Housing Policy Explainer

Missing Middle Housing Typology Terms

Land use reforms, modest densification, and missing middle housing strategies are introducing more terms into the housing policy discussion. Some of these land use terms, such as the “-plex,” butt up against existing terminology building code language on attached dwellings. This whitepaper breaks down the sometimes confusing terms and housing types. 

Attached vs. Detached 

No matter the housing type and the number of lots and units, the central question is whether it is attached or detached housing.

Attached Housing is any unit that is physically connected to another unit. This includes twin homes, duplexes, townhomes, and any so-called “-plexes” being legalized as a part of missing middle strategies. It can also include so-called “in-law suites” or “granny flats. ”Modern building codes prescribe fire separation for attached units.

Detached Housing is any unit that is not physically connected to another structure. This includes traditional single-family homes and accessory dwelling units. Terms such as “detached townhomes” have been used to describe detached housing with smaller setbacks. 


Twin Home vs. Duplex

Twin homes and duplexes are both examples of attached housing with two units, but they are not the same thing. 

Twin Homes are two attached units on two separate lots with separate utility services running to each property. Each lot is recorded as a separate property, with the units butting up to one another on the lot line. 

Duplexes are two attached units on the same lot. These are generally, but not always, stacked on top of one another. 

Note: An accessory dwelling unit on the same lot as another structure is not a duplex, as this is a second, detached structure. 


Townhome vs. Rowhouse vs. -Plex

Townhomes are attached single-family housing units on individual platted lots. A homeowners association is often required to maintain common property, including roofs. Like twin homes, each unit will have its own lot and utility service coming from the street. Some building codes may treat twin homes as two-unit townhomes.

-Plexes (tri-plex, four-plex, etc.) are attached housing units on a single lot. Unlike townhomes, there is a single utility service entering the structure, which is then parsed into individual units. 

Rowhouses are similar to townhomes in that they are attached homes on individual lots. The difference is that they are built more as individual structures, with varying heights, articulation, and materials. In this way, it is treated more like a single-family detached home and does not have common property as each unit has separate (but adjoining) exterior walls and roofs. 


ADU vs. “Granny Flat”

An accessory dwelling unit (ADU) is a secondary structure, detached, housing another dwelling unit. This includes what are also known as “tiny homes” but also a second unit above a detached garage (often referred to as a carriage house).

An “in-law suite” or “granny flat” is when a home includes a smaller attached unit. Local planning and zoning controls will state if this second unit classifies as a duplex or a “suite.” Sometimes this type of housing will include passage between units.


Multi-Unit Occupancy Types

Occupancy type is a growing part of the multifamily discussion when it comes to affordability and access, as many types include what can be considered “common property” shared by all occupants.

Owner Occupied: The property owner lives in one unit and rents the remaining units to other parties. The property owner is responsible for all common property. 

Rental: This is a typical apartment building in which all units are rented to other parties. The property owner is responsible for all common property. 

Common Interest Communities: Condominiums (condos) fall under the common interest community model of housing. Unlike apartments, these are multifamily buildings that are ownership-based. Owners purchase individual units within a larger structure and a homeowners association manages any shared property.

Cooperative Housing: Often referred to as “co-op housing,” this is a residential model where residents collectively own and manage the property, typically through a nonprofit corporation. In a co-op, members purchase shares in the corporation, granting them the right to occupy a unit within the property and participate in decision-making processes regarding its governance and maintenance.

Co-ops vs. Condos: In co-ops, decisions about the property are typically made by the cooperative’s board of directors, elected by the members, while condo owners typically have more autonomy over their units, and decisions are made collectively through the condo’s homeowners association, often with the assistance of a property management company.