ADU Rules in Sammamish, Washington

Sammamish (population ~70,000) is a King County city on the Sammamish Plateau, incorporated in 1999 and one of the fastest-growing and most affluent cities in Washington State. Located between Lake Sammamish to the west and the Cascade foothills to the east, Sammamish is a highly desirable residential community with a household income among the highest in the nation. The city is characterized by master-planned neighborhoods, extensive trail systems, and a very high proportion of HOA-governed communities. Sammamish is subject to Washington's statewide ADU reform laws.

Washington State ADU Law: HB 1337 (2023)

Washington HB 1337 (2023) is the foundational statewide ADU law that applies to Sammamish. 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 Sammamish to allow duplexes and middle housing in residential zones — a significant change for a city historically characterized exclusively by large single-family homes.

Sammamish is subject to the Washington Growth Management Act (GMA) and has a comprehensive plan with an Urban Growth Area designated under King County's regional growth framework.

Sammamish Local ADU Code

Sammamish's Community Development Department administers local zoning and building permits. Local ADU regulations must comply with HB 1337.

Permitted ADU Types

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

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

Size Limits

State law prohibits Sammamish from requiring ADUs smaller than 1,000 square feet. Confirm current maximum size limits for your zoning district with Sammamish Community Development.

Setbacks

HB 1337 limits Sammamish's ability to impose setbacks that would prevent reasonable ADU construction. Setback requirements vary by zoning district. Verify current standards with the Permit Center.

No Owner-Occupancy Requirement

State law prohibits owner-occupancy mandates. Sammamish cannot require property owners to live on-site to rent an ADU.

HOA CC&Rs — Critical Consideration in Sammamish

Sammamish has an exceptionally high proportion of properties subject to HOA (Homeowners Association) Covenants, Conditions & Restrictions (CC&Rs). While Washington HB 1337 overrides government restrictions on ADUs, HOA CC&Rs are private contractual agreements, not government regulations, and state law does not automatically override them. Many Sammamish HOA CC&Rs were written decades ago and may:

• Restrict or prohibit detached accessory structures
• Limit the use of property for rental purposes
• Regulate exterior appearance and materials

Always review your HOA CC&Rs before designing or building an ADU. Consult an attorney if your CC&Rs contain provisions that may conflict with your ADU plans. There is an ongoing legal and legislative discussion about whether state ADU laws preempt HOA restrictions — seek current legal advice on this issue.

Critical Areas Ordinance (CAO)

Sammamish's CAO protects numerous streams draining from the Sammamish Plateau to Lake Sammamish and Lake Washington (including Evans Creek, Pine Lake Creek, and others — many salmon-bearing), wetlands, steep slopes on plateau edges, landslide hazard areas, and critical aquifer recharge areas. CAO buffers from streams and wetlands can be 50–200+ feet and significantly restrict ADU siting on affected lots. Sammamish's GIS mapping system identifies CAO-regulated features. Review CAO constraints early in your project.

Lake Sammamish — Shoreline Master Program

Lake Sammamish is regulated by Washington's Shoreline Master Program (SMP), extending shoreline jurisdiction 200 feet inland from the lake's ordinary high water mark. Properties near Lake Sammamish require a Shoreline Substantial Development Permit for qualifying projects. Check whether your property falls within shoreline jurisdiction before planning an ADU.

Seismic Hazard

Sammamish and the Puget Sound/Lake Washington region face significant earthquake risk from the Cascadia Subduction Zone (CSZ). The plateau has generally good soils, but stream valleys and lower elevation areas near the lake can have softer materials. All ADU construction must meet Washington State Building Code seismic requirements.

Permit Process

1. Check your property's zoning, CAO constraints, and shoreline jurisdiction at Sammamish's GIS portal
2. Review your HOA CC&Rs for any applicable restrictions
3. Schedule a pre-application meeting with Sammamish Community Development for complex sites
4. Prepare construction plans meeting Washington State Building Code requirements
5. Submit permit application at sammamish.us/permits or in person at 801 228th Avenue SE
6. Plan review and permit issuance
7. Construction inspections and Certificate of Occupancy

Contact Sammamish Community Development: (425) 295-0500 | sammamish.us

Disclaimer: This guide provides general information based on Washington HB 1337 (2023) and Sammamish's municipal code as of April 2026. HOA CC&R information is for general awareness only — consult an attorney about your specific HOA documents and the current status of state law preemption of HOA restrictions. Always verify current zoning requirements with Sammamish Community Development before making development decisions. This is not legal advice.