Everything homeowners ask about building an ADU in Sammamish — answered with 2026 data.
Yes, ADUs are allowed in All six residential zones — R-1, R-4, R-6, R-8, R-12 and R-18 — on any lot with one single-family detached residence (SMC 21.05.010.C, development condition (5)(a), Ord. O2024-578, effective January 1, 2025). RCW 36.70A.681(1)(c) makes two ADUs mandatory in every urban-growth-area zone allowing single-family homes, but RCW 36.70A.681(4) withdraws the state protections on lots designated with critical areas or buffers (SMC 21.03.020), common here. Confirm your parcel via free Project Guidance. in Sammamish, WA. The maximum size for a detached ADU is 1,000 sq ft of floor area: "The ADU must not exceed a floor area of 1,000 square feet" (SMC 21.05.010.C(5)(a)(iii)) — the floor RCW 36.70A.681(1)(f) sets, not a foot more. ADUs are listed as "Exempt" from the 50 percent residential floor area ratio in R-1 through R-8, and SMC 21.04.030.C condition (10) exempts them from hardscape coverage (30 percent in R-1, 40 percent in R-4) and FAR calculations on a single-family lot. The 3,000 sq ft maximum building footprint per building still applies. Attached ADUs can be up to 1,000 sq ft, or more when the unit is inside the house: where an ADU is "wholly contained within the existing residence" its floor area may not exceed 50 percent of the existing unit or 1,000 sq ft, "whichever is larger". Sammamish has an ADU-friendliness score of 8/10. Check our full Sammamish ADU rules page for complete zoning details.
ADU construction in Sammamish typically costs $325,000-$550,000. The cost per square foot averages $400-$650. Garage conversions are more affordable at $100,000-$225,000, while prefab ADUs cost $300,000-$450,000. Permit fees add Value-based (2026 Development Fee Schedule): $1,278.51 on the first $100,000 plus $8.65 per $1,000 above it, plus plan review at 65 percent, a $1,578 planning review, $544 plan-review support ($2,248 on a critical-area site) and a 15 percent technology fee — about $11,150 on a $400,000 ADU. to your budget. See our detailed Sammamish ADU cost guide.
Detached ADUs in Sammamish can be up to 1,000 sq ft of floor area: "The ADU must not exceed a floor area of 1,000 square feet". Attached ADUs are limited to 1,000 sq ft, or more when the unit is inside the house: where an ADU is "wholly contained within the existing residence" its floor area may not exceed 50 percent of the existing unit or 1,000 sq ft, "whichever is larger". Junior ADUs (JADUs) have a cap of Not a category in Washington — neither RCW 36.70A.681 nor the Sammamish Development Code recognises a junior ADU. The nearest equivalent is an ADU wholly contained within the existing residence, treated as an attached ADU with the larger-of-50-percent-or-1,000-sq-ft allowance at SMC 21.05.010.C(5)(a)(iii). Maximum height is 24 ft for a stand-alone detached ADU in every residential zone (City of Sammamish ADU page, Option 1 standards) — exactly the floor RCW 36.70A.681(1)(g) permits. A detached ADU combined with another structure such as a garage (Option 2) may go to 35 ft but takes the larger accessory-structure setbacks. Height is measured from average existing grade under SMC 21.04.030.J as rewritten by Ord. O2025-583. Handout No. 310 (dated 2023-09) still shows 18 ft; it predates the state law and is superseded. You are allowed up to Two ADUs per lot: "Only two accessory dwelling units permitted per lot with one single-family detached residence," in four configurations — two attached on a qualifying lot.
Rear setback: 5 ft for a stand-alone detached ADU in all six zones (SMC 21.04.030.C, Detached Accessory Dwelling Setbacks, conditions (4) and (7)). The house itself keeps 30 ft in R-1, 20 ft in R-8, R-12 and R-18, and a 12 to 20 ft minimum in R-4 and R-6 by building size. Condition (7): the 5-ft standard "only applies to stand-alone detached accessory dwelling units" — a DADU combined with a garage or covered patio takes the accessory-structure setback: 10 ft in R-1 through R-8, 5 ft in R-12 and R-18. Side setback: 5 ft for a stand-alone detached ADU in all six zones (SMC 21.04.030.C, Detached Accessory Dwelling Setbacks). The house's own side yard is 25 ft in R-1, 10 ft in R-8, 5 ft in R-12 and R-18, and a 5 to 10 ft minimum in R-4 and R-6 by building size — in R-1 the DADU gains 20 ft. Combined with a garage or other structure, the Option 2 accessory-structure setback applies: 10 ft in R-1 through R-8, 5 ft in R-12 and R-18. RCW 36.70A.681(1)(h) bars stricter ADU setbacks, except on critical-area lots. Front setback: No ADU reduction at the front — the accessory-structure front yard applies: 20 ft in R-1, 15 ft in R-4 and R-6, 10 ft in R-8, R-12 and R-18 (SMC 21.04.030.C, Accessory Structure Setbacks; city ADU page, Options 1 and 2), and development condition (5) makes it 30 ft along any arterial street. In R-1 through R-8 a driveway must be at least 12 ft wide and no more than 30 percent of parcel width up to 20 ft, and required parking spaces must sit at least 20 ft back from the street property line. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None if the parcel has four or more spaces, garage and driveway included (SMC 21.05.010.C(5)(a)(iv); city ADU page). SMC 21.06.030.D(3)(a) also sets no minimum for single-family homes, ADUs and middle housing within 1/4 mile of an existing or planned public transit stop. The state half-mile rule (RCW 36.70A.681(2)(a)(i)) needs a "major transit stop" under RCW 36.70A.030, which Sammamish lacks; the state caps ADU parking at 1 space per unit under 6,000 sq ft, 2 above. Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Not required. SMC 21.05.010.C(5)(a) as rewritten by Ord. O2024-578 (effective January 1, 2025) has no owner-occupancy condition, recorded notice or conversion-or-removal clause; the pre-2025 code required one unit to be owner-occupied and a notice recorded with King County. RCW 36.70A.681(1)(b) bars reinstating it: a city "may not require the owner of a lot on which there is an accessory dwelling unit to reside in or occupy the accessory dwelling unit or another housing unit on the same lot.". Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Sammamish is typically Not published — the city sets no ADU-specific review clock, and in September 2026 warns of reduced service levels during a staff transition. Project Guidance is answered within one week; ask the Permit Center (425-295-0531) for the current building-permit review target before you submit. Plan check: Standard building plan review through MyBuildingPermit.com, plus critical-areas review on many Sammamish sites: the Building Plan Review Support fee is $2,248 for a "Single family house/ADU - Critical Area" against $544 otherwise, a third-party critical area report review takes a $5,000 deposit, and wetland, stream, landslide, erosion, habitat and aquifer-recharge worksheets are on the city's forms page. A minor clearing-and-grading permit (up to 100 cubic yards) is billed at $267 planning plus $543 public-works review. Tree removal is separately permitted ($139), and unlawful removal is charged at $1,500 per inch of diameter at breast height. Concurrency testing for a "Single family house/ADU" is $280 plus $144 processing and a $187 administrative fee. Pre-approved plans availability: None. As of September 20, 2026 the city's ADU page and its Applications and Forms index carry no pre-approved ADU plan set — only the new single-family residence submittal checklist (updated July 1, 2026) and the residential plan-set coversheet — and no King County or state pre-approved plan program was found that applies to permits inside the city. Budget for a full custom design set and confirm with the Permit Center at 425-295-0531.
Average ADU rental income in Sammamish: 1-bedroom units rent for $2,900-$3,300/month, 2-bedroom units for $3,400-$3,850/month. Short-term rentals: Confirm with the city (425-295-0531) before you list. No short-term rental chapter was found on the city's site, and the codified SMC is behind a bot wall, so a rule elsewhere cannot be ruled out. The ADU code sets none. Expected ROI: 5-8% annual return.
The minimum lot size for an ADU in Sammamish is None ADU-specific; RCW 36.70A.681(1)(e) requires an ADU on any lot meeting the principal unit's minimum. Sammamish's zones set a minimum lot width (35 ft in R-1, 30 ft in R-4 through R-18) and a maximum density (1, 4, 6, 8, 12 and 18 units per acre), not a lot area (SMC 21.04.030.C). SMC 21.05.010.C(5)(a)(v) counts ADUs as one dwelling unit for density; RCW 36.70A.681(1)(c) still requires two ADUs on any single-family UGA lot. Ask the Permit Center in writing before designing two ADUs in R-1. This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Sammamish, a garage conversion typically costs $100,000-$225,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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