King County septic vs sewer: ADU implications
Why your sewer status drives 30% of the feasibility decision on unincorporated King County ADUs.
Where the line is drawn
Most parcels inside Seattle, Bellevue, Kirkland, Redmond, and the dense Eastside ring are on public sewer. Most parcels in unincorporated King County — Sammamish Plateau outer edges, Snoqualmie Valley, Vashon, Maury, and large pieces of north and east King — are on private septic.
The status determines what is feasible. Public Health — Seattle & King County governs septic capacity, and adding a dwelling unit to a parcel triggers a Type 1 OSS (on-site sewage system) review.
What the OSS review looks at
(1) Existing system capacity — gallons-per-day design flow vs. proposed total bedrooms on the parcel including the ADU. Most older 3-bedroom homes are on a system sized for 360 gpd, which accommodates one or two additional ADU bedrooms only if the soil and drainfield capacity were originally designed with margin.
(2) Reserve area — King County code requires a 100% reserve drainfield area on the parcel for any new or expanded system. Many wooded lots cannot site one without removing protected trees.
(3) Setbacks — wells (own and neighbor's), surface water, slopes. These often eliminate otherwise plausible DADU sites on rural parcels.
Cost delta
If the existing septic is adequate: zero added cost. If it needs an upgrade to a larger conventional system: $18,000–$32,000. If it needs a Glendon-style mound or a treatment system (Advantex, MicroFAST): $35,000–$60,000. If the lot cannot site a compliant system at all: the DADU is not feasible without a public-sewer connection (rarely available).
Feasibility on septic parcels starts with the existing as-built and a soil-log review, not with design. We have walked away from beautifully buildable lots because the septic math killed the project.
