ADU Knowledge
January 27, 2024

Washington State ADU Regulations: A Comprehensive Guide

Argi Avetisyan
Argi Avetisyan
Gather ADU expert
Washington State ADU Regulations: A Comprehensive Guide
Not sure where to start with your ADU project?

Schedule a free appointment with one of our ADU experts.

Book a call

In recent years, Accessory Dwelling Units (ADUs) have gained popularity as a solution to the housing crisis and a means for homeowners to maximize their property's potential. If you're a homeowner in Washington State, understanding the regulations surrounding ADUs is crucial. In this comprehensive guide, we'll delve into the Washington State ADU regulations, explore their benefits, discuss the permitting process, financing options, and provide real-life case studies to inspire you. Let's dive in!


Before we delve into the regulations, let's establish a clear understanding of what ADUs are. Accessory Dwelling Units are secondary housing units located on the same lot as the primary residential unit. They can be attached, detached, or even conversions of existing structures such as garages or basements. ADUs offer a range of benefits, including increased housing options, the potential for rental income, and the ability to accommodate multigenerational living.

Benefits of ADUs in Washington State

Washington State's ADU regulations provide homeowners with several advantages. First and foremost, ADUs offer increased housing options, allowing homeowners to provide affordable housing to family members, friends, or tenants. Additionally, ADUs can generate rental income, helping homeowners offset their mortgage payments or supplement their income. Lastly, ADUs facilitate multigenerational living, allowing families to live together while maintaining privacy and independence.

Overview of Washington State ADU Regulations

To navigate the world of ADUs in Washington State, it's essential to understand the regulations they entail. Key aspects of these regulations include zoning requirements, size and design restrictions, and parking and setback regulations. Zoning requirements determine whether ADUs are allowed in specific areas, while size and design restrictions dictate the maximum size and appearance of ADUs. Parking and setback regulations ensure that ADUs comply with local standards for parking availability and distance from property lines.

Accessory Dwelling Units in Washington State

According to state law (RCW 36.70A.680 and 36.70A.681), all local governments planning under the Growth Management Act (GMA), including counties (unincorporated urban growth areas) and cities, are required to revise their regulations within six months after their periodic update due date. This revision is necessary to conform with specific requirements, regardless of population size.

  • Minimum ADU requirement: In all GMA urban growth areas, a minimum of two ADUs per lot should be allowed, in addition to the principal unit, for lots meeting the required minimum size. Local regulations should permit ADUs to be attached, detached, or a combination of both. Furthermore, conversion of an existing structure, such as a detached garage, must be allowed.
  • Maximum ADU size: Local governments may not mandate ADUs to be smaller than 1,000 gross square feet.
  • Dimensional standards: A local government may not impose setback requirements, yard coverage limits, tree retention mandates, or restrictions on entry door location that are more restrictive than those required for the principal unit.
  • Street improvements: A local government may not demand street improvements as a condition of permitting accessory dwelling units.
  • Owner occupancy: A local government may not insist on owner occupancy for a principal unit or ADUs.
  • Design review: Local governments cannot enforce aesthetic standards or design review requirements that are stricter for ADUs than for principal units.
  • Required parking: Restrictions apply to the amount of on-site parking that can be mandated, with a sliding scale for smaller lots. No on-site parking standards can be imposed on ADUs within a half-mile of a major transit stop.
  • Impact fees: Impact fees for ADUs are capped at 50% of those assessed to the principal housing unit.
  • Common Interest Communities: New “Common Interest Communities” (e.g., a new subdivision with a homeowners association) are prohibited from adopting covenants, conditions, and restrictions (known as “CC&Rs”) that would restrict the construction of ADUs on any lot. The new law does not affect existing CC&Rs, which can remain in effect.
adu washington state

Understanding the Permitting Process

Obtaining permits is a crucial step in building an ADU in Washington State. The permitting process involves several steps, including obtaining the necessary documentation, submitting applications, and obtaining approvals. However, it's important to note that navigating the permitting process can be challenging. Potential challenges may arise due to local regulations, neighborhood concerns, or specific property requirements. Overcoming these challenges requires patience, persistence, and a thorough understanding of local regulations.

Financing Options for ADUs

Financing an ADU project is an important consideration for homeowners. Fortunately, there are various financing options available. Homeowners can explore grants and incentives offered by local governments or non-profit organizations to offset construction costs. Additionally, financing alternatives such as home equity loans or construction loans can provide the necessary funds for building an ADU. It's crucial to explore these options and select the one that best fits your financial situation.

Backyard cottages and in-law suites

Backyard cottages and in-law suites are gaining popularity in certain parts of Washington, with the increasing interest offering insight into the potential impact of a new statewide ADU law. According to Bertolet, while Seattle's ADU growth may not be typical for most Washington cities, it does indicate a widespread appeal. Following the relaxation of ADU regulations in Seattle, the number of building permits surged, with 768 units approved in 2021 and 959 in 2022. Similarly, Spokane experienced an increase in ADU construction after easing restrictions, with 40 permits approved in 2021 and 51 in 2022. Last year, Spokane lifted some restrictions on ADUs, including the requirement for owners to live in the main house and parking mandates, while also expanding the allowable size of ADUs.


Understanding the regulations surrounding ADUs is essential for homeowners in Washington State. ADUs offer a range of benefits, from increased housing options to rental income potential and multigenerational living. By following the comprehensive guide outlined in this blog post, you'll be well-equipped to navigate the regulations, secure the necessary permits, explore financing options, and embark on a successful ADU project. Now is the time to leverage the opportunities provided by ADUs and make a positive impact on your property and community. Happy building!

gather adu team

Not sure where to start with your ADU project?


Not sure where to start with your ADU project?

Book a free call