workforce units.

Finance Models Supporting Small-Scale Developers

Frolic - Frolic is paving the way for new multifamily housing cooperatives to be built on lots with single-family homes in cities across the US. Their homes require down payments of $10,000 to $30,000, creating opportunities for low-income renters and buyers to enter homeownership.

Ivory Innovations also contributes to finance policy research that affects entry-level housing. In partnership with the Urban Institute, Ivory Innovations leaders have examined how HUD could expand access to small-dollar mortgages, a financing gap that affects buyers of modestly priced homes, including many missing middle units.

Organizations working on missing middle housing, whether through construction methods, policy reform, or financing, are encouraged to apply for the Ivory Prize. Nominations for the 2027 Prize open in September 2026 and close October 30, 2026. Researchers and practitioners can also explore more than 700 related solutions in the Housing Innovation Database.

Frequently Asked Questions About Missing Middle Housing

Is Missing Middle Housing the Same as Affordable Housing?

Not necessarily. "Affordable housing" often refers to income-restricted units subsidized through public programs, while missing middle housing describes a building form. However, because missing middle types use less land per unit and can often be built with lower construction costs than larger buildings, they tend to offer more moderately priced options than detached single-family homes in the same neighborhood.

How Many Units Qualify as Missing Middle Housing?

There is no single legal definition, but missing middle housing generally includes buildings with two to roughly a dozen units that are designed at the scale of a house. Many state reforms focus specifically on two to four units, since those properties typically qualify for residential mortgage financing.

Can Missing Middle Housing Be Built in Any Neighborhood?

That depends on local zoning. In states and cities that have adopted missing middle reforms, such as Oregon and Minneapolis, these building types are allowed on many residential lots. In jurisdictions that still reserve most land for single-family homes, property owners may need a rezoning or variance. Checking with the local planning or zoning department, or reviewing resources like the National Zoning Atlas, is the best place to start.