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ADU Rules in Sammamish, Washington

Complete guide to building an Accessory Dwelling Unit in Sammamish — zoning, setbacks, permits, costs, and more.

Max Size: 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. Setbacks: 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. Parking: Varies Verified: 2026-09-20

Researched by ADU Zoning Hub · verified 2026-09-20 against Sammamish planning code and Washington ADU law · methodology · 23 sources cited ↓

ADU Score: 8/10
Very ADU-friendly · how we score

Are ADUs allowed in Sammamish, WA? [1][2][3][4][5][6][7]

Yes. Sammamish allows ADUs on most residential lots. A detached unit can be up to 1,000 sq ft, set 5 ft back from the rear line. Parking varies by lot and unit size. Owner-occupancy is not required, so the unit can be rented out. A building permit is required and applications are reviewed through the standard permit process.

Maximum size1,000 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.
SetbacksRear 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 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.
ParkingNone 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.
Owner-occupancyNot 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."

Do I need a permit to build an ADU in Sammamish? [8][9][10][11][12][13][14][15][16]

Yes. Every ADU in Sammamish needs a building permit. Review process: A building permit only — no conditional use permit, no hearing and, since Ord. O2024-578, no recorded notice; SMC 21.05.010.C development condition (9) exempts ADUs from the approval otherwise required for more than one dwelling on a lot. All applications are submitted online at MyBuildingPermit.com; the Permit Center (425-295-0531, PermitTech@sammamish.us; walk-ins Monday and Friday 9am-noon, Wednesday 1-4pm, virtual appointments with plans examiners, planners and review engineers) answers questions but issues no permits at a visit. Start with the free Project Guidance request, answered within one week, which reports setbacks, critical-area designations and next steps for your parcel — the step that decides most plateau projects, because RCW 36.70A.681(4) removes the state protections on a lot designated with critical areas or their buffers. The city may not require public street improvements as a condition of an ADU permit (RCW 36.70A.681(1)(l)). In September 2026 the city warns of reduced service levels during a staff transition..

Permit feesValue-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.
Typical timelineNot 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.
Official Sammamish links:

Three things to know before you build in Sammamish

  • The 5-ft 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-ft side and rear standard and takes the 10-ft accessory-structure setbacks in R-1 through R-8 (SMC 21.04.030.C condition (7)). Decide early whether the extra height of the combined option (35 ft) 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, and at 1,750 sq ft or less it is still exempt from the SMC 21.10 affordable-housing in-lieu fee
  • Check for critical areas before anything else, and use the free Project Guidance request to do it. RCW 36.70A.681(4) turns off every state protection on a lot designated with critical areas or their buffers, and the fee schedule shows how common they are: critical-area plan-review support costs $2,248 against $544

Last verified 2026-09-20 against Sammamish planning code. How we verify

Yes — Sammamish allows two accessory dwelling units on any lot with one single-family detached house, in all six residential zones, under SMC 21.05.010.C development condition (5)(a) as rewritten by Ordinance O2024-578 (effective January 1, 2025) to meet RCW 36.70A.681. Each ADU may be up to 1,000 sq ft (an ADU inside the house may be the larger of 1,000 sq ft or half the house), a stand-alone detached ADU keeps a 5-foot side and rear setback in every zone and a 24-foot height, no owner occupancy is required, and ADUs are exempt from the floor-area-ratio and hardscape limits. It takes a building permit through MyBuildingPermit.com, no hearing and no recorded notice; a 1,000 sq ft unit valued at $400,000 runs about $11,150 in permit and review fees plus $10,570 in the per-square-foot parks and transportation impact fees that Ordinance O2026-595 introduced on July 1, 2026 (minimum $8,275.85 even for a tiny unit), plus any school fee. Two things decide most projects here: whether the lot is designated with critical areas or their buffers, which switches off every state ADU protection under RCW 36.70A.681(4) and adds $1,704 in review fees, and the free Project Guidance request that tells you so within a week.

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.

Outside city limits? If your property is in unincorporated King County (not within Sammamish's boundaries), different rules apply — see our King County (unincorporated) ADU rules.

Zoning & Size Requirements [1][2][3][4][5][6][7]

Yes — Sammamish allows ADUs. A detached ADU can typically be up to 1,000 sq ft under current Washington and Sammamish zoning rules.

See what this means for your lot — ask Sammamish ADU builders for a quote →

Allowed ZonesAll 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.
Max Size (Detached)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.
Max Size (Attached)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" (SMC 21.05.010.C(5)(a)(iii)) — 1,500 sq ft on a 3,000 sq ft house. Dwelling units of 1,750 sq ft or less, "including ADUs," are exempt from the SMC 21.10 affordable-housing in-lieu fee of $34.45 per sq ft (SMC 21.10.100(A), Ord. O2026-597, effective August 29, 2026).
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, 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).
Max Height24 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.
Max UnitsTwo ADUs per lot: "Only two accessory dwelling units permitted per lot with one single-family detached residence," in four configurations — two attached (basement, attic, garage, home expansion), two detached in one or two structures, one attached and one detached, or a conversion of an existing structure such as a detached garage (SMC 21.05.010.C(5)(a)(i)-(ii)). RCW 36.70A.681(1)(j) requires conversions to be allowed "even if they violate current code requirements for setbacks or lot coverage."

Setback Requirements [1][2][3][4][5][6][7]

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 Setback5 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 Setback5 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 SetbackNo 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.
Parking RequiredNone 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.
Owner OccupancyNot 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."
Min Lot SizeNone 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.

Permit Process & Fees [8][9][10][11][12][13][14][15][16]

An ADU permit in Sammamish goes through a standard permit review. Review time: 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. Permit fees: 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.

Approval ProcessA building permit only — no conditional use permit, no hearing and, since Ord. O2024-578, no recorded notice; SMC 21.05.010.C development condition (9) exempts ADUs from the approval otherwise required for more than one dwelling on a lot. All applications are submitted online at MyBuildingPermit.com; the Permit Center (425-295-0531, PermitTech@sammamish.us; walk-ins Monday and Friday 9am-noon, Wednesday 1-4pm, virtual appointments with plans examiners, planners and review engineers) answers questions but issues no permits at a visit. Start with the free Project Guidance request, answered within one week, which reports setbacks, critical-area designations and next steps for your parcel — the step that decides most plateau projects, because RCW 36.70A.681(4) removes the state protections on a lot designated with critical areas or their buffers. The city may not require public street improvements as a condition of an ADU permit (RCW 36.70A.681(1)(l)). In September 2026 the city warns of reduced service levels during a staff transition.
Review TimelineNot 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.
Permit FeesValue-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.
Impact FeesPer square foot since July 1, 2026 (Ord. O2026-595, new SMC 21.06.045, adopted to meet SB 5258's amendment of RCW 82.02.060(1)): parks $3.13 and transportation $7.44 per sq ft, each with a 783 sq ft minimum ($2,450.79 and $5,825.06 — $8,275.85 together) and a 3,500 sq ft maximum ($10,955 and $26,040). A 1,000 sq ft ADU pays $10,570. School fees are per dwelling unit by district — Lake Washington SD $10,914.75, Issaquah SD $0.00, Snoqualmie Valley SD $19,690.27 at the single-family rate, plus a $266 administration fee; the schedule lists no ADU category, and RCW 36.70A.681(1)(a) caps any ADU impact fee at 50 percent of the principal unit's (at most $5,457.38 in Lake Washington SD), except on critical-area lots (subsection (4)). Fees are due at permit issuance but may be deferred to occupancy for up to 18 months against a recorded lien (SMC 21.06.045(I)); parks and transportation rates are indexed to the ENR Construction Cost Index each first quarter.
Plan CheckStandard 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 PlansNone. 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.

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Construction Costs [17][18][19][20]

Building an ADU in Sammamish typically costs $325,000-$550,000 ($400-$650 per sq ft); a garage conversion runs $100,000-$225,000.

See what this means for your lot — ask Sammamish ADU builders for a quote →

Average Build Cost$325,000-$550,000
Cost per Sq Ft$400-$650
Garage Conversion$100,000-$225,000
Prefab ADU (Installed)$300,000-$450,000

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Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders.

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Rental Income Potential [21][22]

A 2-bedroom ADU in Sammamish rents for around $3,400-$3,850/mo, an estimated 5-8% annual return.

Avg Rent (1 Bedroom)$2,900-$3,300/mo
Avg Rent (2 Bedroom)$3,400-$3,850/mo
Short-Term RentalConfirm 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.
Est. Annual ROI5-8% 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 and unamended since: at least two ADUs on all lots in urban-growth-area zones that allow single-family homes, no owner-occupancy requirement, no floor-area cap under 1,000 sq ft, no roof-height limit under 24 ft, no setbacks or design rules stricter than the principal unit's, mandatory conversions of existing structures including garages that violate current setbacks, impact fees capped at half the principal unit's, no street improvements as a condition — none of it applying to lots designated with critical areas or their buffers (subsection (4))
  • Ordinance O2024-578, passed December 10, 2024 and effective January 1, 2025 with the 2024 Comprehensive Plan code updates, rewrote SMC 21.05.010.C(5)(a): two ADUs per lot in four configurations; the owner-occupancy requirement, the conversion-or-removal clause, the recorded notice, the one-entrance rule and the family occupancy cap were deleted; an ADU wholly inside the house may be the larger of 1,000 sq ft or 50 percent of the house; the detached-ADU height rose from 18 to 24 ft; a 5-ft side and rear setback for stand-alone DADUs was added to SMC 21.04.030.C; ADUs became exempt from FAR and hardscape coverage (condition (10)); and the middle-housing overlays MHO-1, MHO-2 and MHO-3 were created
  • Ordinance O2025-583, adopted June 3, 2025 (first round of Title 21 housekeeping amendments), rewrote the building-height measurement method at SMC 21.04.030.J (average existing grade on flat sites, a segmented plane on sloped sites), amended the critical areas chapter 21.03.020 and the affordable housing chapter 21.10, and left the ADU conditions at 21.05.010.C(5)(a) and the DADU setback rows untouched
  • Ordinance O2026-594, passed February 3, 2026 and effective February 14, 2026 (second housekeeping round), redefined an accessory dwelling unit in SMC 21.04.040 as a unit "on the same lot as a single-family or multifamily dwelling unit" and re-labelled the 3,000 sq ft building-footprint row of SMC 21.04.030.C with a one-time 300 sq ft upper-floor expansion allowance; it made no change to 21.05.010.C(5)(a)
  • Ordinance O2026-595, passed May 19, 2026 and effective July 1, 2026, repealed SMC 21.08.030, 21.08.040, 21.08.050 and 21.09.090 and adopted SMC 21.06.045, replacing the flat per-unit parks ($6,739) and transportation ($14,204.27) impact fees with per-square-foot rates of $3.13 and $7.44 carrying a 783 sq ft minimum and a 3,500 sq ft maximum, on the FCS Impact Fee Study of March 27, 2026 (Resolution R2026-1142) and in response to SB 5258's amendment of RCW 82.02.060(1); it also codified an occupancy deferral of up to 18 months and annual ENR-index adjustments
  • Ordinance O2026-597, passed August 18, 2026 and effective August 29, 2026, amended SMC 21.10.100 so that new dwelling units of 1,750 sq ft or less (up from 1,500), "including ADUs and cottage housing," garage and unfinished-space conversions and additions are exempt from the inclusionary affordable-housing requirement and its $34.45 per sq ft in-lieu fee, retroactive to January 1, 2025 with refunds to applicants who paid
  • Critical areas are the local trap: RCW 36.70A.681(4) exempts lots designated with critical areas or their buffers from every state ADU protection, and SMC 21.03.020 (rewritten by Ord. O2024-571 and amended by O2025-583) regulates wetlands, streams, habitat areas, landslide and erosion hazard areas and critical aquifer recharge areas across the plateau; the fee schedule prices a critical-area single-family or ADU review at $2,248 against $544
  • Stale guidance to ignore: the city's printed handout No. 310, "Detached Accessory Dwelling Units (DADUs)," dated 2023-09, still says one DADU per residence, an 18-ft height, two parking spaces unless the property has four, a facade-match requirement and a family occupancy cap — every point repealed by O2024-578 or preempted by RCW 36.70A.681(1)(h). The city's Accessory Dwelling Units web page carries the current standards
  • The codified Sammamish Municipal Code (sammamish.municipal.codes) sits behind a bot-verification wall. The text in this entry comes from the city's own PDF of the adopted code updates (Ord. O2024-578), the adopted ordinances O2025-583, O2026-594, O2026-595 and O2026-597 on the city's document center, the city's ADU page and the 2026 Development Fee Schedule, all read September 20, 2026

Tips for Building an ADU in Sammamish

  • The 5-ft 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-ft side and rear standard and takes the 10-ft accessory-structure setbacks in R-1 through R-8 (SMC 21.04.030.C condition (7)). Decide early whether the extra height of the combined option (35 ft) 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, and at 1,750 sq ft or less it is still exempt from the SMC 21.10 affordable-housing in-lieu fee
  • Check for critical areas before anything else, and use the free Project Guidance request to do it. RCW 36.70A.681(4) turns off every state protection on a lot designated with critical areas or their buffers, and the fee schedule shows how common they are: 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. A bus stop within a quarter mile also removes the minimum under SMC 21.06.030.D(3)(a), a broader rule than the state's half-mile major-transit-stop test, which no Sammamish lot meets
  • Worked permit fee, 1,000 sq ft DADU valued at $400,000: building permit $3,873.51, plan review (65%) $2,517.78, planning review $1,578, plan-review support $544, energy code $136, mechanical $314, plumbing $314, concurrency $280 + $144 = $9,701.29, plus the 15% technology fee = about $11,156. A critical-area site adds $1,704 before the technology fee; a surface water development charge of $3,798 may apply to a new dwelling unit
  • 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; a 1,000 sq ft unit pays $10,570. You can defer payment to occupancy (up to 18 months) with the Impact Fee Deferral Affidavit and a recorded lien
  • 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 at the single-family rate — and RCW 36.70A.681(1)(a) caps what an ADU can be charged at half the principal unit's figure; put that statute in your application
  • 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 ft, two parking spaces 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
  • Cost basis: build cost, cost per sq ft, garage conversion and prefab ranges are the Seattle-metro industry basis used for the Bellevue, Kirkland and Bothell entries (four dated 2025-2026 Seattle ADU cost guides, read September 19, 2026), not city data; rents are HUD FY2026 and FY2027 Small Area Fair Market Rents for ZIP codes 98074 and 98075 (1BR $2,920-$3,270, 2BR $3,410-$3,820), and ROI is gross rent on that build cost less typical vacancy, tax and upkeep, before financing

ADU Builders in Sammamish

Looking for an ADU builder in Sammamish? ADU projects need a contractor experienced with local zoning, the permit process, and utility connections — not every general contractor has built one. We match Sammamish homeowners with experienced local ADU contractors who provide free, no-obligation quotes. Getting at least three bids typically saves 20-40% on the same scope.

Sources & References

All data on this page is sourced from official government records and verified market data. View our full methodology.

  1. Sammamish Municipal Code 21.05.010.C, Residential land uses, development condition (5)(a) — Accessory dwelling units; 21.04.030.C District Standards—Density and Dimensions (DADU setbacks, FAR, hardscape, conditions (4), (5), (7), (10)); 21.06.030.D(3)(a) parking (City of Sammamish, Adopted 2024 Comprehensive Plan Code Updates, Ord. O2024-578, effective January 1, 2025) Municipal Code — Accessed 2026-09-20
  2. Accessory Dwelling Units (ADUs) — City of Sammamish Community Development (current ADU standards; Option 1 and Option 2 setback and height tables) Planning Department — Accessed 2026-09-20
  3. Ordinance O2026-594 — Amending Title 21 of the Sammamish Municipal Code (second-round housekeeping amendments; ADU definition; 21.04.030.C footprint row), passed February 3, 2026, effective February 14, 2026 Municipal Code — Accessed 2026-09-20
  4. Ordinance O2025-583 — SMC Title 21 Amendment Ordinance (first-round housekeeping; 21.04.030.J height measurement, 21.03.020 critical areas, 21.10), adopted June 3, 2025 Municipal Code — Accessed 2026-09-20
  5. 2024 Comprehensive Plan and Regulations — City of Sammamish (adopting ordinances O2024-577, O2024-578, O2024-571; regulations effective January 1, 2025) Planning Department — Accessed 2026-09-20
  6. RCW 36.70A.681 — Accessory dwelling units, limitations on local regulation (2023 c 334 s 4, HB 1337) state_statute — Accessed 2026-09-19
  7. RCW 36.70A.030 — Definitions ("major transit stop") state_statute — Accessed 2026-09-19
  8. 2026 Fee Schedule — Development Review, Permits, and Related Fees, City of Sammamish (building permit valuation table, plan review 65%, impact fees under SMC 21.06.045, school fees, Building Plan Review Support, concurrency, clear and grade, surface water development charge) Planning Department — Accessed 2026-09-20
  9. Ordinance O2026-595 — Repealing SMC 21.08.030, 21.08.040, 21.08.050 and 21.09.090 and adopting SMC 21.06.045 Impact Fees, passed May 19, 2026, effective July 1, 2026 (Attachment A: deferral (I), exemptions (D), rate updates (H)) Municipal Code — Accessed 2026-09-20
  10. July 1, 2026 Updated Impact Fees — City of Sammamish (per-square-foot parks and transportation rates, minimums and maximums) Planning Department — Accessed 2026-09-20
  11. Ordinance O2026-597 — Amending SMC 21.10.100 Exemptions (units of 1,750 sq ft or less including ADUs exempt from the inclusionary affordable housing in-lieu fee; retroactive to January 1, 2025), passed August 18, 2026, effective August 29, 2026 Municipal Code — Accessed 2026-09-20
  12. Permit Center — City of Sammamish (MyBuildingPermit.com submittal, Project Guidance, walk-in hours, 425-295-0531) Planning Department — Accessed 2026-09-20
  13. Project Guidance Request Form — City of Sammamish (free; response within one week; September 2026 service notice) Planning Department — Accessed 2026-09-20
  14. Getting Started and FAQs — City of Sammamish Permit Center (online-only applications, submittal vs issuance fees, construction hours SMC 16.05.030) Planning Department — Accessed 2026-09-20
  15. Applications and Forms — City of Sammamish Permit Center (no pre-approved ADU plans; Impact Fee Deferral Affidavit and New Single-Family Residence checklist dated 07/01/26; critical area worksheets) Planning Department — Accessed 2026-09-20
  16. Handout No. 310 — Detached Accessory Dwelling Units (DADUs), City of Sammamish, dated 2023-09 (superseded; retained as a record of the pre-2025 standards) Planning Department — Accessed 2026-09-19
  17. Cost to Build a DADU in Seattle (2025-2026 Update) | Ballard Backyard Cottages industry — Accessed 2026-09-19
  18. How Much Does It Cost to Build an ADU in Seattle? 2026 Guide (Jan. 8, 2026) | Kolmo industry — Accessed 2026-09-19
  19. Seattle ADU Cost 2026 by Type and Size (updated May 12, 2026) | ADU Glossary industry — Accessed 2026-09-19
  20. Prefab ADU 2026: Brands, True Costs (updated May 19, 2026) | OwnADU industry — Accessed 2026-09-19
  21. HUD FY 2026 and FY 2027 Small Area Fair Market Rents, Seattle-Bellevue, WA HUD Metro FMR Area, ZIP codes 98074 and 98075 Government Data — Accessed 2026-09-20
  22. Sammamish Business Portal — City of Sammamish (business licensing; no short-term rental rule published) Planning Department — Accessed 2026-09-20
  23. Washington State Office of Financial Management — April 1, 2026 Population of Cities, Towns and Counties (Sammamish 68,500; file returned 403 on re-verification, figure retained from the 2026-09-01 read) Government Data — Accessed 2026-09-01

Frequently Asked Questions

Can I build an ADU in Sammamish?

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. 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.. 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 $325,000-$550,000. Garage conversions run $100,000-$225,000, and prefab ADUs cost $300,000-$450,000.

How much can I rent an ADU for in Sammamish?

Average ADU rents in Sammamish are $2,900-$3,300/mo for a 1-bedroom and $3,400-$3,850/mo for a 2-bedroom unit.

Do I need parking for an ADU in Sammamish?

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.. 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. 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. Permit fees: 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.

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ADU Guides for Sammamish Homeowners

FAQ

Sammamish ADU FAQ

Answers to every common question about building an ADU in Sammamish — costs, rules, permits, rental income.

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Sammamish ADU Costs

Detailed cost breakdown: construction $325,000-$550,000, garage conversion $100,000-$225,000, plus ROI analysis.

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Garage Conversion ADU Guide

Complete guide to costs ($80K-$180K), impact fee savings, and permits for garage-to-ADU conversions.

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ADU vs Mother-in-Law Suite

Legal differences that affect what you can build, how you can use it, and rental rights.

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About this information: This page is an informational summary of publicly available regulations - it is not legal, financial, or professional advice, and it is not a substitute for confirming requirements with your local planning department. Rules change frequently; always verify current requirements before designing, budgeting, or building. Last verified: 2026-09-20. Contractors we introduce are independent businesses - verify license and insurance directly before hiring. Spotted an error? Tell us - we verify against primary sources and correct quickly.

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