Short definition

Routing a downspout into the sewer is restricted or prohibited in many WA cities — particularly Seattle’s combined-sewer-overflow (CSO) basins. The current direction of regulation across Puget Sound is toward separating roof runoff from the sanitary sewer and discharging to natural ground, a drywell, a rain garden, or a cistern. Discharging to the yard at least 4 feet from the foundation is the safe default in most jurisdictions.

What it is

For decades, WA cities (especially pre-1990 Seattle) routinely connected residential downspouts directly to the sanitary sewer. In a combined sewer area, that downspout connection sends rainwater into the same pipe carrying sewage — and during heavy rain, the system overflows into Puget Sound, the Duwamish, Lake Washington, or the Sound. That’s the “combined sewer overflow” problem WA cities have been working to reduce.

The current state, by jurisdiction:

  • Seattle (CSO basins) — Disconnection is strongly preferred and incentivized through the RainWise rebate program. Discharge to natural ground, rain gardens, drywells, or cisterns is the encouraged alternative.
  • Seattle (separated sewer basins) — Discharge to the storm drain may be allowed; verify with Seattle Public Utilities. Separated basins have a dedicated stormwater main not connected to the sanitary sewer.
  • Tacoma, Bellevue, Bellingham — most jurisdictions prohibit storm-to-sanitary connections in new construction; legacy connections may be grandfathered or required to be disconnected during major renovations.
  • Rural and unincorporated counties — usually allow discharge to natural ground or a properly sized drywell. Verify with county health.

The decision tree for a homeowner:

  1. Identify the current connection — visible above ground? Underground to where? Sometimes a sewer scope is needed to find it.
  2. Determine your basin type — CSO, separated, or none. The utility website usually has a map.
  3. Consult the jurisdiction-specific rule — changes to the connection often require a permit.
  4. Plan the discharge path:
    • Natural-ground discharge at least 4 feet from foundation, sloping away.
    • Drywell sized to the catchment area.
    • Rain garden or cistern (RainWise-eligible in Seattle CSO basins).

Why it matters to a homeowner

A legacy downspout connection to the sewer is one of those things that’s invisible until something triggers a code review — a major remodel, a sewer-line replacement, or a permit-pulling process. At that point, the city may require disconnection, which can run $1,500–$5,000 per downspout depending on access and the destination chosen.

For Seattle homeowners in CSO basins, this is also where the RainWise rebate becomes relevant. RainWise pays for rain gardens and cisterns that disconnect downspouts from the combined sewer. Verify the current rebate rate directly at the program source — different references cite different per-square-foot amounts and the program has updated over time.

When a plumber or sewer contractor says you need to “disconnect the downspouts,” they’re describing the work to comply with stormwater code. Push for clarity on (a) which basin you’re in, (b) which downspouts are actually connected to the sewer, and (c) what the alternative discharge plan is — natural ground, drywell, or rain garden each have different costs.

Common failure modes

  • Legacy combined-sewer connection — adds clean rainwater to overflow events; modern code-review trigger.
  • Buried legacy pipe to nowhere — eventually fails and creates sinkholes or wet areas.
  • Connection to sanitary sewer (not combined) — overloads treatment during storms; jurisdiction may require disconnection.
  • DIY disconnect without permit in a regulated jurisdiction — code violation that surfaces at sale.

Washington note

A few specifics by area:

Seattle: Per the 2021 Stormwater Code (effective July 2021), Seattle Department of Construction and Inspections plus Seattle Public Utilities jointly enforce stormwater rules, including downspout discharge. CSO-basin homeowners can participate in the RainWise rebate program (run jointly by King County Wastewater Treatment Division and SPU) to install rain gardens and cisterns that disconnect downspouts from the combined sewer. Eligibility is tied to specific CSO basins shown on the program’s eligibility map. Verify the current rebate amount directly at 700milliongallons.org before relying on a specific dollar-per-square-foot figure — different sources have cited different amounts and the program rate is updated.

Tacoma: Tacoma Public Utilities and the City of Tacoma have separate stormwater rules; new construction typically requires storm and sanitary to be separated. Verify with the Tacoma Permit Center.

King County unincorporated and small cities (Sammamish, Issaquah, Redmond): The King County Surface Water Design Manual is the standard most jurisdictions adopt by reference. Check the specific jurisdiction’s online stormwater code.

Rural unincorporated counties — Most allow discharge to natural ground or a perc-tested drywell; setbacks from septic drain fields and water wells apply.

Common variants

  • Sanitary sewer (sewage) vs. storm sewer (rainwater) vs. combined sewer (both, legacy).
  • Yard discharge (natural ground) vs. drywell (subsurface) vs. rain garden (designed feature) vs. cistern (collected for reuse).
  • Disconnection requirement (some Seattle basins) vs. permitted connection (other basins, often with a backwater valve).