French Drains & Drainage Systems in SeaTac, WA

Drainage & French drains in SeaTac: what to know

SeaTac’s stormwater chapter adopts the King County manual, 2021 edition, along with a City addendum. We confirm which parts of that manual reach a yard drain and its outlet, since its requirements depend on the project and the site rather than on the drain alone.

The clearing code adds its own checks. A project that constructs or modifies a 12-inch or larger storm drain or ditch triggers review, and so does one that receives runoff from a 12-inch or larger pipe or ditch. A project next to a flood hazard area, erosion hazard area, steep slope, landslide or seismic hazard area, or critical drainage area also triggers review, so a yard drain near any of those features gets checked before the trench is laid out.

Permits and rules in SeaTac

We confirm with the city whether a drain and its outlet trigger clearing and grading review, measured against the triggers the city lists. We also confirm whether an outlet that touches the street, a ditch, or a storm drain needs a Right-of-Way Use Permit, since the city lists drainage systems for stormwater among its right-of-way examples. The answers set the outlet design, so we settle them first.

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

How we approach it

Map the lot against the critical drainage area and the other STE trigger lines
Size the drain and outlet with the 12-inch storm drain or ditch trigger in view
Confirm the discharge point with the city before any tie-in to a street or ditch
Probe the trench for standing water and soil type before choosing the drain

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

Questions from SeaTac homeowners

Does a yard drain in SeaTac need a permit?

It may. We confirm with the city whether a drain and its outlet trigger clearing and grading review, and whether the outlet touches the right-of-way, which needs a Right-of-Way Use Permit. The city’s right-of-way examples include stormwater drainage systems.

What makes a SeaTac drainage project trigger review?

The STE triggers include 2,000 square feet of new or replaced impervious surface, a disturbed area of 7,000 square feet or larger, 50 cubic yards or more of cut or fill, and a 12-inch or larger storm drain or ditch. A project in a flood, erosion, steep slope, landslide, or seismic hazard area, or a critical drainage area, also triggers review.

Does a SeaTac drain near a sensitive area need review?

It can. The city’s STE page lists an activity in a sensitive area, or its buffer regulated area under SMC Title 15, as a trigger. We check the city’s maps for your parcel before any outlet is set, since Chapter 15.700 covers critical areas.