In Washington State, the size of an Accessory Dwelling Unit (ADU) is shaped by a mix of state directives and local zoning rules. While state law aims to standardize access to ADUs, cities and counties retain authority over maximum square footage, height, and setbacks. This article outlines how big an ADU can be, what factors influence size, and how to verify limits in a specific jurisdiction.
Understanding Washington State Rules
Washington has progressively encouraged ADUs as a housing option by requiring jurisdictions to allow and permit them with clear standards. State guidance focuses on streamlining approvals, permitting, and ensuring ADUs are accessory to a primary residence. However, the state does not impose a single statewide cap on ADU size. Instead, size limits are generally determined by local zoning codes, variances, and overlay districts managed by cities and counties.
Key takeaway: plan for local limits, not a universal statewide maximum. Prospective builders should consult the specific city or county code to determine exact square footage allowances, maximum height, and setback requirements for attached and detached ADUs.
Typical Size Limits In Practice
Across Washington, common patterns emerge in ADU sizing. While values vary, several defaults appear frequently in municipal codes and rural jurisdictions:
- Detached ADUs: Common maximums range from 800 to 1,200 square feet. Some jurisdictions cap at 1,000 square feet to maintain scale with the primary dwelling and lot size.
- Attached ADUs: Often limited to a percentage of the main dwelling or a fixed square foot cap, frequently 50% of the primary home’s size or up to 1,000–1,200 square feet, whichever is smaller.
- Height limits: Many areas cap ADU height around 16 to 18 feet for detached units and 12 to 18 feet for attached units, depending on roof form and story count.
- Story count: Most jurisdictions allow at least one story for detached ADUs; some permit two-story designs with stricter setbacks or design review.
Because these ranges are common but not universal, a specific city like Seattle, Spokane, or Bellingham may deviate. Rural counties sometimes allow larger footprints on larger lots, while dense urban areas may impose stricter limits to preserve neighborhood character.
Factors That Affect ADU Size
Several elements influence the permissible size of an ADU beyond a simple square footage cap:
- Lot size and zoning district: Larger lots in rural or suburban zones often permit bigger ADUs, while urban zones may emphasize compact designs.
- Setbacks and coverage limits: Rules on setbacks, lot coverage (the portion of the lot that can be built upon), and impervious surface can constrain total square footage.
- Housing type and design standards: Some jurisdictions segregate ADUs by design style, parking requirements, and energy standards, which can indirectly affect viable size.
- Affordability and density targets: State and local incentives or mandates aimed at increasing housing supply may ease size restrictions in certain districts.
- Historical or neighborhood overlays: Historic districts or overlay zones may impose additional dimensional constraints that limit ADU footprint or height.
In practice, developers should review zoning maps, dimensional tables, and any overlay provisions to understand how these factors shape the largest feasible ADU for a given parcel.
Planning and Permitting Process
Understanding the permitting workflow helps ensure the chosen ADU size complies with local standards. The typical sequence includes:
- Pre-application review: Some jurisdictions offer early feedback on proposed ADU size, design, and site placement.
- Site plan and design: Submittals usually require floor plans, elevations, and site plans showing setbacks, driveways, and utility connections.
- Compliance checks: Plan reviewers verify adherence to maximum size, height limits, and lot coverage.
- Permitting and inspections: After approval, construction permits are issued, with inspections at key stages (framing, electrical, final).
Tip: Gather property documents (survey, plat map), existing home square footage, and any neighborhood design guidelines before applying. This data helps determine the most practical and compliant ADU size upfront.
Practical Examples By City
To illustrate, here are typical patterns observed across several Washington jurisdictions. Note that exact figures can change with code updates, so always confirm with the local permitting department.
- Seattle: Commonly allows an attached ADU up to about 1,000–1,200 sq ft or 50% of the primary dwelling, depending on lot size and zoning. Detached ADUs often cap around 1,000–1,200 sq ft, with height and setback rules shaping design feasibility.
- Spokane: Many residential zones permit detached ADUs near or below 1,200 sq ft, with favorable processing for compliant sizes to encourage ADU construction.
- Tacoma: Similar ranges to Seattle, with attached ADUs frequently limited to a portion of the main house and detached units often capped around 1,000–1,200 sq ft.
- Bellevue and Redmond (Eastside): Suburban cities typically enforce caps near 1,000–1,200 sq ft for detached ADUs, and similar or slightly smaller limits for attached ADUs, subject to lot size and neighborhood character considerations.
These examples show a common theme: while a few jurisdictions push toward 1,200 sq ft, many prefer 800–1,000 sq ft for practical scale on standard city lots. Always check the current code for the exact limit in the parcel’s zoning district.
How To Verify Your Parcel’s ADU Size
For homeowners and builders, the most reliable method to confirm true size limits is direct consultation with the local planning or permitting department. Steps include:
- Identify the parcel’s zoning district using the city’s GIS map or zoning code index.
- Review the dimensional table for ADUs, noting maximum square footage, height, setbacks, and lot coverage.
- Ask about any overlays (historic, urban villages, or environmental overlays) that may affect size or design.
- Discuss recent code updates or amendments that could alter ADU allowances.
Investing time in this due diligence prevents design changes or permit delays later in the process. A precise understanding of local size limits helps optimize the ADU’s layout while staying compliant.