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Issaquah ADU and detached ADU rules

Issaquah ADU and DADU rules, without the guesswork

Issaquah allows two accessory dwelling units in attached or detached combinations, but the useful answer is not simply “yes.” The principal-unit size, zone, parking, utilities, access, geologic hazards, trees, and impervious-surface limits still decide whether a detached unit is practical.

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 detached ADU can be viable on many Issaquah lots. Each ADU is normally capped at 1,000 square feet, the principal unit must generally be at least 25 percent larger than the largest ADU, and a new structure follows the applicable development standards. Projects outside geologically hazardous areas may qualify for Issaquah's self-certified DADU program.

Up to 2ADUs per eligible lotAny attached or detached combination, subject to the lot and unit-density rules.
1,000 sq ftNormal maximum per ADUNonliving area is excluded; qualifying conversions can exceed the cap.
Level 1Land-use reviewA separate building permit and site-specific documents are still required.

What is allowed on one lot

Two ADUs, in any combination of attached or detached units, may accompany one detached single-family residence or middle housing when the lot satisfies the principal-unit lot-size and applicable density rules. Legally nonconforming single-family lots may also qualify, subject to Issaquah's nonconforming provisions.

  • Two attached ADUs.
  • One attached ADU and one detached ADU.
  • Two detached ADUs, including units associated with a garage or separate structures.
  • Conversion of a legally established principal or accessory structure.

Official sources: IMC 18.500.090: accessory dwelling unit standards · RCW 36.70A.681: limits on local ADU regulation

Size, height, setbacks, and site capacity

Maximum size
Each ADU is normally limited to 1,000 square feet of living area. Garages, sheds, and decks are excluded unless they contain the proposed living space. The principal unit must generally be at least 25 percent larger than the largest ADU. A qualifying internal conversion or conversion of an existing accessory structure may exceed the cap without an exterior expansion other than minimal work needed to function as a separate residence.
Height
An ADU follows the height limit for the principal unit in the applicable zone; there is no one citywide DADU height number. Height also uses Issaquah's grade and roof measurement rules.
Setbacks
New attached additions and detached structures follow the applicable zone and development standards. A new ADU has no rear-yard setback where the rear lot line abuts a public alley that the city does not routinely plow. A four-foot-wide walkway is required from the address side of the right-of-way or driveway to the ADU's main entrance.
FAR, lot coverage, or impervious area
An ADU associated with a detached single-family home does not count toward lot density; an ADU associated with middle or multifamily housing does. The physical work still consumes applicable building area, impervious surface, tree, drainage, and critical-area capacity.

Official sources: IMC 18.500.090: accessory dwelling unit standards · IMC 18.404: form and intensity

Parking, occupancy, renting, and separate sale

Parking
Parking follows IMC Chapter 18.604, and tandem parking may be used. State law prohibits an ADU parking requirement within a half-mile walking distance of a major transit stop and caps what can be required elsewhere. Confirm the parcel-specific count rather than assuming the ADU needs a new stall.
Owner occupancy
No owner-occupancy requirement applies. Washington law prohibits Issaquah from requiring the owner to live in either the ADU or another unit on the lot.
Short-term rental
The ADU section does not itself create a short-term-rental permission or prohibition. Check current city business, rental, and nuisance rules and any recorded private restrictions before planning stays under 30 days.
Separate sale
Issaquah may not prohibit an ADU's separate conveyance as a condominium solely because it began as an ADU. A fee-simple lot split is a separate land-division question.

Official sources: IMC 18.500.090: accessory dwelling unit standards · RCW 36.70A.681: limits on local ADU regulation

Utilities, fire access, and the permit path

The city reviews water, gas, electric, fixture count, and fire-flow capacity. The ADU may share the principal unit's utility connections and meters when capacity is adequate. Sewer treatment capacity charges and provider requirements can still apply.

  1. Complete the zoning, critical-area, tree, utility, access, and impervious-surface screen.
  2. Apply for the ADU land-use review and building permit through MyBuildingPermit.
  3. For a detached unit outside a geologically hazardous area, determine whether the city's architect-led self-certification path is available and submit its affidavit if used.
  4. Provide the site plan, architectural and structural plans, energy documents, utility information, and any required soils, drainage, tree, or critical-area material.
  5. Close every related permit and inspection before occupancy.

Official sources: Issaquah detached ADU permit page · Issaquah ADU submittal requirements

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 geologically hazardous-area designation removes the project from the self-certified DADU program and can require a soils or geotechnical report.
  • Critical areas and buffers are exempt from portions of the statewide ADU mandate; mapped features need field and city confirmation.
  • Tree retention, canopy, replacement, and protection can determine where a detached unit and utility trench can go.
  • The four-foot pedestrian route, fire access, hydrant/fire-flow conditions, driveway, and construction access must all fit.
  • King County sewer treatment capacity charges and the serving utility district's connection rules are separate from the land-use allowance.

Official sources: Issaquah detached ADU permit page · IMC 18.500.090: accessory dwelling unit standards

Prepared for a future address check

What a reliable DADU feasibility screen must verify

Issaquah is ready for a detailed automated screen once the parcel's zone, unit configuration, geologic-hazard status, principal-home floor area, and surface/utility constraints are joined in one record.

Jurisdiction and zoning

Confirm the address is actually governed by Issaquah 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.

A screening result should mark self-certification eligibility separately from basic DADU eligibility. It must never infer that an unmapped critical area, adequate fire flow, sewer capacity, or a protected-tree impact has been professionally cleared.

Primary sources

Official Issaquah 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.

IMC 18.500.090: accessory dwelling unit standardsCurrent count, size, conversions, height, alley, walkway, parking, utility, address, and condominium rules.IMC 18.404: form and intensityZone-specific height, setbacks, impervious surface, and principal-unit standards.Issaquah detached ADU permit pageMyBuildingPermit submission, self-certification eligibility, fees, and current submittal links.Issaquah ADU submittal requirementsCurrent plan and application content required for an ADU permit.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 Issaquah 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.