Seattle’s Critical Areas Ordinance (CAO) protects environmentally sensitive areas and significantly affects what can be built on lots containing or adjacent to those areas. If your property is near a steep slope, wetland, stream, or the shoreline, here is what you need to know.
What Are Critical Areas?
Seattle defines five types of critical areas:
- Wetlands — including seasonally wet areas and wetland buffers (typically 25-300 ft depending on category)
- Steep slopes — areas with 40%+ grade or known landslide hazards
- Wildlife habitat — designated urban wildlife habitat corridors
- Flood-prone areas — within the 100-year flood plain
- Shoreline jurisdiction — within 200 feet of Puget Sound, Lake Washington, Lake Union, or major waterways
How Critical Areas Affect Your Project
Development within or adjacent to critical areas requires additional study and review. This may include a geotechnical report, wetland delineation, biological assessment, or shoreline substantial development permit. Each adds time and cost to a project.
Buffers
Required buffers from critical areas limit where you can build on your lot. A wetland buffer of 100 feet means nothing can be constructed (including an ADU) within 100 feet of the wetland boundary — not the property line.
Shoreline Master Program
Properties within Seattle’s Shoreline jurisdiction have additional overlay requirements under Seattle’s Shoreline Master Program. Substantial development (over $7,047 in 2026 threshold) within shoreline jurisdiction requires a Shoreline Substantial Development Permit.
Piper Cole Architects has designed numerous projects on sensitive sites including waterfront, hillside, and wetland-adjacent properties. Free consultation — call 425-753-6452.
Ready to Start Your Project?
Piper Cole Architects offers a free initial consultation for all project types — residential, commercial, ADU, and renovation. No obligation. Based in Kirkland, WA. Serving the entire Seattle metro area since 2000.