Mercer Island residential zoning
Mercer Island residential zoning and setbacks by zone
Mercer Island's base residential zones share several standards, but side yards depend on lot width and lot coverage depends on slope. Shoreline, critical areas, access easements, and net-lot-area exclusions can materially reduce the initial screen.
Reviewed against official Mercer Island sources in August 2026Mercer Island residential base-zone table
| Zone | Minimum lot | Width / depth | Front | Rear | Side yards | Height | Gross floor area |
|---|---|---|---|---|---|---|---|
| R-8.4 | 8,400 sq ft | 60 ft / 80 ft | 20 ft | 25 ft | 15 ft combined up to 90 ft wide; width formula above 90 ft | 30 ft | 40% |
| R-9.6 | 9,600 sq ft | 75 ft / 80 ft | 20 ft | 25 ft | 15 ft combined up to 90 ft wide; width formula above 90 ft | 30 ft | 40% |
| R-12 | 12,000 sq ft | 75 ft / 80 ft | 20 ft | 25 ft | 15 ft combined up to 90 ft wide; width formula above 90 ft | 30 ft | 40% |
| R-15 | 15,000 sq ft | 90 ft / 80 ft | 20 ft | 25 ft | 15 ft combined up to 90 ft wide; width formula above 90 ft | 30 ft | 40% |
Parcel-specific checks
What can change the table
- For a lot over 90 feet wide, combined side yards equal 17% of lot width. Each side must be at least 33% of the combined requirement.
- Maximum lot coverage generally steps down with average slope: 40% below 15%, 35% from 15% to under 30%, 30% from 30% to 50%, and 20% over 50%.
- Hardscape is a separate allowance, generally 9% of net lot area. Gross floor area is generally 40% of net lot area, subject to definitions and exclusions.
- A structure generally must stay 10 feet from a public right of way or vehicular access easement. Shoreline and critical-area rules can require more.
How to use this guide
Turn a zone row into a useful screen
- 1
Find the parcel on the city's current zoning map and copy the complete zone name.
- 2
Read the full row, table footnotes, and definitions. Do not use only the front setback.
- 3
Check corner-lot yards, access easements, critical areas, shorelines, trees, overlays, and recorded restrictions.
- 4
Use a boundary and topographic survey to place the setbacks against the actual lot.
- 5
Have the city or project team confirm any uncertainty before the design is fixed.
Official resources