Guide · Mercer Island · King County · Updated July 2026
How many units can you build in Mercer Island?
If you own or are evaluating a residential lot in Mercer Island, the first question is almost always the same: how many homes can legally go on it? Since Washington's HB 1110 middle-housing law took effect, the answer for most Mercer Island lots is now more than one. Mercer Island allows 2 units per lot by right, rising to 4 near light rail or with one affordable unit.
Mercer Island at a glance: 2 units per lot by right, up to 4 with the city's bonus, plus up to 2 ADUs under HB 1337. Source: MICC 19.02.020 (interim Ord. 25C-02).
What Mercer Island's adopted code allows
Mercer Island allows 2 units per lot by right, rising to 4 near light rail or with one affordable unit. These middle-housing rules are still interim and being finalized.
The details: 4 within a quarter-mile of the light rail station, or on any residential lot with one affordable unit. These are Mercer Island's city-wide standards, per MICC 19.02.020 (interim Ord. 25C-02); the exact figure for a specific parcel — its zone, lot size, transit proximity, and critical areas — is what a Civexa report resolves.
What actually fits: floor area, height, and setbacks
HB 1110 and the city code set the right to a number of units; the zone's building envelope sets what physically fits. Floor-area ratio, lot coverage, height, and setbacks together decide whether those homes pencil as detached cottages, a townhouse row, or a stacked-flat building. Civexa computes this envelope for Mercer Island zone by zone, so the unit count you see is one the zoning can actually hold.
What can shrink it: critical areas
Steep slopes, wetlands, streams, and flood zones can override the unit math on a specific parcel. A lot that qualifies for the maximum on paper may be constrained once environmentally critical areas are mapped. Civexa screens FEMA flood, slope, and county critical-area layers for every Mercer Island address so you find this out before you make an offer, not after.
Get the exact number for your Mercer Island parcel
The figures above are Mercer Island's city-wide rules. The number that matters is the one for your address — its zone, lot size, transit proximity, and critical areas. Civexa turns that into a full feasibility report in about a minute: unit count, buildable envelope, a preliminary pro forma, utilities, and permit path.
Run a Mercer Island feasibility report →
Frequently asked
How many units can I build on a residential lot in Mercer Island?
Mercer Island allows 2 units per lot by right, rising to 4 near light rail or with one affordable unit. That is per MICC 19.02.020 (interim Ord. 25C-02). The exact number for a specific parcel still depends on its zone, lot size, transit proximity, and critical areas — run a Civexa report for the parcel-level figure.
Can I add ADUs on top of that in Mercer Island?
Washington's HB 1337 allows up to two accessory dwelling units on a residential lot. In some cities they count toward the middle-housing total and in others they are separate — your Civexa report applies Mercer Island's specific rule.
What unlocks the maximum in Mercer Island?
4 within a quarter-mile of the light rail station, or on any residential lot with one affordable unit.