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Encouraging Neighborhood-Friendly, Residential Infill Development

Encouraging Neighborhood-Friendly, Residential Infill Development

If your community is experiencing a moderate or high rate of residential development activity, you have probably grappled with the question of how to accommodate the new growth. Or perhaps you want to create a diversity of housing options to accommodate the differing needs and household income levels of your residents.

For communities with lots of vacant land within its boundaries, it is primarily an issue of proper zoning and finding a way to pay for the needed infrastructure. For a city or town that is more built-out, however, the focus often shifts on how to accommodate the new growth within its existing borders.

While upzoning of land is always one tool that can be used by a local government, it is often a very controversial approach due to the public’s concern about the compatibility of multifamily next to single-family homes. As an alternative, several communities are looking at “gentler” methods to encourage residential infill development that is smaller scale and more compatible with adjacent properties.

The Missing Middle

This gap between single-family residences and mid-rise, multifamily development is often referred to as “Missing Middle Housing.” This concept includes accessory dwelling units, duplexes, triplexes, fourplexes, townhouses, and other types of low-scale development that could occur within single-family areas and along the edges of commercial/mixed use areas, thereby serving as a transition between those districts and single-family residential neighborhoods.

But encouraging infill residential development carries its own challenges. It is critical that the reasons for considering actions to encourage this type of development be clearly articulated and explained to the public in order to address citizens’ concerns about changes to their neighborhood. Olympia created a useful Missing Middle Housing webpage, which has links to a lot of information, including numerous photographs and visual examples of hypothetical cases.

It is also very important that infill development regulations be paired with some type of design standards/guidelines in order to ensure that new infill development enhances the physical and visual character of a neighborhood or community.

Taking a phased approach may be appropriate in some cases. For example, Tacoma has established a Residential Infill Pilot Program to test out how regulations will work for a limited number of infill development projects before deciding whether to have them apply citywide.

If you are interested in promoting neighborhood-compatible infill development in your community, the following options are some different approaches to consider.

Accessory Dwelling Units

ADU phot 2

Encouraging construction of accessory dwelling units, sometimes referred to as “ADUs” or “mother-in-law apartments,” is one approach used by several communities to add additional dwelling units in single-family, residential areas. State law (see RCW 43.63A.215 and RCW 36.70A.400) requires that certain cities and counties adopt ordinances to encourage the development of ADUs in single-family zones, but there is some latitude about how to do so.

Attached ADUs — when the extra unit is within or attached to a primary residence — is the most common approach taken by Washington cities, towns, and counties. Detached ADUs, sometimes referred to as “backyard cottages,” are increasingly being allowed by local governments such as Ferndale, Bellingham, and Portland, Oregon.

Many of the issues and details related to the regulation of ADUs may be found in my prior blog post, Accessory Dwelling Units under the Microscope, and MRSC’s publication, Accessory Dwelling Units.

New Houses on Small, Existing Lots


In most communities, there are always some existing, small lots that were created prior to current minimum lot size standards, with the result being that they are now nonconforming lots.

Instead of having those small lots stand vacant, some cities have taken steps to encourage development on them. Portland has been evaluating its standards for infill development, including those on small, skinny lots (see Portland’s Residential Infill Project Summary). Some cities, such as Spokane, allow for housing units on individual lots to be built with their frontage on a private drive, rather than a street, which allows for a more compact development pattern.

Duplexes, Triplexes, and Fourplexes


Many people think of multifamily development as mid-rise buildings of 3-5 stories, with 10-60 dwelling units, but there is a lot of opportunity for smaller-scale, residential development (such as duplexes, triplexes, and fourplexes) to fill in the gaps between this mid-rise type of multifamily development and single-family residential areas.

Good design can soften the visual impact of the extra number of dwelling units. As demonstrated in the photograph above, this is an example of a well-designed, residential fourplex that would visually fit in and be accepted within most single-family neighborhoods. Gig Harbor’s Design Manual includes design standards for duplexes, as does Sammamish’s development regulations (which also address townhouses, cottages, and multifamily dwellings).



In certain parts of the U.S., a large percentage of a city’s multifamily dwelling units are made up of townhouses. Until recently, this housing type has not historically been as popular in Washington State.

Townhouses are usually individual dwelling units that share walls with other residential units but have their own front stoop/porch and backyard, and are usually owner-occupied. To maximize a positive connection with the surrounding neighborhood, it is important that townhouses be oriented towards the public street: For example, having the primary entrance doors face the street and not towards an interior parking area.

SeaTac has some good townhouse regulations.

Cottage Houses


Cottage housing usually consists of a grouping of small, single-family dwelling units clustered around a common outdoor space and developed as part of a master site plan. Early successful examples of this development type in Washington State may be found in the cities of Langley, Shoreline, and Redmond.  Spokane recently amended its development regulations to encourage the development of cottage housing.

Courtyard Housing


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.


Taking steps to increase options for infill development by a local government may sound easy, but that is usually not the case. Neighborhood concerns about density, increased traffic, and parking impacts are likely to be voiced. Development costs for these new types of infill development may be greater than expected, resulting in a low number of units being constructed.

However, if you have want to diversify your housing options and/or accommodate new residents into your community, exploring ways to achieve well-designed infill development is an important task to undertake.

If you are interested in hearing more about this topic, be sure to register for Encouraging Neighborhood-Compatible, Residential Infill Development. This webinar is scheduled for June 21, 2018, from 12:00-1:00 p.m., and will feature the cities of Ferndale and Olympia as case studies.

Questions? Comments?

If you have thoughts about this blog post, please comment below or email me. If you have questions about this or other local government issues, please use our Ask MRSC form or call us at (206) 625-1300 or (800) 933-6772.

MRSC is a private nonprofit organization serving local governments in Washington State. Eligible government agencies in Washington State may use our free, one-on-one Ask MRSC service to get answers to legal, policy, or financial questions.

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About Steve Butler

Steve joined MRSC in February 2015. He has been involved in most aspects of community planning for over 30 years, both in the public and private sectors. He received a B.A. from St. Lawrence University (Canton, New York) and a M.S. in Urban and Regional Planning from the University of Wisconsin-Madison. Steve has served as president of statewide planning associations in both Washington and Maine, and was elected to the American Institute of Certified Planner’s College of Fellows in 2008.