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Sammamish ADU and detached ADU rules

Sammamish ADU and DADU rules by zone and structure type

Sammamish allows two ADUs, but a freestanding DADU and an ADU attached to a garage use different setback and height tables. That distinction, together with the site's hardscape, trees, drainage, critical areas, and septic or sewer conditions, must be part of any fit test.

Reviewed August 14, 2026 against adopted city code and official city guidance

Quick definitions. An accessory dwelling unit (ADU) is a complete additional home on the same property. A detached ADU (DADU) is in its own building. FAR or GFA limits how much enclosed floor area the property may contain.

Bottom line

What the current rules mean

A standalone DADU can be up to 1,000 square feet, uses five-foot side and rear yards, and is limited to 24 feet. An ADU attached to a garage or another structure uses the other-structure standards, including 35-foot height and larger side/rear yards in lower-density zones. ADU area is excluded from residential FAR, but its physical footprint and paving are not free of other site limits.

Up to 2ADUs per eligible lotTwo attached, two detached, or one of each.
1,000 sq ftDetached ADU maximumAttached ADU size follows the larger-of formula.
FAR-exemptADU floor areaHardscape, trees, setbacks, and drainage still apply.

What is allowed on one lot

Up to two ADUs are allowed on a lot with a detached single-family residence.

  • Two attached ADUs, such as basement, addition, or above an attached garage.
  • Two detached ADUs in one or two detached structures, including units attached to each other or a detached garage.
  • One attached and one detached ADU.
  • Conversion of an existing structure.

Official sources: Sammamish accessory dwelling unit guidance · RCW 36.70A.681: limits on local ADU regulation

Size, height, setbacks, and site capacity

Maximum size
A standalone detached ADU may contain up to 1,000 square feet. An attached ADU may be up to 50 percent of the principal single-family residence or 1,000 square feet, whichever is larger.
Height
A standalone DADU is limited to 24 feet. If the ADU is attached to another structure such as a garage, the other-structure table applies and allows 35 feet.
Setbacks
Standalone DADUs use a five-foot side and rear yard in R-1 through R-18, with front yards of 20 feet in R-1, 15 feet in R-4/R-6, and 10 feet in R-8/R-12/R-18. An arterial front yard is at least 30 feet. An ADU attached to a garage or other structure uses 10-foot side/rear yards in R-1 through R-8 and five feet in R-12/R-18.
FAR, lot coverage, or impervious area
ADU floor area does not count toward the principal residence's FAR. The building, driveway, parking, walkways, patios, utilities, and grading still affect hardscape, drainage, tree retention, and critical-area compliance.

Official sources: Sammamish accessory dwelling unit guidance · SMC 21.04.030: residential zone standards

Parking, occupancy, renting, and separate sale

Parking
No additional parking is required when the property already has four or more spaces, including garage and driveway spaces. For other conditions, confirm the current required count through Sammamish Project Guidance rather than relying on the city's superseded 2023 DADU handout.
Owner occupancy
No owner-occupancy requirement applies under the current statewide rule.
Short-term rental
The current city ADU page does not establish a DADU-specific short-term-rental rule. Confirm current city business, rental, and nuisance requirements and recorded private restrictions before planning stays under 30 days.
Separate sale
State law prevents a citywide prohibition on separate condominium conveyance solely because a unit began as an ADU. A unit-lot or other fee-simple division remains a separate land-use process.

Official sources: Sammamish accessory dwelling unit guidance · RCW 36.70A.681: limits on local ADU regulation

Utilities, fire access, and the permit path

Applications are filed through MyBuildingPermit. Use Sammamish's free Project Guidance for property-specific answers and coordinate building, site development, water/sewer or septic, drainage, fire, critical areas, and tree requirements.

  1. Use the Sammamish Property Tool and submit free Project Guidance for the exact parcel and proposed configuration.
  2. Choose the standalone-DADU or other-structure standards before drawing the envelope.
  3. Prepare the MyBuildingPermit application plus site, architectural, structural, energy, drainage, tree, critical-area, fire, and utility documents required for the site.
  4. Coordinate sewer or septic and water capacity before treating the unit count as feasible.
  5. Complete every building, site, utility, fire, and related inspection before occupancy.

Official sources: Sammamish Permit Center · Sammamish applications and forms

Rules that can change the answer for one property

A citywide ADU allowance does not prove that a detached unit fits on a particular lot. Resolve these items before buying plans or fixing the DADU location.

  • A DADU attached to a garage is not entitled to the standalone five-foot-yard table; it uses the other-structure standards.
  • Critical drainage areas near Pine or Beaver Lake or landslide hazards can trigger review for a new ADU even at small impervious-area thresholds.
  • Tree retention/protection and the utility trench can control placement even though ADU floor area is FAR-exempt.
  • Septic capacity or a required sewer connection can make an otherwise buildable envelope impractical.
  • Arterial frontage increases the front setback to at least 30 feet.

Official sources: Sammamish accessory dwelling unit guidance · SMC 21.03.050: surface water management

Prepared for a future address check

What a reliable DADU feasibility screen must verify

Sammamish's future DADU screen must first classify the unit as standalone or attached to another structure, then apply the zone row, FAR exemption, hardscape, trees, drainage basin, critical areas, parking supply, and utility system.

Jurisdiction and zoning

Confirm the address is actually governed by Sammamish and select the correct base-zone standards.

Existing and proposed units

Count the principal dwelling, existing legal ADUs, middle-housing units, and the proposed attached or detached configuration.

Surveyed parcel geometry

Use legal lot lines, front/rear designation, alleys, access easements, and existing structures rather than a tax-map rectangle.

Capacity calculations

Test the ADU against the rules that apply locally: FAR or GFA, building or lot coverage, impervious or hard surface, and landscaping.

Mapped and field constraints

Screen critical areas and buffers, shorelines, slopes, flood hazards, significant trees, drainage, and recorded easements.

Utilities and emergency access

Verify water, sewer or septic, fire flow, hydrant and apparatus access, power, meters, and trench routes.

The automated result must not use the city's older 18-foot/one-DADU handout. It should distinguish current official web guidance from any older PDF still indexed online and send unresolved parking or septic questions to Project Guidance.

Do not rely on the older rule

The city's 2023 DADU handout still appears in search results and says one DADU, an 18-foot height limit, and two parking spaces. The current city ADU page instead allows two ADUs, gives a 24-foot standalone-DADU height, and says no added parking where four spaces already exist.

Primary sources

Official Sammamish and Washington references

These are the sources used for this guide. Check them again before filing because ordinances, fee schedules, and permit forms can change.

Sammamish accessory dwelling unit guidanceCurrent count, configurations, size, FAR exemption, parking trigger, setbacks, and height tables.SMC 21.04.030: residential zone standardsCurrent zoning, FAR, hardscape, setback, height, and table notes.Sammamish Permit CenterMyBuildingPermit submission, permit-status tools, free Project Guidance, and staff contact.Sammamish applications and formsCurrent building, site development, critical-area, septic, fire, and supporting forms.SMC 21.03.050: surface water managementDrainage review triggers, including ADUs in critical drainage areas.RCW 36.70A.681: limits on local ADU regulationWashington's current baseline for two ADUs, size, height, parking, owner occupancy, conversions, alleys, fees, and critical-area exceptions.

A practical first-pass checklist

  1. 1

    Confirm the parcel is inside Sammamish city limits and identify its current zone.

  2. 2

    Map the principal home, lot lines, easements, trees, utilities, driveway, critical areas, and buffers on one current survey.

  3. 3

    Test the ADU footprint against setbacks, height, floor-area or coverage limits, hardscape, parking, and construction access.

  4. 4

    Ask the utility providers to confirm water, sewer, power, meter, side-sewer, and capacity requirements before fixing the design.

  5. 5

    Have the city confirm the complete land-use, building, fire, right-of-way, tree, and utility permit list in writing.