Everything homeowners ask about building an ADU in Sammamish — answered with 2026 data.
Yes, ADUs are allowed in The residential zones — R-1, R-4, R-6, R-8, R-12 and R-18 — on any lot with one single-family detached residence (Sammamish Municipal Code 21.05.010.C, Residential land uses, development condition (5)(a)). State law makes that mandatory rather than optional: RCW 36.70A.681(1)(c) requires a city to "allow at least two accessory dwelling units on all lots that are located in all zoning districts within an urban growth area that allow for single-family homes," and Sammamish lies wholly inside the King County urban growth area. The exception is the one that matters most here — RCW 36.70A.681(4) provides that "[t]he provisions of this section do not apply to lots designated with critical areas or their buffers as designated in RCW 36.70A.060," and Sammamish is dense with wetlands, streams, fish and wildlife habitat conservation areas, landslide and erosion hazard areas and critical aquifer recharge areas regulated under SMC 21.03.020. On such a lot you are left with whatever the local code gives you — which, usefully, is still two ADUs, because SMC 21.05.010.C(5)(a) contains no critical-area carve-out of its own. Confirm your parcel with the Permit Center at 425-295-0531 or PermitTech@sammamish.us, and submit for Project Guidance before you design anything in Sammamish, WA. The maximum size for a detached ADU is 1,000 sq ft. "The ADU must not exceed a floor area of 1,000 square feet" (Sammamish Municipal Code 21.05.010.C(5)(a)(iii)) — exactly the floor RCW 36.70A.681(1)(f) sets, since a city "may not establish a maximum gross floor area requirement for accessory dwelling units that is less than 1,000 square feet." Sammamish has matched the minimum rather than exceeded it. Two things stop that 1,000 sq ft from being eaten by other rules: ADUs are shown as "Exempt" from the maximum residential floor to area ratio in R-1 through R-8, where a single-family house is held to 50 percent, and SMC 21.04.030.C development condition (10) provides that "Accessory Dwelling Units are exempt from hardscape coverage and floor area ratio calculations when constructed on a lot with a single-family home" and built to SMC 21.05.010.C(5)(a). Hardscape coverage is capped at 30 percent in R-1 and 40 percent in R-4, so that exemption is worth a great deal on a large-lot plateau parcel. The separate 3,000 sq ft maximum building footprint still applies per building. Attached ADUs can be up to 1,000 sq ft, or more where the unit is inside the house: "When one of the ADUs is wholly contained within the existing residence, then the floor area shall not exceed 50 percent of the floor area of the existing unit, or 1,000 square feet, 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 $265,000-$480,000. The cost per square foot averages $320-$530. Garage conversions are more affordable at $130,000-$260,000, while prefab ADUs cost $190,000-$365,000. Permit fees add Building permit fees run on a project-value sliding scale (2026 Development Fee Schedule). For a project valued between $100,001 and $500,000 the fee is $1,278.51 on the first $100,000 plus $8.65 for each additional $1,000 — so a $300,000 ADU is about $3,008 in permit fee. Plan review is charged at 65 percent of the permit fee on top, roughly $1,955 on the same project, and a 15 percent technology fee is applied to each item except impact fees, stormwater rates, surface water development charges and code penalties. Add the Planning Division's Building Application Plan Review for New Construction at $1,578; an Energy Code fee of $136 for a new single-family residence or $91 for a remodel/addition; flat mechanical and plumbing fixture fees of $314 each for a new residence; concurrency testing at $280 plus $144; and Building Plan Review Support, which is $544 for new construction generally but $2,248 where the site is a critical area. A surface water development charge of $3,798 applies to a new residential dwelling unit with up to 3,500 sq ft of impervious coverage, and the residential stormwater rate is $504.68 to your budget. See our detailed Sammamish ADU cost guide.
Detached ADUs in Sammamish can be up to 1,000 sq ft. "The ADU must not exceed a floor area of 1,000 square feet". Attached ADUs are limited to 1,000 sq ft, or more where the unit is inside the house: "When one of the ADUs is wholly contained within the existing residence, then the floor area shall not exceed 50 percent of the floor area of the existing unit, or 1,000 square feet, 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, which is treated as an attached ADU and gets the larger-of-50-percent-or-1,000-sq-ft allowance at SMC 21.05.010.C(5)(a)(iii). Maximum height is 24 feet for a stand-alone detached ADU, against 35 feet for structures generally in R-1 through R-8 and 60 feet in R-12 and R-18 (City of Sammamish, Accessory Dwelling Units page, Option 1 development standards). Twenty-four feet is precisely the statutory floor: RCW 36.70A.681(1)(g) provides that a city "may not establish roof height limits on an accessory dwelling unit of less than 24 feet." Sammamish has matched the minimum, not exceeded it. If you want more height, the city's Option 2 gets you there — a detached ADU combined with another structure such as a garage takes the ordinary structure standards, 35 feet, at the price of the larger setbacks described below. Note that the city's own printed handout No. 310, dated 2023-09, still shows an 18-foot DADU height limit; that predates the state law and is superseded. You are allowed up to 2 ADUs per lot. "Only two accessory dwelling units permitted per lot with one single-family detached residence," in any of four configurations: two attached ADUs, such as a unit in a basement, attic, garage or home expansion; two detached ADUs, comprised of either one or two detached structures; one attached and one detached; or a conversion of an existing structure into an ADU, such as a detached garage on a qualifying lot.
Rear setback: 5 feet for a stand-alone detached ADU in every residential zone — R-1, R-4, R-6, R-8, R-12 and R-18 alike (Sammamish Municipal Code 21.04.030.C, Detached Accessory Dwelling Setbacks, development conditions (4) and (7)). Compare what the house must keep: 30 feet in R-1, 20 feet in R-8, R-12 and R-18, and 15 to 25 feet in R-4 and R-6 depending on building size. The catch is condition (7): the 5-foot standard "[o]nly applies to stand-alone detached accessory dwelling units. Does not apply to detached accessory dwelling units that are combined with other structures or improvements such as pool houses, outdoor kitchens, detached garages, covered patios, etc. Standard minimum structure setbacks apply" — which are 10 feet at the rear in R-1 through R-8 and 5 feet in R-12 and R-18. Attach the ADU to a garage and you trade 5 feet for 10. State law separately bars setbacks more restrictive than for principal units (RCW 36.70A.681(1)(h)), so Sammamish is comfortably inside the law here — but that protection lapses on a critical-area lot under RCW 36.70A.681(4), where the buffers in SMC 21.03.020 do the real work. Side setback: 5 feet for a stand-alone detached ADU in all six residential zones (Sammamish Municipal Code 21.04.030.C, Detached Accessory Dwelling Setbacks). The house's own side yard is 25 feet in R-1, 10 feet in R-8, 5 feet in R-12 and R-18, and 5 to 12 feet in R-4 and R-6 by building size. In R-1 that is a twenty-foot gain, and it is the single most valuable number in Sammamish's ADU rules. Combine the ADU with a garage or other structure and the ordinary accessory-structure side setback of 10 feet applies in R-1 through R-8, 5 feet in R-12 and R-18. Front setback: The accessory-structure front yard: 20 feet in R-1, 15 feet in R-4 and R-6, and 10 feet in R-8, R-12 and R-18 (Sammamish Municipal Code 21.04.030.C, Accessory Structure Setbacks) — with development condition (5): "The front yard setback along any arterial streets shall be 30 feet." There is no reduced front setback for a detached ADU, so unlike the side and rear yards there is nothing extra to claim at the front. Note also that a driveway must be at least 12 feet wide, no more than 30 percent of parcel width up to 20 feet, and required parking spaces must sit at least 20 feet 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 your parcel already has four spaces — and for most Sammamish homes it does. "No additional off-street parking space shall be required when the parcel contains four or more parking spaces" (Sammamish Municipal Code 21.05.010.C(5)(a)(iv)), and the city counts the garage and the driveway toward that total. Separately, SMC 21.06.030 requires no minimum parking for "Single-Family, ADUs, and Middle Housing within 1/4 mile of an existing or planned transit stop." Outside both, the baseline is 2.0 spaces per dwelling unit for a single detached dwelling. Do not expect help from the state transit rule here: RCW 36.70A.681(2)(a)(i) bars a parking requirement only "within one-half mile walking distance of a major transit stop," and RCW 36.70A.030 defines that as a high-capacity transit stop, a commuter rail stop, a rail or fixed guideway stop, or a bus rapid transit stop — none of which Sammamish has. What the statute does cap is the number: not more than one space per unit on lots smaller than 6,000 sq ft, and not more than two per unit on larger lots (RCW 36.70A.681(2)(a)(ii)-(iii)). Confirm your space count with the Permit Center at 425-295-0531 before you assume the four-space rule covers you. 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, and recently repealed. The current SMC 21.05.010.C(5)(a) contains no owner-occupancy condition, no recorded notice and no conversion-or-removal clause. The previous version of the code did: it required that "[t]he primary dwelling unit or the accessory dwelling unit shall be owner occupied," that "[t]he accessory dwelling unit shall be converted to another permitted use or shall be removed if one of the dwelling units ceases to be owner occupied," and that the applicant file a notice with the records and elections division running with the land before any permit would issue. All of it is gone. Sammamish could not reinstate it: "The city or county 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" (RCW 36.70A.681(1)(b)). 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 Confirm with the Sammamish Permit Center at 425-295-0531 — the city publishes no ADU-specific review clock, and Washington's permit review timelines under the 2023 permitting reform legislation vary by project type. Two items add real calendar time and are worth starting early: concurrency testing, which the fee schedule bills for a "Single family house/ADU" at $280 for the test and $144 for processing and informational analysis, and critical-areas review where any of the site is designated. Plan check: Standard building plan review, plus critical-areas review on most Sammamish sites. Budget for the Building Plan Review Support fee, which quadruples from $544 to $2,248 for a "Single family house/ADU - Critical Area," and for a clear and grade permit, which starts at $543 for up to 100 cubic yards of earth. If your existing house or an existing detached ADU already sits inside a wetland, stream, habitat or landslide hazard buffer, SMC 21.03.020 allows a one-time footprint increase of up to 1,000 sq ft within that buffer or setback, subject to a critical areas study and, for landslide hazard areas, a demonstration of no increased risk to life or property — but no further expansion is allowed where the structure sits over or within the wetland, stream or landslide hazard area itself. Tree removal is separately regulated, and unlawful removal is charged at $1,500 per inch of diameter at breast height. Pre-approved plans availability: None found in a City of Sammamish source as of September 1, 2026 — the city publishes handouts and a Master Builders Association member directory but no pre-approved ADU plan gallery of the kind Kirkland runs. Confirm with the Permit Center at 425-295-0531 before commissioning a full custom set.
Average ADU rental income in Sammamish: 1-bedroom units rent for $2,100-$2,750/month, 2-bedroom units for $2,650-$3,500/month. Short-term rentals: Confirm with the City of Sammamish at 425-295-0531 before you list. No short-term rental chapter could be verified for this entry — the Sammamish Development Code text reviewed here contains no short-term rental provision, and the code host blocks automated access, so a chapter elsewhere in the Municipal Code cannot be ruled out. SMC 21.05.010.C(5)(a) itself imposes no minimum tenancy and no rental restriction on an ADU. What the code does regulate separately is the bed and breakfast guesthouse, permitted only as an accessory to the operator's permanent residence, with meals limited to breakfast and no more than five guests a night (ten where the structure meets R-1 occupancy standards). Long-term letting of an ADU is unrestricted: owner occupancy was repealed and there is no cap on renting the unit. Expected ROI: 4-7% annual return.
The minimum lot size for an ADU in Sammamish is No ADU-specific minimum, and state law requires a city to allow an ADU on any lot that meets the minimum lot size for the principal unit (RCW 36.70A.681(1)(e)). Sammamish's residential zones set a minimum lot width — 35 feet in R-1, 30 feet in R-4 through R-18 — and a maximum density rather than a minimum lot area: 1 dwelling unit per acre in R-1, 4 in R-4, 6 in R-6, 8 in R-8, 12 in R-12 and 18 in R-18 (Sammamish Municipal Code 21.04.030.C). Density is where the ambiguity sits: SMC 21.05.010.C(5)(a)(v) says that "[f]or the purposes of calculating residential density, accessory dwelling units count as one dwelling unit," which on a large R-1 lot could in principle be read to squeeze the second unit. RCW 36.70A.681(1)(c) requires two ADUs regardless in any district within an urban growth area that allows single-family homes — except, again, on a critical-area lot. Put the question to the Permit Center in writing at PermitTech@sammamish.us before you commit to a two-ADU design 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 $130,000-$260,000. 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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