French Drains & Drainage Systems in Fall City, WA

Drainage & French drains in Fall City: what to know

The county’s residential drainage guidance sets its first lines at 2,000 square feet of new and replaced impervious surface and 7,000 square feet of land disturbing activity. Patios, driveways, and parking areas count as non-structure surfaces under that guidance. Trench work changes soil cover, so each run is totaled as land disturbing activity, and a run near the line is put to King County in writing before digging.

Outlets decide whether a drain works. On the parcel we use a planted area or a soakage area, and we test the soil before choosing an infiltration outlet. A run that approaches the county road or its right-of-way is marked on the plan and reviewed before digging starts. Water is not sent onto a neighbor’s lot.

Permits and rules in Fall City

Before we quote, we check each alignment against the parcel lines and the county road frontage, total the disturbance against the 7,000 square foot threshold, and confirm in writing with King County any outlet that reaches public land before excavation. A line that crosses the road shoulder waits for the county’s reply. The plan keeps the fall and the outlet point for every run.

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

How we approach it

Survey the grade and record the fall before any trench
Total each run’s disturbance against the 7,000 square foot line
Test the soil before choosing an infiltration outlet
Keep every outlet on the parcel unless a right-of-way permit covers it

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

Questions from Fall City homeowners

Does a Fall City French drain need a county permit?

A drain kept entirely on the parcel is still counted toward the county’s drainage and disturbance lines. We total the new hard surface and the trench disturbance, then ask King County in writing whether any run needs its own review before we dig.

Can a Fall City yard drain empty toward the road?

Not without a right-of-way use permit when the outlet sits in county right-of-way. The county requires one for private improvements or uses in its road right-of-way, and limited permits lapse one year after issue, so an outlet that reaches the road shoulder waits for that permit.

How deep can a Fall City drain trench go before the county reviews it?

The clearing and grading guidance uses a 5-foot cut and a 3-foot fill, each measured in any one location, and more than 100 cubic yards of cumulative earth moving as its lines. We check the trench depth and total volume against those lines before the first cut.