You are here: Home Making it Easier to Build Green! Code Innovations Blog Accessory Dwelling Units under the Microscope

Accessory Dwelling Units under the Microscope

Accessory dwelling units (ADUs) have been around for decades. In many parts of Washington State, the concept is accepted and local governments have revised their regulations to accommodate such housing. Even so, the number of ADUs created in accordance with local standards has remained relatively low, due in part to the difficulty in meeting those regulations and the associated costs related to them. In response, a few local governments are relooking at their standards and discussing how to make them easier to meet. The potential easing of existing ADU regulations, however, is causing neighborhood homeowners to take notice. Originally published in MRSC Insight Blog, July 21, 2016 by Steve Butler, Policy Director

By Steve Butler, Policy Director.  Municipal Research Services Center.  Originally published in MRSC Insight Blog, July 21, 2016

Accessory dwelling units (ADUs) have been around for decades. In many parts of Washington State, the concept is accepted and local governments have revised their regulations to accommodate such housing. Even so, the number of ADUs created in accordance with local standards has remained relatively low, due in part to the difficulty in meeting those regulations and the associated costs related to them. In response, a few local governments are relooking at their standards and discussing how to make them easier to meet. The potential easing of existing ADU regulations, however, is causing neighborhood homeowners to take notice.

What is an Accessory Dwelling Unit (ADU)?

An accessory dwelling unit (ADU) is a small, self-contained residential unit located on the same lot as an existing single-family home. They are sometimes referred to as "mother-in-law apartments" or "granny flats." An ADU 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.

There are two types of ADUs:

1. Attached ADU, which may be created as either:
    • 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 its own entrance); or
2.  Detached ADU, created in a separate structure on the lot (such as a converted garage or a new “backyard cottage”).

Reasons for Allowing ADUs

[Washington] State law (see RCW 43.63A.215 and 36.70A.400) requires that certain cities and counties adopt ordinances to encourage the development of ADUs in single-family zones by incorporating the model ordinance recommendations prepared by the Washington Department of Commerce. In addition to just meeting a statutory mandate, however, ADUs have also helped local jurisdictions meet their GMA goals related to affordable housing, and provide a variety of housing densities and types, while still preserving the character of single-family neighborhoods. From a planning perspective, it is considered by many to be a “kinder and gentler” method for accommodating population growth in a community, as compared to upzoning land to do so.

Standard ADU Regulations

Most local ADU regulations have standards to address the following issues:
  • Maximum Unit Size
  • Owner-Occupancy
  • Dedicated Off-Street Parking
  • Attached ADUs Only
  • Maximum Number of Dwelling Units on One Lot
  • Separate Entrances /Only One Visible from the Street
  • Other Design Standards (especially for detached ADUs) for such items as roof pitch, window style, and exterior material
  • Maximum Number of Occupants
  • Minimum Lot Size
  • Building Code and Other “Life/Safety” Requirements

Communities Starting to Reconsider Their ADU Requirements

Some local governments in Washington and elsewhere are re-examining their existing ADU requirements and questioning the rationale behind them, especially given the low production of new ADUs. As a result, some communities are considering changes to their ADU regulations, such as:
  • Unit Size: Most current ADU standards set a maximum size (for example, 800 square feet), but some communities are considering an increase to their limit to provide more flexibility.
  • On-site Parking: Some local governments are looking at a reduction or elimination of standards requiring on-site parking spaces for the ADU’s occupants, especially in areas where there is adequate on-street parking. Such a change may face stronger opposition in neighborhoods where street parking is at a premium.
  • Detached ADUs - Most codes only allow attached ADUs, but more communities are expanding their regulations to permit detached ADUs (which are usually required to be placed in the back half of a residential lot). Even if allowed, the high cost of constructing “backyard cottages” may limit the number that actually get built.
  • Owner-Occupancy - Most codes require that the property owner needs to occupy either the primary or accessory unit, but some communities (such as Seattle) are considering removing this requirement.
  • Allowing More than Two Dwelling Units – A cutting-edge regulatory change is to increase the maximum number of dwelling units on a single family lot to three (by allowing one primary dwelling unit, one attached ADU, and one detached ADU). In Seattle, the city council is currently considering proposed code revisions that would include an increase to three units on one lot.

Discussion about these types of changes has caused anxiety for some homeowners, who are concerned about the impacts on neighborhood character and property values. On the other hand, affordable housing advocates consider changing existing regulations as a way to effectively increase the number of legal ADUs.
Regardless of how local governments decide to regulate them, ADUs may be a viable approach to address your community’s growth and affordable housing policies in a manner that is acceptable to your residents (especially if they consider the alternatives). Just be sure your regulations and development review process aren’t so burdensome that property owners end up either not creating these dwelling units or doing so surreptitiously, without obtaining the required permits.

Recommended Resources

Photo courtesy of AccessoryDwellings.org.

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. 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.
Document Actions
  • Print this Print this