Septic system and well for a new home on a rural King County lot: what to settle before the house is designed

Short answer: before anyone draws a house on a King County lot with no sewer, three things have to be settled with Public Health Seattle & King County and the state: that the soil can hold a septic system and a full-size reserve area, where the drinking water will come from, and how many bedrooms the system will be designed for. The septic design belongs to a licensed designer or engineer, and the house plan has to fit what they find.

Piper Cole designs and administers construction; I do not build. This page is a reading guide to the rules, not septic or well design. The sources for this page were read on 9 November 2026: King County Board of Health Code Title 13 (text updated June 11, 2025), chapter 246-272A WAC (version effective April 1, 2025), the county’s septic and permit pages, Ecology’s well pages, and the RCW and WSR text cited below. Rules and review times change; confirm each item with the agency named.

Who does what

Who	What they do	Source
Licensed on-site sewage system designer or professional engineer	Evaluates soil and site, designs the system, signs and stamps the application, and inspects the installation. A soil scientist may do the soil evaluation only.	WAC 246-272A-0220, 0230; BOH 13.20.020
Public Health Seattle & King County	Approves the site design, issues the installation permit, inspects, and reviews building applications for septic impacts.	BOH 13.28.010, 13.20.010
Certified master installer	Normally applies for the installation permit and builds the system. The applicant may not also be the designer, except for a failure repair.	BOH 13.20.010
Licensed well driller	Constructs the well. Ecology says a license is required to construct a well, with two exceptions, and that a notice of intent must be submitted at least 72 hours before drilling begins.	Ecology well pages
Architect	Draws the house, decks, driveway and bedroom layout that the septic design must fit. The septic design is reserved to the licensed designer or engineer.	BOH 13.20.020

Sewer or septic

The county’s on-site sewage code applies to “all lots, parcels, and tracts not served by public sewers” (BOH 13.04.040). Inside the Urban Growth Area, new residential construction “shall connect” to a public sewer “if the sewer utility permits such connection” (BOH 13.04.050). Inside a city the connection follows the policies of that city or the local sewer utility.

For an urban lot the county’s design application asks for a sewer letter stating whether the nearest sewer is more or less than 200 feet from the property line. If the lot is inside a city such as Snoqualmie or North Bend, start with that city and its sewer utility; the page on Snoqualmie and North Bend covers what those cities’ own documents decide.

Water comes first

Title 13 says no system “may be constructed or expanded” if the fixtures draining to it are not supplied from an approved water source (BOH 13.04.070). The approved sources are:

• A public water source. King County’s drinking water policy page says that if one is available to a new development, “they should connect to it.”
• A private individual well, only on a lot of five acres or more, or a lot created before May 18, 1972, and only if it meets the conditions below. Critical area acreage may count toward the five acres (BOH 13.04.070(E)).
• Rainwater catchment as the only drinking source, only where the health officer finds that connecting to a public source or a well would cause undue hardship. A private spring follows the same lot rule.

For example, a 2 acre lot created after 1972 does not meet the lot test for a private individual well, so a new house on it would need public water or another approved source. That one rule matters when you are choosing a lot; the design application asks for proof of the lot’s segregation date when the source is a proposed well and the property is under 5 acres.

The order of steps

This order is assembled for this page from the county’s application checklist and Title 13; it is not a county checklist.

1. Confirm sewer and water availability. A sewer letter for an urban lot; a water availability letter in a public water system area, or a plan for an approved well.
2. Soil and site evaluation. The county’s checklist lists a minimum of 5 soil logs: 2 for the drainfield, 2 for the reserve area and 1 for the tank or tanks. A percolation test is not the baseline; the health officer may require one “where identification of soil absorption characteristics is in question” (BOH 13.28.050).
3. Critical areas designation. The county’s checklist lists it as required for a new design. The code adds that a system in a critical area needs documentation that the jurisdiction’s critical area review is complete (BOH 13.28.030(E)).
4. Site design application. The designer submits it online through the Public Health Permit Center. The county’s checklist has nineteen lines, among them a vicinity map, site plans, hydraulic calculations, soil log data, well documents and, where it applies, proof of lot segregation date.
5. Building application approval. The county says this approval is “required before most building permits can be issued,” and its single family permit page lists the Public Health approval and, for a well, a well permit from Public Health.
6. Installation permit. It requires the approved design and evidence that the building permit has been issued. The county says “Construction must not begin until the installation permit is posted on the site/property.”
7. Inspections and record drawing. After the foundation and plumbing stub-out are in, the designer inspects before the system goes in. After installation the designer certifies completion with a record drawing within 30 days of the installer’s notice (BOH 13.56.020 and 13.56.030).

What in the house plan depends on it

The septic design and the house share one piece of ground. These are the points in the county code and state rule where the house plan reaches the septic plan. The designer applies them to your lot, and the health officer may require more.

• Bedroom count. It sets the design flow, counted next.
• Where the house can stand. Minimum distances from the drainfield and reserve area are in the table below.
• Driveways and paving. The state rule allows a soil dispersal component only where the area is not subject to cover by impervious material or vehicular traffic (WAC 246-272A-0210(5)). The county adds that no drainfield may be installed under driveways, roadways, parking areas or paved areas (BOH 13.48.010(I)), and defines the reserve area as ground “protected and maintained for replacement” of the system.
• Slope. The county does not allow a system on slopes over 40 percent; over 30 percent the drainfield must be pressure distribution with trenches no wider than 2 feet (BOH 13.28.030(B)). The state rule allows a soil dispersal component only on a slope of less than 45 percent, and where the two differ the more stringent provision applies (BOH 13.04.045).
• Fill and grading. No part of the drainfield may sit in fill or disturbed soil (BOH 13.28.030(P)). The site plan shows the maximum building footprint, so an undrawn later addition can collide with the reserve area.
• Roof and footing drainage. No downspout or footing drain may connect to the system (BOH 13.28.030(H)), so the house drainage has to land somewhere else.
• Plumbing stub-out. The design drawings show its elevation, and the designer checks it against the built foundation before installation.
• Well location. A proposed new well is shown on the septic site design, with the reserve area, structures and drainage on one plot plan.

Item	Minimum from the edge of the drainfield or reserve area
Private well	100 feet
Surface water, from the ordinary high water mark	100 feet
Building foundation	10 feet up-gradient; 15 feet plus the foundation cut height down-gradient (30 feet at most)
Property or easement line	10 feet
Footing, curtain and interceptor drains, down-gradient	30 feet

These distances come from the county’s Table 13.28-2, which carries reductions the health officer may grant and increases the officer may require. The state’s Table IV is less strict in places, such as 5 feet to a property line, and the county code controls.

Bedrooms and design flow, counted

The three plans in the table are hypothetical examples, not a lot. The state defines design flow as “the maximum volume of sewage a residence… is estimated to generate in a 24-hour period.” Its minimum is 120 gallons per bedroom per day (WAC 246-272A-0230). King County is stricter: at least 150 gallons per bedroom per day, with a minimum of three bedrooms (BOH 13.28.070). A reserve area “equal to at least one hundred percent of the primary” drainfield area is required (BOH 13.28.030(C)).

Plan	County design flow	Arithmetic
2 bedroom cabin	450 gallons per day	Counted as 3 bedrooms: 3 x 150
4 bedroom house	600 gallons per day	4 x 150
4 bedroom house and a 1 bedroom ADU on the same system	750 gallons per day	(4 + 1) x 150; each ADU bedroom is added to the primary house count

The state minimum for the 4 bedroom house would be 480 gallons per day (4 x 120), so the county figure governs. An ADU with no bedroom counts as one bedroom (BOH 13.28.030(M)), and each bedroom added later adds 150. The county defines a bedroom as “a room used for sleeping and that includes a window, a door, and a closet,” and excludes a room under 70 square feet with a closet. Before you label a den or loft on a plan, ask the designer how the county would count it.

The well and Ecology side

Where the county allows a private well, Title 13 attaches conditions (BOH 13.04.070(B)):

• The health officer approves the well location before the well is built.
• A recorded covenant keeps potential contamination sources at least 100 feet from the well.
• Quantity: a four hour pump test showing at least 400 gallons per day in known or suspected low-production areas, otherwise information such as neighboring well logs.
• Quality: two samples with no coliform, a nitrate and arsenic test within state limits, and the driller’s report.
• Construction to chapter 173-160 WAC.

Ecology says a license is required to construct a well, with two exceptions, and that a notice of intent must be submitted at least 72 hours before drilling begins; the notice “is not a permit, certificate, or water-right application.” State law exempts groundwater withdrawals “for single or group domestic uses in an amount not exceeding five thousand gallons a day” from the permit requirement (RCW 90.44.050). For a new house the county’s policy page says domestic permit-exempt wells “are limited to 950 gallons per day, per residence.” The statute states the limit as a maximum annual average of 950 gallons per day per connection. For each new domestic use that relies on a permit-exempt well in the watersheds it lists, RCW 90.94.030(4) requires that limit, a fee of $500 paid to the permitting authority, and recording of the water supply restrictions on the property title, until rules for the watershed are adopted. The listed watersheds include WRIAs 7 (Snohomish), 8 (Cedar-Sammamish), 9 (Duwamish-Green) and 10 (Puyallup-White). The $500 fee is set in state law, as read on 9 November 2026. Ask the county which WRIA your parcel is in. Ecology put the same requirements into its rules for WRIA 7, WRIA 8 and three other watersheds (WSR 26-12-038, effective June 27, 2026), which apply to permit-exempt wells constructed after that date.

A building permit for a building that needs potable water requires evidence of an adequate water supply, and an application for a water right is not sufficient proof (RCW 19.27.097).

Timing and expiry dates

• Review time. The state rule says the local health officer must respond to an application within 30 days (WAC 246-272A-0200). The county’s septic program page says review timelines are currently longer than in the past, because of a new permitting system, staffing changes and high volume. This page prints no current review time; ask the county for it.
• Winter water table. Where a high winter water table is likely, the health officer may require monitoring, with the plan submitted no later than December 1. The officer decides on the monitoring results within 12 months of receiving the application, assuming typical precipitation (BOH 13.28.060).
• Expiry. Design approval expires three years from approval unless a valid building permit application has been accepted (BOH 13.28.010). An installation permit expires three years from issue (BOH 13.20.010).

If a house already stands on the lot

Title 13 requires additions and alterations that impact the system to be served by a compliant system, unless the health officer waives it because the work does not adversely affect the system or its reserve area (BOH 13.64.020). The county’s remodel page asks owners adding a bedroom to call the Septic Expert first, and gives tearing a house with an older system down to the foundation and rebuilding as an example that would require a new septic system. For slopes and buffers see hillside and view lots.

Where to confirm each item

Item	Ask	Contact
Sewer or septic, design application, installation permit	Public Health Seattle & King County, on-site sewage program	Environmental Health Services 206-477-8050 (phone only; hours are on the county page)
Technical septic questions, bedroom changes	King County Septic Expert	206-477-8177 (voicemail; leave name, phone and parcel number or address)
Well location approval and well permit	Public Health, then your licensed driller	County pages; Ecology for the notice of intent
Water rights and WRIA	Department of Ecology	ecology.wa.gov

Where to start

1. Find out whether the lot is in a sewer area, and its creation date and acreage.
2. Decide the water source: public water, or a well if the lot qualifies.
3. Hire a licensed designer or engineer for the soil and site evaluation, and give them your bedroom count and any ADU.
4. Then place the house, driveway and decks on the ground that is left.

Questions

Do I need a perc test for a new house on a rural King County lot?

Not as a rule. King County’s code bases the design on soil logs, which are test pits examined in the drainfield, reserve and tank areas. It says a percolation test or particle size analysis may be required by the health officer where identification of soil absorption characteristics is in question (BOH 13.28.050). Ask your designer which tests the county will want on your lot.

Who can design a septic system in King County?

A licensed on-site sewage system designer under chapter 18.210 RCW, or a professional engineer licensed under chapter 18.43 RCW (BOH 13.20.020; WAC 246-272A-0230). A resident owner may design a system for their own residence only if the county’s conditions are met, including a gravity system, at least four feet of original permeable soil, a property not adjacent to a shoreline and a soil evaluation by a licensed designer, engineer or soil scientist (BOH 13.20.040).

Can I drill a well on any rural lot?

Not for a house served by a septic system. Title 13 accepts a private individual well as the water source only on a lot of five acres or more, or a lot created before May 18, 1972, and only with county location approval, a recorded protective covenant, a water quantity and quality showing and a driller’s report (BOH 13.04.070). A lot that does not qualify needs a public water source or another approved source.

How many bedrooms is my septic system designed for?

At least three. King County uses a minimum design flow of 150 gallons per bedroom per day with a minimum of three bedrooms, and 150 more for each additional bedroom (BOH 13.28.070). A 4 bedroom house is 4 x 150 = 600 gallons per day. Each bedroom in an ADU served by the same system is added to the count (BOH 13.28.030).

Does an approved septic design let me start building the system?

No. The county requires both a site design approval and an installation permit when a new system is built, and says construction must not begin until the installation permit is posted on the site. The installation permit application needs the approved design and, for a new system serving a building, evidence that the building official has issued the building permit (BOH 13.20.010).

Can I put a driveway, pool or deck over the drainfield or reserve area?

Ask the designer before you draw it. The state rule allows a soil dispersal component only where the area is not subject to cover by impervious material or vehicular traffic, and the county lists minimum distances from the drainfield or reserve area to foundations, pools, decks, property lines and drains (WAC 246-272A-0210; BOH Table 13.28-2). The county defines the reserve area as ground protected for replacement of the system.

How long does Public Health take to review a septic design?

No current review time is printed here. The state rule says the local health officer must respond to an application within 30 days (WAC 246-272A-0200). The county’s septic program page says review timelines are currently longer than in the past because of a new permitting system, staffing changes and high volume. Ask the county for the current timeline before you plan a schedule.

What does a septic system and a well cost?

This page gives no price for the system or the well. The county’s permit fees are in its published fee schedule, and RCW 90.94.030 sets a $500 fee paid to the permitting authority for a new domestic use that relies on a permit-exempt well in the listed watersheds (read on 9 November 2026). Designer, installer and driller prices depend on the lot, so get written scopes from each before you compare them.

Do the same septic rules apply inside a city such as Snoqualmie or North Bend?

Title 13 applies to lots not served by public sewers, but inside the Urban Growth Area a new house has to connect to a public sewer if the sewer utility permits, and within cities that connection follows the city’s or utility’s policies (BOH 13.04.050). Start with the city and its sewer utility, then Public Health if the lot will use septic.