Seattle Steep Slope and Landslide ECAs
If you recently discovered your property falls within a steep slope erosion hazard area (commonly called a steep slope ECA) or a landslide-prone area (commonly called a landslide ECA), you’re not alone.
Many Seattle homeowners and architects encounter these Environmentally Critical Areas (ECAs) while planning additions, remodels, accessory dwelling units, or new construction. The City of Seattle’s ECA rules are complex, but with the right geotechnical guidance, your project can move forward.
At Cross Reiter, we help clients navigate ECA permitting. Our Seattle-based geotechnical engineers know what SDCI reviewers are looking for and how to prepare clear, defensible documentation that keeps your project on track.
What Are ECAs?
The City of Seattle regulates certain environmentally sensitive areas to protect people, property, and the environment. Two of the most common ECAs that affect development are:
Steep Slope Erosion Hazard Areas
A steep slope erosion hazard area is any slope 40 percent or steeper over a vertical elevation change of at least 10 feet (Seattle Municipal Code 25.09.012.A.4 and A.3.b.5).
These areas require 15-foot buffers from the top and toe of the slope (SMC 25.09.090.C.1).
To visualize this: a 40 percent slope (2.5H:1V) rising 10 feet covers 25 feet horizontally.
Landslide-Prone Areas
Landslide-prone areas are defined in SMC 25.09.012.A.3 and include slopes or landforms that have failed before or are susceptible to failure because of geology, groundwater, or topography.
You can check whether your property is affected by either designation using the Seattle Department of Construction & Inspections (SDCI) Property Information Map.
What This Means for Your Project
If your property lies within a steep slope ECA or landslide ECA, you’ll likely need to work with a licensed geotechnical engineer and go through Seattle’s ECA permitting process before you can begin construction.
There are two common permitting pathways for most smaller projects:
Relief from Prohibition on Development
Development within steep slope ECAs and their buffers is prohibited unless your project qualifies for relief under one of the following four conditions (SMC 25.09.090.B.2). If your project meets one of these criteria and demonstrates no adverse impact on slope stability, SDCI may grant relief.
The project stays entirely within the footprint of existing legal structures or paved areas.
The slope was created by previous legal grading (e.g., road cuts, rockeries, retaining walls).
The slope is less than 20 feet tall and at least 30 feet from other steep slopes.
The project is the minimum necessary to stabilize an active landslide hazard.
Small Project Waivers
Small Project Waivers (SMC 25.09.090.D) can allow limited development in steep slope erosion hazard areas or buffers on certain older lots if:
The total development area is less than 750 square feet, with no more than 300 square feet within the ECA or buffer.
It’s impractical to build elsewhere on the lot.
The project is designed to minimize impacts to the slope.
Do You Need a Geotechnical Report?
Almost certainly.
SDCI typically requires a geotechnical engineering report to:
Verify the extent and characteristics of the ECA,
Evaluate potential impacts of proposed grading or construction, and
Demonstrate the project will have no adverse effect on slope stability or erosion.
You may also need a topographic survey from a licensed surveyor to map the slope and buffers accurately.
In landslide-prone areas, the City requires that all disturbed portions of the site be completely stabilized. Your geotechnical engineer will identify the specific measures needed to achieve stability and comply with SMC 25.09.080.