ADU Rules in Redmond, Washington

Redmond (population ~75,000) is a King County city on Seattle's Eastside, globally recognized as the world headquarters of Microsoft. The city also serves as a major hub for Nintendo of America and numerous other technology companies. Redmond is bisected by the Sammamish River, and the scenic Sammamish River Trail connects Redmond to Marymoor Park and the broader regional trail system. Lake Sammamish State Park is on the city's eastern edge. Redmond faces extreme housing cost pressures driven by its tech economy — ADUs are a critical tool for adding housing supply on existing residential lots.

Washington State ADU Law: HB 1337 (2023)

Washington HB 1337 (2023) is the foundational statewide ADU law that applies to Redmond. Key requirements:

• Cities must allow up to 2 ADUs per lot — one attached/internal + one detached — by right on any lot with a single-family home
• No owner-occupancy requirement permitted
• No ADU-specific impact fees allowed
• Cities may not require ADUs smaller than 1,000 square feet
• State law limits how restrictive local setback, lot coverage, and height standards for ADUs can be

Washington HB 1110 (2023) accompanies HB 1337, requiring Redmond to allow duplexes and middle housing in residential zones — a significant policy shift for an Eastside city historically dominated by large single-family homes.

Redmond is subject to the Washington Growth Management Act (GMA) and has a comprehensive plan with an Urban Growth Area coordinated with King County.

Redmond Local ADU Code

Redmond's Permit Center and Planning Division administer local zoning and building permits. Local ADU regulations must comply with HB 1337 and may not be more restrictive than state law allows.

Permitted ADU Types

• Detached ADU: A standalone structure in the rear or side yard, separate from the primary home
• Attached ADU: An addition to the primary dwelling sharing at least one wall
• Internal/Converted ADU: Conversion of existing garage, basement, or other interior space

Lots may have up to 2 ADUs (one attached/internal + one detached) simultaneously.

Size Limits

State law prohibits Redmond from requiring ADUs smaller than 1,000 square feet. Confirm current maximum size limits for your zoning district with Redmond's Planning Division.

Setbacks

HB 1337 limits Redmond's ability to impose setbacks that would effectively block ADU construction. Setback requirements vary by zone. Verify current standards with the Permit Center before designing.

No Owner-Occupancy Requirement

State law prohibits owner-occupancy mandates. Redmond may not require property owners to live on-site to rent an ADU.

Critical Areas Ordinance (CAO)

Redmond's CAO protects the Sammamish River and its tributaries (including salmon-bearing Bear Creek and Evans Creek, critical for salmon recovery in Lake Washington), wetlands, frequently flooded areas (the Sammamish River valley has extensive floodplains), steep slopes, landslide hazard areas, and seismically hazardous zones. CAO buffers from streams and wetlands — often 50–150+ feet — can substantially restrict ADU siting. Redmond's GIS mapping system identifies CAO-regulated features. Review CAO constraints early in your project planning.

Lake Sammamish and Shoreline Master Program

Lake Sammamish, on Redmond's eastern edge, is regulated by Washington's Shoreline Master Program (SMP), which extends jurisdiction 200 feet inland from the ordinary high water mark. Properties near the lake require a Shoreline Substantial Development Permit for qualifying projects. Check Redmond's SMP regulations for properties near the lake.

Seismic Hazard

Redmond and the Puget Sound/Lake Washington region face significant earthquake risk from the Cascadia Subduction Zone (CSZ). The Sammamish River valley has alluvial soils susceptible to liquefaction. USGS and Washington DNR seismic hazard maps identify higher-risk areas. ADU foundations in the valley should incorporate geotechnical assessment. All construction must meet Washington State Building Code seismic requirements.

Permit Process

1. Check your property's zoning, CAO constraints, and any shoreline jurisdiction using Redmond's GIS portal
2. Schedule a pre-application meeting with Redmond Planning for complex or constrained sites
3. Prepare construction plans meeting Washington State Building Code requirements
4. Submit permit application at redmond.gov/permits or in person at 15670 NE 85th Street
5. Plan review and permit issuance
6. Construction inspections
7. Final inspection and Certificate of Occupancy

Contact Redmond Permit Center: (425) 556-2700 | redmond.gov/permits

Disclaimer: This guide provides general information based on Washington HB 1337 (2023) and Redmond's municipal code as of April 2026. Zoning regulations change frequently. Always verify current requirements with Redmond's Planning Division before making development decisions. This is not legal advice.