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Infill Housing Approaches: Targeting the Missing Middle and Accessory Dwellings

Greenwood Avenue Cottages. Photo courtesy of The Cottage Company

Affordable housing is a major problem faced by communities around Washington State. Exacerbating this challenging situation is the fact that the demand for housing will increase in the years ahead: Preliminary projections from the Washington State Department of Commerce (Commerce) indicate a total need for nearly one million new housing units across the state over the next 23 years. 

Increasing housing supply by allowing a greater variety of housing types is one of several solutions being pursued. While Missing Middle Housing (MMH) and Accessory Dwelling Units (ADU) are not the entire answer to our housing crisis, they are two viable options for Washington local governments looking to increase their local housing supply.

On September 27, 2022, MRSC hosted a webinar highlighting the efforts taken by the cities of Bellingham, Kirkland, Olympia, Spokane, and Wenatchee to encourage MMH (register to view it at our On-Demand Webinars). This blog will highlight efforts raised in the webinar, along with those from other Washington local governments, and is a part 2 companion to my recent MMH-related blog.

A Refresher on MMH and ADUs

The term Missing Middle Housing was created by Daniel Parolek, a co-founder and principal with Opticos Design, Inc., based in Berkeley, CA. MMH is viewed as being able to fill the gap between single-family residences and mid-rise, multi-family development found in many communities, and it includes the following housing types: (1) duplexes; (2) triplexes; (3) fourplexes (and up); (4) townhouses; (5) cottage housing; (6) courtyard housing; and (7) live/work units.

Another type of residential infill development is an Accessory Dwelling Unit, which is a small, self-contained residential unit located on the same lot as an existing single-family home. ADUs can be either attached to a primary residence (for example, an addition to the home or a converted garage) or they can be a detached structure (sometimes referred to as a “backyard cottage”).

While MMH will not address the housing affordability needs of lower income households, these units are typically more affordable to moderate income households (for example, 80-100% area median income, or AMI) than more traditional single-family housing units. ADUs, on the other hand, are usually more affordable (e.g., at 50-80% AMI, and sometimes even lower) than MMH units.

State Law and “Safe Harbor” Provisions

For almost 30 years, state law has required many Washington cities and counties to adopt ordinances encouraging the development of ADUs in single-family zones (see RCW 43.63A.215 and RCW 36.70A.400).

More recently, a new MMH-oriented statute was enacted that encourages, but does not require, cities to increase their residential building capacity by authorizing “at least one duplex, triplex, quadplex, sixplex, stacked flat, townhouse, or courtyard apartment on each parcel in one or more zoning districts that permit single-family residences…” (see RCW 36.70A.600 subsection (1)(c)) or “…on one or more parcels for which they are not currently authorized” (see RCW 36.70A.600 subsect. (1)(d)).

In exchange for adopting ordinances, regulations, and other related actions described in the previous paragraph (along with other activities included in the statute), local governments are provided a “safe harbor” against administrative (SEPA and GMHB) or judicial appeals, per RCW 36.70A.600 subsections (3) and (4).

A few cities, including Olympia, have taken advantage of those “safe harbor” provisions to enact MMH-related regulations (see the Expanding Olympia's Residential Building Capacity Under HB 1923 blog for more information).

Options for Encouraging MMH

Taking specific actions to encourage the construction of MMH has proven to be successful for several Washington cities. As of June 2022, Bellingham had 132 MMH units completed, 310 approved, 197 units with preliminary approval, and 152 units in the application stage. During the same time period, Kirkland had close to 80 building permits issued for MMH units, and Wenatchee has seen a significant increase in townhome construction since they updated their development regulations.

The following are a few steps that some local governments in Washington have taken to encourage MMH.

Change your zoning code and development regulations

Several communities have amended their zoning codes to allow MMH in more locations within their jurisdictional borders. There are two basic steps related to this recommendation. First, revise the zoning code’s use charts to allow the different types of MMH in more zones. Second, make MMH a “permitted” use that is allowed outright in your zoning code’s use charts, and not as a conditional use (see my blog A New Approach for Dealing with Conditional Uses in Your Zoning Code for more details).

Spokane and Tacoma are two examples of cities that took this action. After lots of internal study and an extensive public engagement process, Tacoma adopted a permanent set of zoning changes. Spokane, however, took the approach of adopting its MMH ordinance on an interim, or “pilot program” basis“. This option will allow Spokane to assess how its MMH regulations are working before making a final decision.

Of course, be sure that any planned zoning map (and development regulation) revisions are consistent with your adopted Comprehensive Plan. If there are any inconsistencies, however, they can be addressed during an annual “no more than once per year” comprehensive plan amendment process.

In addition to changing your zoning code’s use charts as described above, another step would be to review and revise your other development requirements to address some specific issues related to the different types of MMH. For example, Spokane’s MMH-related interim regulations included changes to the following city standards:

  • Front yard setbacks,
  • Landscaping,
  • Maximum impervious pavement,
  • Outdoor areas,
  • Screening, and
  • On-site parking requirements.

Focus on design

Having a strong set of design standards is a very important step to making sure that new MMH is compatible with your community’s vision, which will increase the potential for community acceptance of this type of housing. It is recommended that you adopt your design regulations as standards that require good design, and not as voluntary “guidelines.” Wenatchee’s design review requirements have been positively received by the local development community.

Sometimes, it is the small details that can turn a development’s design from good to great. Spokane’s design standards address the following issues:

  • Placement of entrance locations,
  • Building articulation, and
  • Garage design (i.e., the garage cannot be more than 50% of the front façade and must be set back at least two-foot from the front façade).

Revise your permitting procedures

It is important that your development project review procedures for MMH proposals (and other types of development proposals) be clear and easy to understand, which will help both applicants and permit staff successfully navigate the development review process.

Some specific suggestions include: (a) allowing MMH to undergo administrative review as much as possible (and not designate them as “conditional uses”); and (b) streamlining the design review process so that it doesn’t unnecessarily slow down the approval process. Bellingham’s Infill Housing Toolkit is an excellent example of a user-friendly guidebook. The City of Wenatchee is considering creation of pre-approved plans for some MMH types to streamline the approval process.

Consider financial incentives

There are some financial incentives that may be applicable for MMH development, especially those with units affordable to lower-income households. One option to consider is the Multi-Family Tax Exemption (MFTE) program. State law limits outright reductions or waivers on impact fees (unless for housing that will remain affordable at set AMI levels for a minimum number of years) while other local fees and charges (such as utility connection charges) may potentially be reduced or waived only if the utility usage by a specific housing type is proportionately lower. Check with your legal counsel before undertaking any such financial incentives.

Engage the public

Creating strong messaging that helps frame the policy issues is critical. The reasons for wanting to encourage MMH and ADU development should be clearly articulated and explained to all interested parties. A robust engagement process that involves both housing stakeholders and the general public and allows them to provide perspectives and suggestions regarding a proposed MMH or ADU program is strongly encouraged.

Wenatchee participated in a “Our Valley, Our Future” regional housing survey, which let local decisionmakers know that alternative housing options were desired by a significant number of their constituents.

ADUs as Another Type of Infill Development

While some would say ADUs are not technically a type of MMH, these small housing units are definitely a viable type of infill housing that more communities in Washington State are encouraging. Given their small size, they are also more likely to be affordable to households in the 50-80% AMI category, with attached ADUs (such as converted garages and attics) sometimes being priced to be affordable at 30-50% AMI levels.

As a result, some local governments in Washington have amended their ADU regulations, making them more flexible and permissive. This includes changes in such regulatory categories as:

  • Increasing maximum ADU size,
  • Eliminating the requirement to provide additional on-site parking spaces,
  • Allowing detached ADUs, instead of only permitting attached ADUs, 
  • Eliminating the owner-occupancy requirement (i.e., no longer requiring that a property owner occupy either the primary or accessory dwelling unit),
  • Allowing both a detached and attached ADUs on the same parcel, and
  • Allowing ADUs to be condominiums, where the primary and accessory units are allowed to be owned separately.

Commerce’s MMH Program

Commerce’s Growth Management Services Division has a team of experienced planners (Shane Hope, Dave Osaki, and Joe Tovar) to develop resources and provide technical assistance that encourages new moderate-density, middle housing options within Washington State. Additionally, there are still Middle Housing grant funds available from Commerce for cities within the central Puget Sound region (i.e., within the counties of King, Kitsap, Pierce, and Snohomish). If your community is interested, you should move quickly to contact Commerce staff and apply for those grant funds, since they need to be spent by June 30, 2023. 


Striving to supply an adequate amount of housing is a difficult and complicated endeavor. While infill development is not the complete answer to Washington’s housing crisis, encouraging MMH and expanding options for ADUs are two approaches that local governments should consider taking.

To learn more about ADUs, register for our Accessory Dwelling Units - Innovative Local Government Approaches webinar, scheduled for December 13, 2022, from 1:00-2:30 p.m. It will feature speakers from the cities of Langley, Leavenworth, and Vancouver, as well as from Skagit County.

The author would like to thank the following individuals for their assistance with this blog: Leonard Bauer, City of Olympia; Chris Behee, City of Bellingham; Jacques Colon, City of Tacoma; Spencer Gardner, City of Spokane; Stephen Neuenschwander, City of Wenatchee, and Adam Weinstein, City of Kirkland.

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.

Photo of Steve Butler

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.