French Drains & Drainage Systems in West Seattle, WA

Drainage & French drains in West Seattle: what to know

SPU’s drainage rule lists approved points of discharge in order: receiving waters, the piped public drainage system, ditch and culvert systems, the public combined sewer, and last on-site infiltration. Small projects can use curb or alley discharge. The rule says the combined sewer is not a public drainage system, and groundwater and footing drains may never enter a sanitary sewer.

Slopes add a second test. SDCI asks for a slope stability analysis when infiltration or dispersion sits above a steep slope or landslide-prone area, and its landslide guidance says never to point water onto a slope. Dense till caps many hilltops and takes water poorly. The sand beneath passes water down until it meets clay.

Permits and rules in West Seattle

Adding a catch basin to an existing service drain is permit work under SDCI’s side sewer rules. In steep slope, landslide-prone, and flood-prone areas a grading permit starts at more than 25 cubic yards of combined excavation and fill, and the 2026 stormwater code reviews permits adding more than 750 square feet of hard surface. We look up the outlet and slope mapping with SDCI and SPU before we quote.

Rules change and vary by parcel, so we confirm the current requirements with the city before we start.

How we approach it

Confirm the outlet against SPU’s ranked discharge points before pricing the trench
Keep concentrated water off the slope below the yard
Probe for till or clay to see where winter water perches
Tightline downspouts in solid pipe, with cleanouts at every bend

For the full process, see our Drainage Systems & French Drains page.

Questions from West Seattle homeowners

Where can a French drain discharge on a West Seattle lot?

SPU ranks the options: receiving waters, the public drainage pipe, a ditch and culvert, the combined sewer, then infiltration on your lot. Small projects can use curb or alley discharge. A sanitary sewer is never an option. We confirm which one your street allows first.

Can drain water soak in above the slope behind my West Seattle house?

Not without analysis. SDCI’s critical area checklist calls for a slope stability analysis when infiltration or dispersion sits above a steep slope or landslide-prone area, and landslide guidance warns against aiming water at a slope. A tightline to a legal outlet is the other route.

Do I need a permit for a catch basin or French drain in West Seattle?

Adding a catch basin to an existing service drain needs a side sewer permit from SDCI. A trench alone may not, but grading permits start at more than 25 cubic yards in steep slope or landslide-prone areas, and at 5,000 square feet of disturbance elsewhere. We check both.