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Missing Middle Housing

This page provides an overview of housing types that are considered part of the "Missing Middle Housing" in Washington State, including descriptions and examples of local codes defining and regulating each housing type.

What is Missing Middle Housing?

"Missing Middle Housing" refers to homes that are on the building spectrum (or "in the middle") between single-family homes and high-density apartment buildings. In the past, it was sometimes referred to as "infill housing." Buildings such as duplexes and townhouses contribute to the diversity of housing options both in form and affordability. Developing Missing Middle Housing increases the housing stock while catering to a variety of different demographic groups, including millennials and multigenerational households.

missing middle housing diagram

Missing Middle Housing term created by Daniel Parolek | Image © Opticos Design, Inc. | For more information visit

This page provides information on the following Missing Middle Housing types: duplexes, triplexes, and fourplexes, cottage housing, townhouses/rowhouses, courtyard apartments, and live/work buildings.

Middle Housing Statutory Requirements in Washington

In 2023, E2SHB 1110 was passed and took effect in July of the same year. This bill updated the Growth Management Act (GMA) mandating that certain GMA planning cities include middle housing types at minimum densities within six months after the periodic update due date. It also requires that specific provisions be made for middle housing in their development regulations. See RCW 36.70A.635.

Below is a table, created and published by Commerce in their Fact Sheet for Implementing E2SHB 1110 (2023), that shows the basic requirements for cities.

  Minimum number of middle housing units that must be allowed per lot in predominately residential zones NEAR A MAJOR TRANSIT STOP: Minimum number of middle housing units that must be allowed per lot within 1/4 mile walking distance of major transit stop in predominately residential zones WITH AFFORDABLE HOUSING: Minimum number of middle housing units that must be allowed per lot with affordable housing in predominately residential zones where density in applicable zone does not otherwise allow this number (See also HB 1110, Sec. 3(2)
TIER ONE: Cities with population of at least 75,000 (E2SHB 1110, Sec. 3(1)(b)) 4 du/lot, unless zoning permits higher densities 6 du/ lot, unless zoning permits higher densities 6 du/lot if at least 2 units are affordable, unless zoning permits higher densities
TIER TWO: Cities with population of at least 25,000 but less than 75,000 (E2SHB 1110, Sec. 3(1)(a)) 2 du/lot unless zoning permits higher densities 4 du/lot, unless zoning permits higher densities 4 du/lot if at least 1 unit is affordable, unless zoning permits higher densities
TIER THREE: Cities with population under 25,000 that are contiguous with a UGA that includes the largest city in a county with a population over 275,000 (E2SHB 1110, Sec. 3(1)(c)) 2 du/lot, unless zoning permits higher densities N/A N/A

Under the 2023 statutory requirements, the Department of Commerce is to provide technical assistance to local governments when they are in the process of creating and adopting new regulations (RCW 36.70A.636). The new legislation also includes a number of other requirements, including design review, SEPA exemption, and certain specific parking requirements.

Resources from the Department of Commerce

Duplexes, Triplexes, and Fourplexes

Duplexes, Triplexes, and Fourplexes are multi-family homes that have two, three, or four units, respectively, in one building. Units can be side-by-side or stacked on top of each other. Each unit has a separate entrance and complete living spaces. Properties are often kept by one owner but occupied by two or more households. These options can be appealing to a number of different household types, including the growing number of multigenerational households in Washington as a way to stay close to older family members who may need help caring for themselves and young families looking for a less expensive housing option. It is notable to mention that each of these housing types may be subject to different zoning laws, and local codes and regulations may vary as to what is considered a multifamily development.

triplex and duplex housing examples

Image credits (triplex and duplex): © Opticos Design, Inc.

fourplex housing example

Image credit (fourplex): Steve Butler, MRSC

Examples of Codes

  • Issaquah Municipal Code
    • Sec. 18.102.080 – “Duplex” means a building, located on one legal lot, containing two dwelling units designed exclusively for occupancy by two single households living independently of each other. A single-family dwelling containing an approved ADU is not a duplex.
    • Sec. 18.102.240 – “Triplex” means a multifamily building, which is located on one legal lot, containing three dwelling units designed exclusively for occupancy by three single households living independently of each other.
    • Sec. 18.102.100 – “Fourplex” means a multifamily building, located on one legal lot, containing four dwelling units designed exclusively for occupancy by four single households living independently of each other.
  • Port Orchard Municipal Code Sec. 20.139.055 – Provides side-by-side duplex and attached house design standards
  • Port Townsend Municipal Code Sec. 17.16.010 – Categorizes and regulates duplexes, triplexes, and fourplexes as single-family homes
  • Puyallup Municipal Code Sec. 20.26.100 – Provides duplex and triplex design standards

Cottage Housing

Cottage housing or cottage clusters are groups of smaller detached housing units, typically 800 to 1,200 square feet, which are oriented around a common open space like a courtyard, garden, or walkway. Parking areas for these developments are located on the side or rear of the cottage to emphasize community space. This community-oriented, close knit, and smaller scale housing is frequently used as an infill option for single family neighborhoods since it can match the existing aesthetics while adding more housing units. Cottage housing provides a degree of privacy and some of the benefits of single-family housing, combined with the lower cost and maintenance of attached housing. The clustered arrangement can also contribute to a strong sense of community within the cottage housing site itself. Although these small units can offer its owners a quality living experience that is less expensive than traditional single-family housing, they can, depending on the neighborhood, be not as affordable as other Missing Middle Housing options.

cottage housing example

Image credit: HUD Kirkland Case Study/Wenzlau Architects

Examples of Codes


Townhouses / Rowhouses

Rowhouses are attached single family homes that generally include a backyard and one-to-three stories of living space. This type of home is sometimes viewed as being distinct from townhomes because rowhouses are not typically set back from the sidewalk, when townhomes can be. However, most municipalities will use the terms interchangeably and have regulated them in the same manner.

Rowhouses tend to vary drastically in size, from hundreds to thousands of square feet. Many, due to their size, luxury features or locations, tend to not be affordable for households making less than 80% of the Area Median Income (AMI). However, there is a trend of seeing smaller townhomes at more affordable price levels.

rowhouses housing example

Image credits: City of Shoreline

Examples of Codes

  • Redmond Municipal Code Sec. 20A.20.190 – “Single-Family Dwelling Unit, Attached. A building designed for occupancy by one family on an individually owned lot where the building abuts one or more property lines and shares a common wall with an adjacent dwelling unit(s), also known as a ’row house’ or ’townhouse.’ For transportation impact fee, trip generation, and concurrency purposes, a townhouse use will be considered as a multi-family use. (Ord. 2482)”
  • Chelan Municipal Code Ch. 17.14 – Sets design standards for townhouses, showing good and bad examples of design
  • SeaTac Municipal Code Ch. 15.505 – Contains a detailed set of design standards for both townhouses and duplex development
  • Wenatchee Municipal Code Sec. 10.47.130 – Regulations for attached single-family dwellings

Courtyard Apartments

Some cities allow the development of “courtyard apartments,” which consist of several attached dwelling units (either rentals or owner-occupied) arranged on two or three sides of a central courtyard or lawn area. This type of housing is usually one or two stories in height and sometimes serves as a buffer between arterial roadways and single-family neighborhoods. Each unit should have access to the courtyard area.

courtyard apartment housing example

Image credits: Steve Butler, MRSC (left photograph), © Opticos Design, Inc.(right photograph)

Examples of Codes


Live-Work Buildings

Live-work spaces typically contain one or more dwelling units attached or detached from a non-residential space. The flexible non-residential or workspace may have a taller height and a shopfront frontage. Live-work options are good solutions for connecting jobs and housing for improving density and facilitating economic activity. As more people decide to permanently work from home, live-work buildings could prove to be an increasingly attractive investment option.

live-work building housing examples

Image credits: City of Tukwila

Examples of Codes


Recommended Resources

Last Modified: February 23, 2024