French Drains & Drainage Systems in Des Moines, WA

Drainage & French drains in Des Moines: what to know

The city’s street standards bar downspouts and driveway runoff from crossing a sidewalk, walkway, or road. In a landslide hazard area, the code also asks for an analysis showing the project will not add surface water or sediment to adjacent properties. A pipe that dumps at the top of a bluff or into a creek ravine is the kind of discharge those rules target.

Where the ground allows, infiltration is a recognized tool. Roof downspout infiltration appears among the flow-control practices in the city’s standards, with design details left to the King County manual. Whether it works depends on the soil on your lot, so we probe before committing to a trench.

Permits and rules in Des Moines

The Public Works surface water division administers stormwater, and the city adopted the 2026 King County Surface Water Design Manual effective July 1, 2026. A drain that stays on your lot generally avoids a building permit, but a long trench can reach the grading code’s 50 cubic yard line, and a culvert or pipe in the street right-of-way needs a right-of-way permit. We confirm the outlet, the volume, and any creek or slope mapping with the city before quoting.

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

How we approach it

Settle the outlet first: on-lot infiltration, dispersion, or an approved connection
Keep drain water off bluff edges, ravine sidewalls, and creek buffers
Tightline downspouts so roof water never crosses the sidewalk or street
Request utility locates two working days ahead, then lay pipe with cleanouts

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

Questions from Des Moines homeowners

Where can a French drain discharge on a Des Moines hillside lot?

On your own lot through infiltration or dispersion, or to an approved connection. Never across a sidewalk or onto a neighbor, and in landslide hazard areas the city requires an analysis showing no added discharge or sediment next door. The outlet decides whether a drain is realistic.

Do I need a permit to put a culvert under my Des Moines driveway?

Where a driveway crosses a roadside ditch, the city’s street standards call for a culvert at least 12 inches in diameter, and at least as large as existing pipes within 500 feet upstream. Work in the right-of-way needs a permit from Public Works.

I live near Des Moines Creek or Massey Creek. Do creek rules limit a yard drain?

They can. The critical areas chapter sets stream buffers of 65 to 115 feet depending on stream type, plus a 10-foot building setback line, and clearing, grading, or filling in a critical area is barred unless a development exception applies. We check the mapping before we design.