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Infill Development

This page provides an overview of infill development for local governments in Washington State, including information on infill development options, examples of plans and ordinances, relevant statutes, and recommended resources.


Infill development is the process of developing vacant or under-utilized parcels within existing urban areas that are already largely developed. Most communities have significant vacant land within their city limits, which, for various reasons, has been passed over in the normal course of urbanization. Communities across the country are increasingly recognizing that the spread-out patterns of growth, which have shaped American communities for the past several decades, cannot be sustained. This combined with the reduced land supply has created new interest in infill development opportunities in central and suburban cities alike.

Successful infill development can increase residential densities to support improved transportation and a wider variety of neighborhood amenities. Infill projects can help to increase the variety of housing types available by promoting small to medium scale housing. This scale of housing, ranging from duplexes to mid-rise apartments, is sometimes referred to as Missing Middle Housing because it is often overlooked in favor of single-family home or high-rise development. Jurisdictions employ a variety of strategies to promote infill development, including addressing barriers to developing on small urban lots, creating infill incentive programs and allowing for a greater variety of housing types. These strategies are often paired with design guidelines to ensure that new infill development matches or enhances the physical character of the community.

More and more communities in Washington are analyzing the capacity of potential infill sites to both accommodate new growth and serve as an alternative to sprawl development.

Types of Infill Development

Infill development may be very small in scale such as a single-family house on a vacant lot within a developed block. It can also occur on a much larger scale such as an entire block or even the redevelopment of hundreds of acres (as in the case of the redevelopment of Denver’s former Stapleton Airport site into the 4,700-acre, mixed-use community of Stapleton). Different types of infill development may be appropriate in different areas depending on the surrounding context and a community's goals for future growth.

The following is a list of different infill development options, focusing primarily on small- to medium-scale infill projects, with more information on each type on the sections below:

  • Residential Development – Provides development options that increase the amount of residential housing options as well as make them potentially more affordable
  • Small-Lot Development – Small-lot infill development is encouraged by designating zones that relax development regulations like minimum lots size and setback requirements.
  • Mixed Use Development – A complementary mix of uses in close proximity such as residential, retail, commercial, employment, civic and entertainment uses - sometimes in the same building.
  • Transit-Oriented Development (TOD) – TOD is an effort to focus residential construction in areas with convenient access to transit options, promoting walkability and use of transit.
  • Brownfield Redevelopment – A brownfield is an abandoned, idled, or underused property where expansion or redevelopment is complicated by the presence or potential presence of contamination. 
  • Greyfield Redevelopment – A greyfield refers to an old, abandoned, or underutilized property used for retail or commercial shopping such as a mall. These properties typically do not involve environmental concerns but can demand the replacement or improvement of old infrastructure.

Residential Development

Infill development serves as a useful way to increase the amount of residential housing options as well as make them potentially more affordable. This can involve adding onto properties that have existing structures on them or building multifamily units on undeveloped land. In either case, diversifying the housing stock with ADUs and missing middle housing options can also address the goals and purpose of transit-orientated and small-lot developments.

Accessory Dwelling Units (ADUs)

An ADU is a small, self-contained residential unit located on the same lot as an existing single-family home. It has all the basic facilities needed for day-to-day living independent of the main home, such as a kitchen, sleeping area, and a bathroom. As the term "accessory" implies, ADUs are generally defined to be smaller in size and prominence than the main residence on the lot. An ADU may be created as a separate unit within an existing home (such as in an attic or basement), an addition to the home (such as a separate apartment unit with separate entrance), or in a separate structure on the lot (such as a converted garage).

For more information, see our page on Accessory Dwelling Units.

Missing Middle Housing

Missing Middle Housing refers to homes that are on the building spectrum between single-family homes and high-density apartment buildings.

For more information, see our page on Missing Middle Housing.

Live-Work Buildings

Live-work spaces 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 great 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 a growing investment option.

For more information, see the section in our Missing Middle Housing page on Live-Work Builidings.

Small-Lot Development

A number of communities have established some small lot zones with reduced minimum lot size requirements, which tend to reduce the cost of land associated with each residence and allow developments with more units per acre. Other local codes provide for attached housing, corner lot duplexes, cottage or clustered housing, or other more flexible arrangements that may prevent potential units from being lost due to constraints, such as a creek, or may allow a small increase in units without giving up a sense of open space.

Examples of Codes

Mixed Use Development

Mixed use development is an important component of successful transit-oriented development, traditional neighborhood development, and smart growth/livable community development schemes. Mixed use developments contain a complementary mix of uses such as residential, retail, commercial, employment, civic and entertainment uses in close proximity - sometimes in the same building. Compatibility issues are addressed through performance standards, transition tools, careful site layout and building design, rather than by separating uses into single use zones.

When a wide variety of uses are located in close proximity to each other, walking and cycling become practical means of travel. For mixed use development to succeed, varied land uses must be within convenience walking distance of each other (one quarter mile, 5-10 minutes) and there must be direct, safe, and convenient connections between the uses. Residents in mixed use developments can take care of many daily needs without having to drive elsewhere. Mixed use development allows convenient access between work, home and other uses and services. In addition, mixed use development can contribute vitality and interest for residents, additional customers for neighborhood businesses, and a variety of housing choices.

Examples of Design Manuals, Best Practices, and Reports

Examples of Codes

Transit-Oriented Development

TOD is designed to increase the number of residents, employees, and potential transit riders that have convenient access to transit. A complementary mix of uses, activities, and services located in close proximity to each other allow TOD residents to commute to work, run errands, recreate, and meet basic needs without needing a car. A variety of moderate and higher density housing options located within easy walking distance from a centrally-located transit station or transit corridor (about 1/4 mile, 10 minutes) are typically a part of the mix.

For more information, see our page on Transit-Oriented Development.

Brownfield Redevelopment

This type of development may be difficult to institute because of the need for environmental assessment and the potential liability issues involved. A property owner may find it more advantageous to leave the property abandoned because development or sale may require a costly clean-up or spur a lawsuit—but this is changing. State and federal environmental laws and policies over the past few years have addressed some of these issues and are now focusing on how to turn brownfields into opportunities for investment.

In 2013, the Washington State Legislature made significant changes in the Model Toxics Control Act (MTCA) to facilitate clean up and redevelopment of brownfields. Changes of interest to local government include providing for model remedies to facilitate development of low risk sites, and granting authority for local governments to establish redevelopment opportunity zones where additional tools and resources, such as a revolving loan fund, will be available. An example of effective implementation of a local government brownfield development program can be seen with Spokane’s Brownfields Program. Notably, Spokane is home to Washington’s first and third Brownfield Redevelopment Opportunity Zones (ROZ).

Laws and Regulations

Greyfield Redevelopment

A greyfield refers to an old, abandoned, or underutilized property used for retail or commercial shopping such as a mall. Developing these properties typically does not involve the environmental concerns seen with brownfield development but can demand the replacement or improvement of old infrastructure. Greyfield development can be advantageous largely because the infrastructure (foundation, plumbing, sewerage, electrical) can be salvaged. Greyfields are often developed into mixed use properties that are also transit-oriented.

Examples of Codes


Infill Development Incentives

If communities are to succeed in promoting infill development, they will need to recognize and overcome impediments to such development. Neighborhood opposition, financing challenges, inflexible building codes and development regulations, lengthy permit processes, substandard infrastructure, and other conditions may need to be addressed to attract infill development.

This section provides examples of how cities in Washington and elsewhere are changing their policies and offering incentives to encourage infill development.

Examples of Incentives to Facilitate Infill Development

Washington State

Outside Washington State

Examples of General Infill Codes

Relevant Statutes

  • Ch. 36.70A RCW – The Growth Management Act
  • RCW 43.21C.299 – Infill development - Categorical exemptions from chapter (SEPA)
  • Ch. 35.100 RCW – Downtown and neighborhood commercial districts (sales and use tax increment financing)

Recommended Resources

Last Modified: January 24, 2023