ECA Hillside Residence I — Design Study

### How we approach this site

A design study showing how a Seattle SDCI Environmentally Critical Area parcel can be developed without triggering the kind of excavation that severs mature root systems.

Foundation logic

Seattle SDCI requires a permit for any deck inside an ECA, regardless of height. For slopes >40%, a geotechnical hazard review is mandatory and stamped drawings from a licensed PE are required.

This approach uses pin-piles driven 12 feet deep to bypass active topsoil. Mature Douglas Firs and their root systems remain intact, which protects both the structural soil stability and the parcel’s tree-retention status under Seattle’s land-use code.

Material logic

Capped polymer PVC (TimberTech AZEK Vintage) is the conservative material choice for ECA and shoreline-adjacent decks — the 100% synthetic core absorbs zero moisture, eliminating the rot risk that wood-fibre composite cores carry on damp, low-clearance sites.

A dry under-deck ceiling with integrated infrared heaters extends usable outdoor hours through the 8-month wet season, addressing the “wasted space” pain point that PNW homeowners flag most often.

For the regulatory background, see [Do I Need a Permit for My Deck in Seattle?](/journal/do-i-need-a-permit-for-my-deck-seattle/) and our [Slope & ECA Decks service page](/services/slope-eca-decks/).