How Does Engineering Permitting Work in Unincorporated Snohomish County, WA?

On any parcel touching a mapped landslide hazard area — ravine lots, bluff-top properties, hillside acreage — the geotechnical investigation is the project. The county expects the geotechnical engineer to characterize the slope, analyze stability under static and seismic conditions, and set the rules the rest of the design obeys: building setbacks from slope crest and toe, grading limits, retaining wall requirements, and surface and subsurface drainage design that keeps water out of the slide mass. Foundations near slopes may require deep systems bearing below potential failure surfaces. The county's review of the geotechnical report is substantive, and peer review or additional analysis can be required on complex slopes. For the owner, the honest message is that some slopes should not carry the project as first imagined — the geotechnical findings may shrink the footprint, push the building to the flatter portion of the lot, or recommend against the purchase entirely. That finding, delivered early, is worth more than any design heroics later. After Oso, nobody in this county treats slope review as paperwork.

The civil design centers on the county drainage manual: collect, treat, and control runoff per the manual's facility sizing, with infiltration testing where dispersion or infiltration is proposed. Glacial soils vary wildly across the county — outwash that drains beautifully in one area, tight till a mile away — so the infiltration testing has to be site-specific, not assumed from the neighbor's lot. The drainage report, calculations, and details are core permit documents. Septic review runs through the health district on unincorporated parcels without sewer, with soil evaluation and reserve-area requirements that compete for usable ground alongside the stormwater facility and the building. Shoreline permits add a separate track along Puget Sound and the major rivers, with their own setbacks and mitigation expectations. Each thread has its own reviewer and its own timeline, and the building permit waits for all of them — the project schedule should be built on the slowest track, not the fastest.