Accessory Dwelling Units Legal Essentials for Property Owners
Unlock your property’s potential with legal guidance tailored to Washington’s evolving ADU laws.
Accessory Dwelling Units (ADUs) are gaining ground in Washington as a flexible solution to the state’s housing shortage. Recent legislation has eased restrictions on these secondary housing units, allowing for greater use of both attached and detached structures on residential lots. This article outlines the legal definition of ADUs, explains the scope and timeline of new state regulations, and highlights the key zoning, design, and occupancy rules that property owners need to understand. It also explores the practical benefits of building an ADU—from increased rental potential to housing family members—and addresses how changing local planning requirements may affect your ability to proceed.
What Is an ADU?
An Accessory Dwelling Unit, or ADU, is a secondary residential structure located on the same lot as a primary home. Under Washington law, ADUs must be self-contained living spaces, meaning they include a place to sleep, a kitchen, and a bathroom. These units come in two primary forms: attached ADUs, such as basement apartments or units built above garages, and detached ADUs (DADUs), such as standalone cottages in a backyard. While they offer additional housing, ADUs do so without altering the single-family classification of the property.
New State Law Opens Doors for ADU Development
In response to the state’s housing crisis, Washington enacted legislation in 2023 that significantly broadens the ability of homeowners to develop ADUs. The law mandates that most cities and counties allow up to two ADUs per residential lot, regardless of whether the units are attached, detached, or conversions of existing structures. Notably, the law permits owners to live off-site and even allows ADUs to be sold separately in some circumstances.
However, the timeline for implementation varies. Local jurisdictions are required to enforce the new rules only after they complete their next scheduled comprehensive plan update. These updates are staggered across the state, with some jurisdictions not due until 2027. This phased rollout means that property owners must still consider local policies and planning timelines when assessing their ADU options.
Check out RCW 36.70A.680-2 for more details.
Legal Highlights for Property Owners
The state law brings several important legal changes. It prevents cities and counties from applying more stringent design standards to ADUs than they apply to primary residences. It also requires jurisdictions to allow ADUs up to 1,000 square feet in size if requested by the property owner, and it modifies off-street parking requirements, particularly for properties near public transit. While local governments retain some discretion, the state law aims to reduce regulatory inconsistency and make ADU development more predictable across Washington.
Why Consider Building an ADU?
- More Affordable Housing: ADUs create lower-cost housing options within established neighborhoods.
- Multi-Generational Living: Ideal for aging parents, young adults, or caregivers who want proximity without shared living space.
- Rental Income: Offers potential for long-term or short-term rental revenue.
- Environmental Sustainability: Smaller footprints and energy-efficient construction can reduce utility usage.
- Neighborhood Diversity: ADUs increase the variety of housing types, which helps address historical inequities in land use planning.
Is An ADU Right For You?
For Washington property owners, ADUs present both opportunities and responsibilities. From zoning compliance to navigating local implementation timelines, the process can involve several legal and practical hurdles. Whether you’re seeking to house a family member, generate rental income, or add long-term value to your property, understanding your rights and obligations under state and local laws is critical. To ensure your project moves forward smoothly and legally, schedule a consultation with Cascade Counsel and get the tailored guidance you need to make your ADU plans a success.
