Sammamish, WA Accessory Structures: ADU Rules (2026)
ADU rules in Sammamish, WA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- ADUs allowed per lot
- One per primary dwelling
- Attached ADU condition
- Lot under 10,000 sq ft, or 2+ primary dwellings
- Floor area cap
- 1,000 sq ft (unless basement/attic)
- Detached ADU siting
- Outside critical area buffers, shoreline setbacks
- Governing section
- SMC 25.07.080(7)
Summary
Within Sammamish's shoreline jurisdiction, SMC 25.07.080(7) allows only one accessory dwelling unit per primary single detached home, caps an attached ADU's floor area at 1,000 square feet unless it sits wholly in a basement or attic, and requires a detached ADU to stay outside critical area buffers and shoreline setbacks with no setback reductions or variances allowed.
(7) Accessory Dwelling Unit (ADU). Only one accessory dwelling is allowed per primary single detached dwelling unit. An ADU is only allowed in the same building as the primary dwelling unit when the lot is less than 10,000 square feet in area or when there is more than one primary dwelling on a lot. One of the dwelling units shall not exceed a floor area of 1,000 square feet except when one of the dwelling units is wholly contained within a basement or attic. A detached ADU shall be located outside of all critical area buffers and/or shoreline setback areas and shall not be subject to any shoreline setback reductions or variances.
Official source re-checked September 8, 2026: no newer edition of the code had been published (publisher’s edition: current through Ordinance O2026-598, and legislation passed through August 18, 2026).
Full Breakdown
080(7) governs accessory dwelling units for property inside Sammamish's shoreline jurisdiction, covering Lake Sammamish, Pine Lake, and Beaver Lake. Only one ADU is allowed per primary single detached dwelling unit. An ADU may share the same building as the primary dwelling only when the lot is under 10,000 square feet in area, or when more than one primary dwelling already sits on the lot; otherwise the code contemplates a detached unit. One of the two dwelling units on the lot, the primary or the ADU, is capped at 1,000 square feet of floor area, except that a unit wholly contained within a basement or attic is exempt from that cap.
020. 010(59), which under WAC 173-27-040(2)(g) can qualify it for the streamlined normal-appurtenance exemption process rather than a full shoreline substantial development permit, provided it otherwise meets these standards. Because this provision sits in the Shoreline Master Program, it applies only to lots within shoreline jurisdiction; the city's general Title 21A ADU standards for inland lots are not part of this record.
Violations & Fines
A second ADU beyond the one allowed per lot, an ADU exceeding the 1,000-square-foot cap without qualifying for the basement/attic exemption, or a detached ADU sited inside a critical area buffer or shoreline setback is a shoreline program violation. It is enforced under the Chapter 173-27 WAC penalty provisions adopted by SMC 25.08.120 and can require a permit revision or removal of the noncompliant unit.
Frequently Asked Questions
How many ADUs can I build on a Sammamish shoreline lot?
Is there a size limit on an ADU near the lake?
Can a detached ADU use the same setback reduction as the main house?
Sources & Official References
Other rules in Sammamish
How Sammamish compares: Easiest Cities to Build an ADU·Compare Sammamish to another location·View the Washington accessory structures overview
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ADU Rules in Nearby Cities
How other cities in King County handle adu rules.