SMA is extra review — not a ban
If your lot is within 200 ft of a designated waterway, the Shoreline Management Act adds permits, setbacks, and review time to your ADU project. But unlike a watershed prohibition, it does not stop you from building — most shoreline lots can still add an ADU with proper permitting.
What Is the Shoreline Management Act?
The Shoreline Management Act (SMA) is a Washington State law that regulates development within 200 feet of the ordinary high water mark (OHWM) of designated waterways. Seattle administers it locally through its Shoreline Master Program.
Within this 200 ft "shoreline jurisdiction," projects — including ADUs — are subject to additional setbacks, environmental standards, and a shoreline permit.
Source: RCW 90.58 (Shoreline Management Act); Seattle Shoreline Master Program (SMC 23.60A)
Which Waterways Are Covered
- Lake Washington (Madrona, Leschi, Mount Baker, Madison Park, Laurelhurst shorelines)
- Lake Union (including Portage Bay and the Montlake Cut)
- Union Bay and the ship canal corridor
- Green Lake
- Puget Sound (Alki / West Seattle, Magnolia, the central waterfront)
- Cedar River and the Duwamish / Green River
- Creeks with significant drainage and their associated buffers
What SMA Means for ADUs
SMA review is not a blanket prohibition. Most shoreline lots can still build an ADU, but the project will generally require a Shoreline Substantial Development Permit or, for certain uses, a Shoreline Conditional Use Permit (smaller projects may qualify for an exemption).
Added Timeline
Expect SMA review to add 4–8 weeks to permitting, sometimes longer for complex sites — on top of the standard Seattle DCI ADU review.
What SMA Requires
Shoreline permit
Most ADU work in shoreline jurisdiction needs a Shoreline Substantial Development Permit, or a Shoreline Conditional Use Permit / exemption depending on scope and cost.
Setbacks from the OHWM
Building setbacks from the ordinary high water mark typically range from 50 to 200 ft depending on the shoreline environment designation assigned to your lot.
Stormwater management
Shoreline projects face heightened stormwater and drainage standards to protect water quality.
Vegetation conservation
Native vegetation buffers along the shoreline often must be retained or restored as a condition of approval.
What SMA Does NOT Do
- It does NOT ban ADUs on all shoreline lots — unlike a watershed prohibition, the SMA is a review process, not a blanket no.
- Many lots within the 200 ft shoreline jurisdiction can still build an ADU with the right permits.
- Most of a typical Seattle neighborhood is well outside shoreline jurisdiction and is unaffected.
Neighborhoods Most Affected
| Waterway | Neighborhoods |
|---|---|
| Lake Washington | Madrona, Leschi, Mount Baker, Madison Park, Laurelhurst |
| Lake Union / Union Bay | Eastlake, South Lake Union, Montlake |
| Green Lake | Green Lake neighborhood |
| Puget Sound | Alki / West Seattle, Magnolia |
Practical Advice
Check your lot's shoreline designation in Seattle's GIS portal. If you're within 200 ft of any waterway, plan for the extra review up front.
Budget roughly 4–8 extra weeks and $5,000–$15,000 for the shoreline permit and associated studies, and engage your designer early so the site plan respects the OHWM setback from day one.
Not sure if your lot is in shoreline jurisdiction? Our free feasibility study identifies SMA jurisdiction automatically using GIS data — so you know before you design whether a shoreline permit is in your future.
Already know you're on the water? Our permit specialists handle shoreline submittals through DCI for you.
Official Sources
- WA Shoreline Management Act (RCW 90.58): app.leg.wa.gov
- Seattle Shoreline Master Program: seattle.gov/sdci
- Seattle DCI: (206) 684-8600
Last updated: June 2026. Not legal advice. Verify current regulations with Seattle DCI.
Building Near the Water? Check Your Shoreline Status Free
Our free feasibility study flags SMA jurisdiction and shoreline setbacks for your specific parcel.
Book Free WalkthroughPrefer to call? (206) 222-2898
