This page provides information about accessory dwelling unit regulations in Washington state.
An accessory dwelling unit or ADU is a self-contained living space within or on the same property as a principal dwelling unit. By definition, ADUs are generally smaller in size and more affordable than traditional homes and blend in well with low and moderate-density development.
What is required in Washington?
Inside urban growth areas, most cities and counties in Washington must:
- Allow two ADUs per residential lot which may be attached, detached, or a combination of both, or conversion of existing structures
- Allow an ADU of at least 1,000 square feet
- Collect no more than 50% of the impact fees charged for the principal unit
- May not require the owner to occupy the property, except if the ADU property is a short-term rental
- Allow sale as independent units
- Modify parking requirements based on distance from a major transit stop and/or lot size
Note: Many counties may also allow one rural detached ADU per lot outside urban growth areas, with certain restrictions.
Guidance materials
- ADU Guidance (PDF) —This guide document aims to assist in the understanding and regulatory requirements of ADUs. The audience is local government staff.
- ADU Checklist for Urban Growth Areas (Word) — A tool that local governments can use to compare their existing code against statutory standards.
- Detached ADU Checklist outside Urban Growth Areas (Word) — A tool that counties can use to review rural detached ADU regulations outside UGAs for consistency with HB 1345.
- Detached Rural ADU Tracking and Reporting Guide (PDF) – A tool for counties that allow detached ADUs outside urban growth areas. Counties can use this tool to track completed detached ADUs, prepare annual reporting to Commerce, and support comprehensive plan accounting for detached rural ADU development.
- ADU Building Permit Checklist (Word) — This document is intended to be used when a local jurisdiction, using the state law on ADUs, receives a building permit. This document may be used in conjunction with an existing local checklist, so long as other standards do not conflict with the state statute.
- For preempted/superseded jurisdictions, this customizable poster may be used to help inform your community about any preemption. Note, the fillable text boxes will appear once the document is downloaded. Any section of local regulation which conflicts with state statute RCW 36.70A.680-.681, is preempted:
Thinking of adding one or two ADUs to your property? Check out Commerce’s Accessory dwelling landowner checklist (PDF) for important questions about zoning, permitting, and financing that you should consider.
Property owners should always check with their local planning department for site-specific ADU requirements. Washington cities are using a variety of creative tools to make the ADU development process more accessible, and requirements may vary from one location to another.
Any homeowner considering building and/or renting out an ADU should be aware of the following
- If you rent the ADU, Washington State Landlord and Tenant laws apply.
- You must report net income on tax returns and may deduct portions of property tax, utilities and other fees.
- What to Know About Accessory Dwelling Units when applying for an FHA loan.
Homeowners may benefit from the 3-year property tax exemption
Property owners may apply for a 3-year property tax exemption when making extensive remodeling to a single-family dwelling, including constructing an accessory dwelling unit. Applications should be submitted before the remodeling is complete. The remodel cost must not exceed 30 percent of the assessed value of the structure prior to remodeling.
The application link is here: Exemption for Physical Improvement to a Single-Family Dwelling (PDF)
Homeowners may choose to sell an ADU independently of a principal unit
State law allows for ADUs in many jurisdictions to be owned independently of a principal unit. This option creates benefits for property owners and communities alike. More information on separate sale of ADUs for homeowners is included in the Separate Sale of ADUs fact sheet (PDF).
Senior citizens may benefit from property tax exemptions for their homes and an ADU
As provided in RCW 84.36.383 senior citizens can receive property tax exemptions for their homes and an ADU. This statute applies to taxes levied for collection in 2025 and after. Senior citizens occupying their primary residence can claim this exemption once taxes have been imposed. This can be used for a person who sells, transfers or is displaced from their residence under certain circumstances.
Cities may operate a self-certification program to allow registered architects to submit ADU building permits which meet all building code requirements, per RCW 36.70A.683.
Any city operating a self-certification program must provide a detailed report to Commerce by July 31, 2028 addressing items identified in RCW 36.70A.683(5).
Audits
Cities must audit permits submitted under this program and report to Commerce any architect who failed an audit. Architects failing an audit will be identified here as those under a permanent prohibition on participation in a self-certification program.
At this time, no architects have been reported to Commerce.
There are several ways a local jurisdiction can support ADU development, including:
- Ensuring that local codes are consistent with state statutes
- Providing preapproved plans, consolidated permits and/or expedited reviews
- Identifying where sewer, water and stormwater facilities are available to support an ADU.
- Defining when an ADU can connect to the principal unit’s sewer, water or other utility connection.
- Clarifying connection requirements and other standards for sale of units – either by condominium or unit lot subdivision processes.
- Consider proportional system development charges for a broader range of ADU sizes
Defining fees and development charges applicable for the full development of the project – no surprise fees in the process.
Local government should also be clear about the local health departments policy for developing ADUs on lots with septic systems or connection requirements for sewer. King County provides an example memo outlining ADUs using on-site sewage systems (PDF).