Washington State · Installation Costs

Septic System Installation Cost in Washington

A new septic system in Washington can run $3,000 or $20,000. Soil conditions on your specific lot usually drive that gap more than which contractor you hire. Here's what each system type costs installed, why your site picks the system for you, and what counties charge separately to review and permit the project.

Independent research, checked against Washington state and county primary sources on July 22, 2026.

The figures below are budgeting ranges pulled from published contractor pricing and public county fee schedules, not quotes. Confirm current numbers with your county environmental health department and a licensed WA designer or installer before you budget a project.

Installed Cost, By System Type

System type is the biggest single driver of price. Below are Washington-specific installed-cost ranges, cheapest to most expensive, published by a Puyallup, WA septic installer — the most Washington-specific contractor pricing cited on this page. Treat these as budgeting ranges, not quotes. (allsepticandsewer.com — Types of septic systems, pros, cons, and costs, accessed July 22, 2026)

Washington installed cost by system type, cheapest to most expensive
System typeInstalled cost (WA)Typically used where
Conventional gravity$3,000–$7,000≥3 ft usable native soil; cheapest when the lot allows it
Chamber$4,000–$10,000High-water-table areas
Recirculating sand filter$7,000–$15,000~1.5 ft native soil
Drip distribution$8,000–$15,000Tight or irregular lots
Mound$10,000–$20,0001–2 ft native soil
Aerobic Treatment Unit (ATU)$10,000–$20,0001–1.5 ft native soil; higher ongoing maintenance

Ranges depend on soil conditions, tank size, and site access. They do not include the county design and permit fees covered further down this page.

Glendon Biofilter: no published Washington installed price. No published Washington installed-cost figure exists for the Glendon Biofilter specifically. The only Washington figure available for it is a permitting-fee line item from Thurston County, covered below. Until you have a real quote, budget a Glendon install using the sand filter or mound ranges above as a proxy, confirmed with a licensed WA designer or installer.

For a rough citywide cross-check, Angi lists overall Seattle septic installation at $4,092–$12,937, averaging $8,514. That blends every system type into one number, so it won't tell you what your specific system costs, but it lands inside the range above. (angi.com — Septic system installation cost, Seattle, WA, accessed July 22, 2026)

National, non-WA-specific cost guides quote wider ranges for mound systems and ATUs, some running to $25,000–$50,000, well above the Washington contractor pricing above. (homeguide.com, theseptic.guide, accessed July 22, 2026) Use the Washington figures first; treat national sites as a rough sanity check only.

Why Your Lot Decides the System, Not Your Budget

System type gets set by how much usable native soil sits above a restrictive layer or the water table on your lot. The Department of Health lays out the thresholds directly.

Native soil depth required, by system type
Native suitable soil above restrictive layer/water tableSystem type
At least 3 ftGravity
2–3 ftPressure distribution
1–2 ftMound
About 1.5 ftSand filter
1–1.5 ftATU (aerobic treatment/pretreatment)

(doh.wa.gov — Types of on-site sewage systems, accessed July 22, 2026)

Less soil depth means a pricier system, and WAC 246-272A-0234 sets the specific requirement behind that. For daily design flows between 1,000 and 3,500 gallons, soil dispersal components can only go in soil types 1 through 5, on slopes under 30 percent, about 17 degrees. The infiltrative surface generally can't sit deeper than 3 ft below finished grade, though a local health officer can approve up to 10 ft under special conditions. At least 6 inches of trench sidewall, or dripline, has to be in original, undisturbed soil. (WAC 246-272A-0234, accessed July 22, 2026)

The rule also spells out who gets the cheapest option. A plain gravity system is allowed in soil type 1 only for a single-family residence on more than 2.5 acres, getting under 25 inches of rain a year, outside the 12 Puget Sound counties, and meeting minimum unsaturated-depth requirements. Large, dry, non-Puget-Sound lots qualify for the cheapest system above. Smaller, wetter, or Puget Sound county lots get pushed toward pressure, mound, sand filter, or ATU systems instead, one reason western Washington projects skew toward the pricier end of that range. (WAC 246-272A-0234, accessed July 22, 2026)

Setbacks eat into your usable lot before you even get to soil

Soil depth is one filter. Distance from your drainfield to everything around your property is the second. WAC 246-272A-0210 sets minimum horizontal setbacks in Table IV.

Minimum setback distances, WAC 246-272A-0210 Table IV
FromTo soil dispersalTo sewage tankTo building sewer
Wells100 ft50 ft50 ft
Surface water (ordinary high-water mark)100 ft50 ft10 ft
Property/easement line5 ft5 ft—
Building foundation10 ft5 ft2 ft
Pressurized water supply lines10 ft10 ft10 ft

Setbacks to public drinking-water sources run more restrictive, 100–200 ft depending on whether the source is a well or surface water. (app.leg.wa.gov — WAC 246-272A-0210, accessed July 22, 2026)

A local health officer can require larger setbacks than the table when site conditions raise contamination risk: excessively permeable soils, unconfined aquifers, shallow or saturated soils, dug wells, and improperly abandoned wells. On a tight or sensitive lot, that pushes toward a smaller drainfield footprint and a pumped or mound-type system built for it. (doh.wa.gov — Site sewage system setbacks, accessed July 22, 2026)

Setback rules are in flux through 2026. A March 2024 rule change increased setbacks near public drinking-water surface-water sources. A November 2025 petition challenged that change, and the State Board of Health filed further proposed amendments on July 21, 2026, with public comment open through September 17, 2026. The numbers above reflect what's codified today and may change again later this year. (doh.wa.gov — Site sewage system setbacks, accessed July 22, 2026)

Critical-area proximity adds its own cost, but not to the new-system application itself. Thurston County's fee schedule adds a $472 fee to a septic tank placement when a parcel sits in, or within 400 feet of, a mapped critical area, or when county records aren't clear; the same trigger applies to a Voluntary Modification of an existing system, but only when it's mapped within a critical area. On a new-system Application for Conditional Site Approval, the $472 Site App Review–Planning fee is standard, already built into the $1,947 total covered with Thurston's other numbers below. (Thurston County — 2026 Septic & CSA common fees, accessed July 22, 2026)

Permit and Design Fees Sit on Top of the Installed Cost

None of the installed-cost ranges above include what the county charges to evaluate your site, review the design, and issue the permit. Those fees apply no matter which installer you hire. Here's what three Washington counties currently charge.

Thurston County: design and site-evaluation fees (2026)

Thurston prices site work separately from the permit itself: OSS Design Review, $720; OSS Application, $580; OSS Onsite Evaluation, $625; a Winter Water Study, which allots 5 hours of review, $1,510; Designer Certified Sewage Site Plan Review, $295; a Certified Water Sample, $239 ($175 site visit plus $32 coliform and $32 nitrate testing). A Winter Water Study is the kind of extra cost a high-water-table lot can trigger before a design is even approved. (Thurston County — 2026 Septic & CSA common fees, accessed July 22, 2026)

Thurston County permit totals, 2026
Permit typeTotal feeWhat's included
New system: Application for Conditional Site Approval$1,947OSS Application $580 + Design Review $720 + Site App Review–Planning $472 + Site App Review–Development $175
Septic Construction Permit$520Construction Permit $490 + O&M $30, after site approval
OSS Repair$1,820Repair Permit App $580 + Design Review $720 + Construction Permit $490 + O&M $30
Glendon Biofilter/Sand Filter/Mound Rebuild/Repair$1,325Application $580 + Construction Permit $490 + O&M $30 + O&M Certificate $225
Septic Tank Placement$610Tank-only line item
Voluntary Modification of Existing System$1,820

Add $60 to a new-system application if the property sits in a mapped flood zone or high-groundwater area. Add $225 if the system requires an Operational Certificate, which applies to ATU, sand filter, and mound-type systems. (Thurston County — 2026 Septic & CSA common fees, accessed July 22, 2026)

Pierce County (Tacoma-Pierce County Health Department)

TPCHD fee schedule, 2026 adopted rates (rev. Dec 2025)
Permit typeFee
Design, Single-Family Residential (new)$1,020
Design, Single-Family Repair$960
Design, Single-Family Residential (additional residence/ADU)$1,140
Septic/Tank Repair$360
Repair, Partial$660
Installation Permit$480
Installation Permit (repair)$480
Installation Permit (tank replacement only)$240
Design, Commercial/School (1,000+ gal/day)$1,800

TPCHD's design fee varies by scope: a new single-family system runs $1,020, a repair design is $960, and a design covering an additional residence on the same system (e.g. an ADU) is $1,140. These are the county's Rev. 12/2025 rates, effective for 2026. (tpchd.org — On-Site Sewage, Individual Well and Water Resources 2026 Fees, accessed July 27, 2026)

Snohomish County

Snohomish County Health Department, 2026 on-site sewage fees (selected)
Permit typeFee
Gravity Distribution, application review$1,130 (valid 2 years)
Pressure Distribution, application review$1,130
All Other Systems (DOH-approved: mound, sand filter, ATU, proprietary/Glendon-type)$1,290
Product Development System (novel/pilot technology)$1,400 base + $235/hr over 6 hrs
Permit for OSS, issuance (all types except community)$385
Repair (licensed designer, single-family)$390
Redesign to repairNo fee
Holding tank, preliminary review$1,115
Holding tank, permit fee$640
Holding tank, annual monitoring$470

These figures come from the county's 2025 fee-adjustment planning document. Snohomish's Board of Health has since finalized rates through Ordinance BOH25-08 (effective Nov. 1, 2025) and Ordinance BOH26-03 (2026 adjustments, approved April 14, 2026); confirm the current line-item amount on the county's fee page before budgeting. (snohd.org — Fee adjustment comparison, accessed July 22, 2026)

Repair costs less than new; replacement is priced like new

Washington draws a real line between "repair" and "new system," and it matters for your budget. WAC 246-272A-0010 defines "repair" as relocating, replacing, or reconstructing a failed system, or a component not on the minor-repair list, to restore it to non-failure status. "Minor repair" covers specific damaged components: control panels, building sewers, risers, baffles, filters, pumps, and inspection boxes. It excludes the sewage tank, treatment component, and soil dispersal component; replacing any of those three counts as a full "repair" or new-system action. A "modification" is an alteration that doesn't expand the system, and the rule states outright that a modification is not a repair. (app.leg.wa.gov — WAC 246-272A-0010, accessed July 22, 2026)

The three county fee schedules above all price that distinction the same way. Snohomish bills a full system "Replacement" at the same rate as a brand-new application, while a "Repair" for a single-family residence with a licensed designer's submittal is a separate $390 line, and a "Redesign to repair" carries no fee at all. Thurston's OSS Repair, $1,820 total, comes in cheaper than its new-system Application for Conditional Site Approval, $1,947, skipping the planning and development review a brand-new siting needs; its tank-only Septic Tank Placement line is cheaper still, at $610. Pierce County's Septic/Tank Repair, $360, and Installation Permit (repair), $480, both undercut its Design Single-Family Residential new-system fee of $1,020.

DOH's own consumer pages cover general maintenance and failure signs, but don't spell out a single statewide bright line for how repair versus replacement gets permitted. The county fee schedules above are the clearest practical evidence of how the WAC 246-272A-0010 definitions get applied day to day; local health jurisdictions keep some discretion, so confirm which track your project falls under with your own county before you budget.

Which Path Fits Your County

This page covers statewide numbers. Where you actually are changes what happens next.

Live in Thurston, Mason, or Lewis County? Live connection

Our sister site, Thurston Septic Guide, has a page built specifically around installation cost in the tri-county area, with local numbers layered on top of the statewide figures here. See tri-county installation costs →

Everywhere else in Washington

This site doesn't currently connect homeowners outside Thurston, Mason, and Lewis County with a local provider. Tell us what you need on the contact form. If we can match you with a local pro free of charge, we will. Go to the contact form →

How to Read These Numbers

Every dollar figure above comes from the cited document. Most county figures are base fees only; per-hour, resubmittal, flood-review, and Operational Certificate fees often apply on top. The installed-cost ranges in the first section came from a Washington installer's published pricing, not permit records, so treat them as budgeting ballparks, not quotes. Pierce County's figures are 2026 adopted rates, effective Jan. 1, 2026 (TPCHD Resolution No. 2025-4903). Snohomish County's on-site sewage fee schedule is no longer in draft; the Board of Health finalized current rates through Ordinance BOH25-08 and Ordinance BOH26-03, covered above.

Primary sources

Sources accessed July 22, 2026, except the Tacoma-Pierce County fee schedule, accessed July 27, 2026. This page is general information about septic installation costs and permitting fees in Washington. It is not a quote and not legal or engineering advice. Costs and fees change over time; confirm current numbers with your county environmental health department and a licensed WA designer or installer before budgeting a project.

Related guides

Questions about septic installation costs? Ask the Septic Guide, our AI assistant.