Sammamish has brought its code into line with Washington's ADU law and, on setbacks, gone well past it. Two accessory dwelling units are allowed on any lot with a single-family detached house, at up to 1,000 sq ft each, with no owner-occupancy requirement — and a stand-alone detached ADU takes a 5-foot side and rear setback in every residential zone, against the 25-foot side and 30-foot rear yard the house itself must keep in R-1. ADUs are also exempt from the floor area ratio and hardscape coverage calculations, which on a heavily wooded plateau lot is often the constraint that actually matters. Three things temper that. Sammamish has no major transit stop as RCW 36.70A.030 defines the term — no light rail, no commuter rail, no bus rapid transit — so the half-mile zero-parking rule at RCW 36.70A.681(2)(a)(i) that helps homeowners in Kirkland and Redmond almost certainly does not reach any lot here; what saves you instead is the city's own rule that no additional space is required when the parcel already has four. Critical areas are everywhere, and RCW 36.70A.681(4) switches off every state ADU protection on a lot designated with them — which is also the difference between a $544 and a $2,248 plan-review support fee. And impact fees changed on July 1, 2026: Ordinance O2026-595 replaced the flat per-unit parks and transportation fees with per-square-foot rates that carry a 783 sq ft minimum, so even a small ADU pays at least $8,275.85 between the two.
In Sammamish you will also hear an ADU called a DADU — short for detached accessory dwelling unit, the usual term across Washington. The rules on this page apply either way.
Zoning & Size Requirements [1][2][3][4][5]
Yes — Sammamish allows ADUs. A detached ADU can typically be up to 1,000 sq ft under current Washington and Sammamish zoning rules.
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| Allowed Zones | 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 |
| Max Size (Detached) | 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 |
| Max Size (Attached) | 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 Municipal Code 21.05.010.C(5)(a)(iii)). Read that carefully — it is whichever is larger, not smaller. On a 3,000 sq ft house an internal conversion may run to 1,500 sq ft, half again what the detached cap allows. This is one of the more generous internal-ADU allowances on the Eastside, and it applies only to a unit wholly contained within the existing residence |
| Max Size (JADU) | 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) |
| Max Height | 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 |
| Max Units | 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 (Sammamish Municipal Code 21.05.010.C(5)(a)(i)-(ii)). That matches RCW 36.70A.681(1)(c) configuration for configuration, and the conversion option matches RCW 36.70A.681(1)(j), which requires a city to allow ADUs converted from existing structures "including but not limited to detached garages, even if they violate current code requirements for setbacks or lot coverage." Note that the state's conversion protection is broader than the local text, and it is the one to cite if an existing garage does not meet today's setbacks |
Setback Requirements [1][2][3][4][5]
Sammamish requires a rear setback of about 5 ft for a detached ADU, and parking requirements vary by unit size or location. Owner-occupancy is not required — you can rent it out without living on the property.
| 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 |
| Parking Required | 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 |
| Owner Occupancy | 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)) |
| Min Lot Size | 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 |
Permit Process & Fees [6][7][8][9]
An ADU permit in Sammamish goes through a standard permit review. 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. Permit fees: 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.
| Approval Process | A building permit, applied for online through MyBuildingPermit.com — Sammamish takes no paper applications and issues no permits over the counter. There is no conditional use permit, no hearing and, since the code update, no recorded notice running with the land. Start with Project Guidance, the city's formal request for zoning and development information about a parcel; on a plateau lot with any chance of a wetland, stream, steep slope or aquifer recharge designation this is the step that decides the project, because RCW 36.70A.681(4) removes every state protection from a lot designated with critical areas or their buffers and SMC 21.03.020 buffers then govern where anything can be built. State law bars the city from requiring public street improvements as a condition of permitting an ADU (RCW 36.70A.681(1)(l)). Permit Center: 425-295-0531, PermitTech@sammamish.us; walk-in hours Monday and Friday 9am-noon and Wednesday 1-4pm, virtual appointments available with building plans examiners, land use planners and development review engineers |
| Review Timeline | 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 |
| Permit Fees | 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 |
| Impact Fees | This changed on July 1, 2026, and the change matters. Ordinance O2026-595, adopted May 19, 2026, created a new SMC 21.06.045 replacing the city's flat per-unit parks and transportation impact fees with per-square-foot rates: Parks Impact Fee $3.13 per square foot, minimum $2,450.79 (783 sq ft) and maximum $10,955.00 (3,500 sq ft); Transportation Impact Fee $7.44 per square foot, minimum $5,825.06 (783 sq ft) and maximum $26,043.91 (3,500 sq ft). Scaling by size helps an ADU, but the 783 sq ft floor means even a 500 sq ft unit pays at least $8,275.85 across the two. School impact fees remain per dwelling unit and depend entirely on which district you are in: Lake Washington School District $10,914.75, Issaquah School District $0.00, Snoqualmie Valley School District $19,690.27, plus a $266 administration fee. Over all of it sits RCW 36.70A.681(1)(a): a city "may not assess impact fees on the construction of accessory dwelling units that are greater than 50 percent of the impact fees that would be imposed on the principal unit." Hold the city to it — and be aware that on a lot designated with critical areas or their buffers, RCW 36.70A.681(4) removes that cap along with everything else in the section |
| 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 | 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 |
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Construction Costs [10]
Building an ADU in Sammamish typically costs $265,000-$480,000 ($320-$530 per sq ft); a garage conversion runs $130,000-$260,000.
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| Average Build Cost | $265,000-$480,000 |
| Cost per Sq Ft | $320-$530 |
| Garage Conversion | $130,000-$260,000 |
| Prefab ADU (Installed) | $190,000-$365,000 |
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Rental Income Potential [11][12]
A 2-bedroom ADU in Sammamish rents for around $2,650-$3,500/mo, an estimated 4-7% annual return.
| Avg Rent (1 Bedroom) | $2,100-$2,750/mo |
| Avg Rent (2 Bedroom) | $2,650-$3,500/mo |
| Short-Term Rental | 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 |
| Est. Annual ROI | 4-7% annual return |
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Recent Law Changes
What's Changed Recently
- Washington HB 1337 (2023 c 334), codified at RCW 36.70A.681, preempts local ADU regulation in cities planning under the Growth Management Act: at least two ADUs on all lots in all zoning districts within an urban growth area that allow single-family homes, no owner-occupancy requirement, no maximum gross floor area under 1,000 sq ft, no roof height limit under 24 feet, no setbacks, yard coverage limits, tree retention mandates, entry door restrictions, aesthetic requirements or design review more restrictive than for principal units, mandatory allowance of conversions of existing structures including detached garages even where they violate current setback or lot coverage rules, detached ADUs at a lot line abutting a public alley, impact fees capped at half the principal unit's, and no public street improvements as a condition
- Sammamish rewrote SMC 21.05.010.C(5)(a) to match. The allowance went from "[o]nly one accessory dwelling per primary single detached dwelling unit" to two per lot in four named configurations; the owner-occupancy requirement, the conversion-or-removal clause tied to it, the recorded notice filed with the records and elections division, the one-entrance-per-street-side rule and the occupancy cap tied to the definition of "family" were all deleted; and the size rule was rewritten so that an ADU wholly contained within the existing residence may be the larger of 1,000 sq ft or 50 percent of the existing unit
- The detached ADU roof height limit was raised from 18 feet to 24 feet — the exact minimum RCW 36.70A.681(1)(g) permits, not a foot above it
- SMC 21.04.030.C development condition (10) now exempts accessory dwelling units from hardscape coverage and floor area ratio calculations outright where they are built on a lot with a single-family home. The previous rule was narrower: a one-time five percentage point increase in maximum lot coverage if an ADU or a covered outdoor living space was built on site
- Ordinance O2026-595, adopted by the City Council on May 19, 2026 and effective July 1, 2026, created SMC 21.06.045 and replaced the flat per-unit parks and transportation impact fees ($6,739 and $14,204.27 per dwelling unit) with per-square-foot rates of $3.13 and $7.44, carrying a 783 sq ft minimum and a 3,500 sq ft maximum. The city describes the change as a response to changes in state law
- RCW 36.70A.681(4) exempts lots designated with critical areas or their buffers under RCW 36.70A.060 from every one of the statute's ADU protections. In Sammamish that is not a marginal category: the city's critical areas regulations at SMC 21.03.020 cover wetlands, streams, fish and wildlife habitat conservation areas, wildlife habitat corridors, landslide hazard areas and critical aquifer recharge areas, the development code routinely calculates density on "net area excluding critical areas," and the fee schedule prices a critical-area single family house or ADU review at $2,248 against $544 for everything else
- Stale guidance to ignore: the city's printed handout No. 310, "Detached Accessory Dwelling Units (DADUs)," is dated 2023-09 and still states that "[o]nly one DADU per primary residence is permitted," gives an 18-foot height limit, requires two parking spaces unless the property already has four, requires the DADU to "match the facade, roof pitch, siding, and windows of the primary residence," and caps combined occupancy by the definition of family. Every one of those points is either repealed or preempted — RCW 36.70A.681(1)(h) bars aesthetic requirements for ADUs more restrictive than for principal units. The city's own Accessory Dwelling Units web page carries the current standards
- The Sammamish Municipal Code is hosted on a platform that blocks automated access. The code text verified for this entry came from the City of Sammamish's own PDF of the adopted 2024 Comprehensive Plan code updates dated February 3, 2025, checked against the city's current ADU page and the 2026 Development Fee Schedule. The hosted code is described as current through Ordinance O2026-595 of May 19, 2026 — confirm anything you rely on with the Permit Center at 425-295-0531
Tips for Building an ADU in Sammamish
- The 5-foot setback is the prize, and you only get it stand-alone. A detached ADU that touches a garage, pool house, covered patio or outdoor kitchen loses the 5-foot side and rear standard and takes the ordinary 10-foot accessory-structure setbacks in R-1 through R-8. Decide early whether the extra height of the combined option is worth twice the setback
- Put the ADU inside the house if you want floor area. A unit wholly contained within the existing residence may be the larger of 1,000 sq ft or half the house's floor area — on a 3,000 sq ft house that is 1,500 sq ft, half again what a detached unit is allowed
- Check for critical areas before anything else, and pay for Project Guidance to do it. RCW 36.70A.681(4) turns off every state protection on a lot designated with critical areas or their buffers, and Sammamish has wetlands, streams, steep slopes and aquifer recharge areas throughout. The city's own fee schedule tells you how common it is: critical-area plan review support costs $2,248 against $544
- Count your existing parking spaces. Four or more — garage plus driveway — and no additional space is required at all. Sammamish has no major transit stop, so the half-mile state parking waiver that helps Kirkland and Redmond homeowners will not help you
- Which school district you are in is worth thousands. School impact fees are $0.00 in the Issaquah School District, $10,914.75 in Lake Washington and $19,690.27 in Snoqualmie Valley — and RCW 36.70A.681(1)(a) caps what an ADU can be charged at half the principal unit's figure
- Small does not mean cheap on impact fees any more. Since July 1, 2026 parks and transportation fees are per square foot but carry a 783 sq ft floor, so a 500 sq ft ADU pays the same $8,275.85 as a 783 sq ft one. If you are close to that line, the extra space is effectively free
- Converting a garage that breaks today's setbacks? The local code lists conversion as an allowed configuration but says nothing about nonconformity. RCW 36.70A.681(1)(j) does: cities must allow ADUs converted from existing structures including detached garages "even if they violate current code requirements for setbacks or lot coverage"
- Ignore handout No. 310. It is dated 2023-09, says one DADU per house, 18 feet, and a facade-match requirement — all superseded. Use the city's Accessory Dwelling Units web page and SMC 21.05.010.C(5)(a) instead
ADU Builders in Sammamish
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Sources & References
All data on this page is sourced from official government records and verified market data. View our full methodology.
- Sammamish Municipal Code 21.05.010.C, Residential land uses, development condition (5)(a) — Accessory dwelling units; and 21.04.030.C, District Standards—Density and Dimensions (City of Sammamish, Adopted 2024 Comprehensive Plan Code Updates, February 3, 2025) Municipal Code — Accessed 2026-09-01
- Accessory Dwelling Units (ADUs) — City of Sammamish Community Development (current ADU standards and detached/attached setback tables) Planning Department — Accessed 2026-09-01
- RCW 36.70A.681 — Accessory dwelling units, limitations on local regulation (2023 c 334 s 4, HB 1337) state_statute — Accessed 2026-09-01
- RCW 36.70A.030 — Definitions ("major transit stop") state_statute — Accessed 2026-09-01
- Title 21: Sammamish Development Code, effective January 1, 2022 (superseded ADU conditions, critical areas provisions at SDC 21.03.020, height maximums table) Municipal Code — Accessed 2026-09-01
- 2026 Fee Schedule — Development Review, Permits, and Related Fees, City of Sammamish Planning Department — Accessed 2026-09-01
- July 1, 2026 Updated Impact Fees — City of Sammamish (Ordinance O2026-595, SMC 21.06.045) Planning Department — Accessed 2026-09-01
- Permit Center — City of Sammamish (MyBuildingPermit.com submittal, Project Guidance, 425-295-0531) Planning Department — Accessed 2026-09-01
- Handout No. 310 — Detached Accessory Dwelling Units (DADUs), City of Sammamish, dated 2023-09 (superseded; retained as a record of the pre-HB 1337 standards) Planning Department — Accessed 2026-09-01
- Angi — ADU Construction Cost Data Market Data — Accessed 2026-09-01
- Zillow Rental Manager — Market Rent Data Market Data — Accessed 2026-09-01
- HUD Fair Market Rents Government Data — Accessed 2026-09-01
- Washington State Office of Financial Management — April 1, 2026 Population of Cities, Towns and Counties (Sammamish 68,500) Government Data — Accessed 2026-09-01
Frequently Asked Questions
Can I build an ADU in Sammamish?
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. 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. Owner-occupancy is not required, so you can rent out the ADU.
How much does an ADU cost in Sammamish?
ADU construction in Sammamish typically costs $265,000-$480,000. Garage conversions run $130,000-$260,000, and prefab ADUs cost $190,000-$365,000.
How much can I rent an ADU for in Sammamish?
Average ADU rents in Sammamish are $2,100-$2,750/mo for a 1-bedroom and $2,650-$3,500/mo for a 2-bedroom unit.
Do I need parking for an ADU in Sammamish?
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. Check with Sammamish planning department for the latest Washington ADU parking rules, as many states have relaxed requirements in recent years.
How long does it take to get an ADU permit in Sammamish?
Sammamish does not publish a standard ADU review timeline — 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. Permit fees: 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.
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