"We hired MNBE to build our ADU in Bellevue, and the entire experience was amazing. They handled everything from foundations and framing to kitchen, siding, windows and HVAC."
Zena Heywood
Bellevue, WA · Attached ADU
FIVE-STAR RATEDRATED ON GOOGLE
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Mercer Island ADU & DADU rules, in plain English
Mercer Island's ADU rules are interim regulations, in force since June 30, 2025. Here is what they allow today, rule by rule. Written by MNBE founder Aharon Ben Elisha. Checked October 28, 2026.
MICC 19.02.030 Interim rules since Jun 30, 2025 2 ADUs per lot
The Update
What changed in Mercer Island.
Mercer Island adopted its HB 1337 rules as interim regulations: Ordinance 25C-02, effective June 30, 2025, renewed since then while the city writes permanent rules (city project page). The rules sit in MICC 19.02.030.
Because the rules are interim, they can change at each renewal. We check the current version before every design.
Rules for other cities are in our ADU rules by city guide.
Mercer Island · MICC 19.02.030
Mercer Island's ADU rules, by the numbers.
The four numbers homeowners ask about first. Check them against your lot with a feasibility study →
2
ADUs per lot
Attached or detached.
1,000sq ft
Size cap
From 220 to 1,000 sq ft, not counting garage area.
30feet
DADU height
The house's limit, so two stories are possible.
25feet
Rear setback
The house setbacks apply, except on a public alley.
The Code
Mercer Island ADU rules, line by line.
The main single-family zones. Zone-specific numbers vary, so we confirm them for each lot.
| Rule | What Mercer Island allows | Source |
|---|---|---|
| ADUs per lot | Up to two ADUs, attached or detached, with a single-family or middle housing dwelling. | MICC 19.02.030 |
| Size | 220 to 1,000 sq ft, not counting garage area. | MICC 19.02.030 |
| Height | A DADU follows the house's limit: 30 feet, including on the downhill side. | MICC 19.02.040 |
| Setbacks | The house setbacks: front 20 feet, rear 25 feet, sides at least 5 feet with a combined minimum. A DADU may sit on the lot line where it meets a public alley. | MICC 19.02.030 |
| Floor area | The lot's gross floor area limit includes the ADU, and ADUs, garages, sheds and other accessory buildings together may use no more than 25% of the allowed floor area, so an existing garage can take much of the room for a DADU. | City ADU key concepts |
| Lot coverage | 40% on flatter lots, stepping down to 35%, 30% and 20% as the slope increases. | MICC 19.02.020 |
| Minimum lot size | The lot must meet the minimum for the house in its zone. Lots created before September 28, 1960 are exempt. | MICC 19.02.030 |
| Parking | None within half a mile walking distance of a major transit stop. Otherwise one space per ADU on lots of 6,000 sq ft or less, and two per ADU on larger lots. | MICC 19.02.030 |
| Owner occupancy | Not required. | MICC 19.02.030 |
| Selling separately | An ADU on a unit lot, or a condominium unit first built as an ADU, can be sold separately. | MICC 19.02.030 |
| Existing structures | Existing structures can be converted, including legally nonconforming ones. | MICC 19.02.030 |
| Design and review | Additions and new detached ADU buildings match the house's roof pitch, siding and windows. A land use review comes before the building permit. | MICC 19.02.030 |
| Street improvements | Frontage improvements are not required for an ADU. | MICC 19.02.030 |
| Trees | Trees 10 inches in diameter and up count toward retention, and exceptional trees, 24 inches and up, must be kept when retention applies. | MICC 19.10 |
Checked October 28, 2026. Sources: MICC Chapter 19.02, the city’s middle housing and ADU page and RCW 36.70A.681. Fire code amendments require sprinklers in new dwellings; we confirm what applies to each ADU design. A plain-English summary, not legal advice. The code governs, and we confirm it for your lot.
Your Lot
How we check a Mercer Island lot for an ADU.
The code sets the outer limits. Your lot sets the real ones. This is the order we work in during the free site visit and feasibility.
01
Zone and overlays
We confirm your zone and check Mercer Island's maps for critical areas, shoreline and slopes, which can move or shrink the buildable area.
Zoning
02
Trees
We locate the trees the code protects and their root zones before we place the ADU.
Tree code
03
Fit the footprint
We test size, height, setbacks, coverage and floor area against your house, and whether an existing building can be converted.
Footprint
04
Utilities
We check whether water, sewer and power can come from the house or need their own connections.
Utilities
05
Design and permit
We draw the plans in house, submit to Mercer Island, answer corrections and carry the project through inspections.
City review
Reviews
What Seattle homeowners say about building with MNBE.
Verified five star reviews from owners we guided through ADU feasibility, permitting, and construction.
"We hired MNBE for a full renovation of our 100 year old home and the results exceeded expectations, structural upgrades, new insulation, a luxury bathroom, a high end kitchen, a new roof and mini split AC."
Dave Gierok
Redmond, WA · DADU
"My mom hired MNBE Construction for her two-bedroom DADU in Seattle, and watching the process up close was impressive. They did it all: design, architectural plans, the building permit, and the full construction. … The communication was excellent and there were never any surprises."
Maryam Z.
White Center, WA · ADU
Schedule Free Estimate
FIVE-STAR RATEDRATED ON GOOGLE
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FAQs · Mercer Island ADU rules
Mercer Island ADU questions we hear on site visits.
Short answers from the code. For your exact lot, call us or book the free site visit.
Up to two, attached or detached, under the interim rules in MICC 19.02.030.
From 220 to 1,000 square feet, not counting garage area. The lot's floor area limit also includes the ADU.
Not within half a mile walking distance of a major transit stop. Elsewhere the code asks for one space per ADU on lots of 6,000 square feet or less, and two on larger lots.
Yes. Additions and new detached ADU buildings match the house's roof pitch, siding and windows.
No. They are interim regulations, renewed while the city writes permanent rules, so we check the current version before every design.
Contact Us · Free Estimate
Tell us about your project.
We'll check your address against Mercer Island's ADU code and get back to you within one business day with a clear next step.
Contact information
Thank you for considering MNBE. Thank you for considering MNBE. Tell us your address and we will tell you, in writing, what Mercer Island's code allows on your lot. We respond within one business day, Monday through Saturday.
(206) 637 0044
info@mnbe-wa.com
2014 Fairview Ave Unit 2308, Seattle, WA 98121