Can I build a backyard home in Mercer Island?
Since 2023, Washington law lets most city lots hold up to two more homes. Mercer Island writes its own rules on top. Here’s what the code says, in plain English — and a free report on what may fit on your lot.
What Mercer Island allows.
The headline standards. Tap any one for the full wording and the code section.
ADUs per lot2: up to two attached or detached ADUs per lotFull wording
2: up to two attached or detached ADUs per lot (MICC 19.02.030.B.2).
Max size1,000 sq ft max, 220 sq ft min (garage not counted)Full wording
1,000 sq ft maximum and 220 sq ft minimum, excluding any garage area (MICC 19.02.030.B.4). A detached ADU building also counts toward the 25% accessory-building share of allowed gross floor area, with an ADU increase allowed (MICC 19.02.040.C.1).
Max height30 ft (the house’s limit)Full wording
30 ft: a detached building containing an ADU must meet the primary building height limit of 30 ft above average building elevation (MICC 19.02.040.C.2.b; 19.02.020).
Front setback20 ft in R-8.4, R-9.6, R-12 and R-15 (same as the house)Full wording
Same as principal: 20 ft in R-8.4, R-9.6, R-12 and R-15 (MICC 19.02.030.B.5; 19.02.020).
Side setback5 ft minimum (same as the house)Full wording
Same as principal: 5 ft minimum, or 33% of the combined side yards if greater. Side yards total 15 ft on lots 90 ft wide or less, or 17% of width on wider lots (MICC 19.02.020). A DADU may sit at a lot line that abuts a public alley (MICC 19.02.030.B.6).
Rear setback25 ft (same as the house)Full wording
Same as principal: 25 ft (MICC 19.02.020). A DADU may be at the lot line if it abuts a public alley (MICC 19.02.030.B.6).
Lot coverage40% on lots under 15% slopeFull wording
40% on lots under 15% slope; 35% at 15–30%, 30% at 30–50%, 20% over 50% (MICC 19.02.020).
Parking1 per unit on lots to 6,000 sq ft, 2 on larger; none near major transitFull wording
1 space per unit on lots of 6,000 sq ft or less, 2 spaces per unit on larger lots, and none within one-half mile walking distance of a major transit stop (MICC 19.02.030.B.9).
Where a city figure reads stricter than the state floor (RCW 36.70A.681), the floor may control — the report flags it for your parcel.
Living there, selling it, plans, time and fees.
Each answer is the code’s own wording, with the section it comes from. “Not published yet” means the city hasn’t posted a figure — the report asks the city, not the internet.
Are backyard homes allowed?
Yes. An ADU may be added to or within the principal unit, or in a detached structure, subordinate to a single-family or middle housing dwelling in the residential zones (MICC 19.02.030.B, B.7).
Minimum lot size
An ADU is allowed where the principal unit meets the zone minimum lot size (MICC 19.02.030.B.1, 19.02.020.A). No separate ADU minimum.
Do you have to live there?
No. Removed by Ord. 25C-02; MICC 19.02.030 contains none.
Can it be sold on its own?
Yes. ADUs on a unit lot may be sold individually from the principal unit, and condominium units originally built as ADUs may be conveyed separately (MICC 19.02.030.E).
Pre-approved plans
None found
How long the city takes
Not published
Impact fees
Resolution 1671 applies the existing impact fee rates to ADUs and middle housing; RCW 36.70A.681 caps ADU impact fees at 50% of the principal unit's.
Plans Mercer Island has already approved.
A pre-approved plan skips the design review: Mercer Island has already checked the drawings, so the permit review is about your lot.
Loading the plans…
Pictures are the official pages’ own. A pre-approved plan is not a permit: the city still reviews your site.
The code is half the answer. Your parcel is the other half.
Lines and easements
Where the lot lines really are, which setbacks apply on each side, and whether a utility or access easement takes the corner you had in mind.
Critical areas and slope
Steep slopes, wetlands and streams come with buffers that no citywide number shows. The report reads the county’s maps for your parcel.
Sewer, septic, water, trees
Whether sewer reaches the lot or it’s on septic, where the water service is, and which trees a city protects — the things that decide where a unit can actually go.
What Mercer Island allows, what could fit on your lot, and a planning cost range. If something won’t fit, the report says so — and shows what might.
Where this page comes from.
Read from the Mercer Island municipal code and the city’s own pages on November 5, 2026. The same table feeds the HousUp report, so the report and this page never disagree. Cities amend their codes; before you design anything, confirm the current wording with the city.
2 sources, read November 5, 2026
Find out what your property could become.
Type your address. Within a few hours: what Mercer Island allows, what could fit, and a planning cost range. Free, no obligation.