Mercer Island ADU and detached ADU rules
Mercer Island ADU and DADU rules for real lots
Mercer Island now allows up to two attached or detached ADUs, but its slope-based lot coverage, gross-floor-area limits, hardscape, trees, shoreline, geologic hazards, and access easements make a parcel-level screen essential.
Reviewed August 14, 2026 against adopted city code and official city guidanceQuick 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
The citywide rule allows up to two ADUs, each between 220 and 1,000 square feet excluding garage area. A new detached unit generally follows the same development standards as the principal unit, so net lot area, average slope, gross floor area, side-yard formulas, hardscape, and tree retention can be more restrictive than the 1,000-square-foot ADU cap.
What is allowed on one lot
Up to two attached or detached ADUs are permitted per lot, subordinate to a single-family or middle-housing dwelling, when the lot meets the minimum size and other applicable standards.
- Two attached ADUs.
- One attached ADU and one detached ADU.
- Two detached ADUs.
- Conversion of an existing or legally nonconforming structure when building-code requirements can be met.
Official sources: Mercer Island City Code, including MICC 19.02.030 · Mercer Island 2026 staff code summary · RCW 36.70A.681: limits on local ADU regulation
Size, height, setbacks, and site capacity
- Maximum size
- Each ADU must be at least 220 and no more than 1,000 square feet, excluding garage area.
- Height
- ADUs follow the principal-unit development standards in MICC 19.02.020 unless the ADU section says otherwise. For residential lots, both the average-building-elevation roof limit and downhill façade/height method can matter.
- Setbacks
- The principal-unit setbacks, including Mercer Island's width- and façade-height-based side-yard rules, generally apply. A detached ADU may sit at the lot line that abuts a public alley. Access easements, shorelines, and critical areas can require additional separation.
- FAR, lot coverage, or impervious area
- ADU living area and any garage must be tested against residential gross floor area, slope-based lot coverage, the separate hardscape allowance, and net-lot-area exclusions. ADUs are also treated as units for the city's middle-housing unit-density calculation.
Official sources: Mercer Island City Code, including MICC 19.02.030 · Mercer Island residential zoning standards
Parking, occupancy, renting, and separate sale
- Parking
- No additional ADU parking is required within a half-mile walking distance of a major transit stop. The current code material applies one space per unit on lots of 6,000 square feet or less and two spaces per unit on larger lots before a zero-lot-line subdivision or lot split; have staff confirm the count and any existing-space credit for the exact proposal.
- Owner occupancy
- No owner-occupancy requirement applies under the current state and local framework.
- Short-term rental
- The ADU section does not by itself establish short-term-rental eligibility. Confirm the current city rental/business rules and recorded private restrictions before using either unit for stays under 30 days.
- Separate sale
- An ADU on an approved unit lot may be sold separately, and a condominium unit that began as an ADU may be conveyed separately. The land-division or condominium process is distinct from the ADU permit.
Official sources: Mercer Island City Code, including MICC 19.02.030 · Mercer Island 2026 staff code summary · RCW 36.70A.681: limits on local ADU regulation
Utilities, fire access, and the permit path
The city requires a land-use application and building permit and reviews water-meter sizing, sewer, fire area, drainage, and site development. Utility-district capacity and charges remain separate approvals and costs.
- Calculate net lot area, average slope, GFA, lot coverage, hardscape, setbacks, trees, and any shoreline or critical-area limits.
- Prepare the Type II ADU land-use application, project narrative, code-compliance matrix, current title report, ownership/agent documents, and plan set.
- Coordinate the land-use review with the building permit and any tree, shoreline, critical-area, drainage, fire, or right-of-way approvals.
- Confirm water-meter sizing, sewer capacity and connection work, power, and trench routes.
- Complete land-use conditions, building inspections, and any recording or separate-sale steps before occupancy or conveyance.
Official sources: Mercer Island 2026 ADU application · Mercer Island City Code, including MICC 19.02.030
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.
- Average slope changes maximum lot coverage, so a flat percentage is not reliable without a surveyed slope calculation.
- Net lot area can exclude rights-of-way, access easements, and other areas, reducing both GFA and coverage capacity.
- Geologically hazardous areas, wetlands or watercourses, shorelines, and their buffers can move or eliminate the apparent DADU area.
- Tree retention and exceptional-tree protection must be overlaid with the building, driveway, fire access, utilities, and construction workspace.
- Recorded covenants, view easements, and shared access or utility rights can be stricter than the general citywide ADU rule.
Official sources: Mercer Island City Code, including MICC 19.02.030 · Mercer Island residential zoning standards · Mercer Island 2026 ADU application
Prepared for a future address check
What a reliable DADU feasibility screen must verify
A Mercer Island DADU screen must calculate capacity from surveyed net lot area and slope, then test the detached footprint, garage, hardscape, GFA, side-yard formula, trees, critical areas, and access together.
Confirm the address is actually governed by Mercer Island and select the correct base-zone standards.
Count the principal dwelling, existing legal ADUs, middle-housing units, and the proposed attached or detached configuration.
Use legal lot lines, front/rear designation, alleys, access easements, and existing structures rather than a tax-map rectangle.
Test the ADU against the rules that apply locally: FAR or GFA, building or lot coverage, impervious or hard surface, and landscaping.
Screen critical areas and buffers, shorelines, slopes, flood hazards, significant trees, drainage, and recorded easements.
Verify water, sewer or septic, fire flow, hydrant and apparatus access, power, meters, and trench routes.
A preliminary map cannot verify average building elevation, net lot area, geologic conditions, a tree-retention plan, utility capacity, or recorded private rights. Those items must remain explicit confirmation steps.
Do not rely on the older rule
Older Mercer Island code PDFs still found in search results describe one ADU, a 900-square-foot cap, and owner occupancy. Those provisions are stale. Use current MICC 19.02.030 and the city's 2026 application material.
Primary sources
Official Mercer Island 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.
A practical first-pass checklist
- 1
Confirm the parcel is inside Mercer Island city limits and identify its current zone.
- 2
Map the principal home, lot lines, easements, trees, utilities, driveway, critical areas, and buffers on one current survey.
- 3
Test the ADU footprint against setbacks, height, floor-area or coverage limits, hardscape, parking, and construction access.
- 4
Ask the utility providers to confirm water, sewer, power, meter, side-sewer, and capacity requirements before fixing the design.
- 5
Have the city confirm the complete land-use, building, fire, right-of-way, tree, and utility permit list in writing.