Buying land or building a home in the Pacific Northwest? Whether you’re in Oregon or Washington, one of the most important infrastructure questions you’ll face is: What kind of septic system will this property need?
Septic systems are critical for wastewater treatment in rural and undeveloped areas across both states. But because of the varied soils, terrain, and water tables common in the PNW, not every property qualifies for a basic septic setup.
In this guide, we break down the key septic system types—standard and alternative—and when you need them, drawing directly from state-compliant installation guides and regulations used in counties like Klamath and Deschutes (Oregon) and beyond.
Why Septic Systems Matter for Oregon and Washington Landowners?
Most rural properties across Oregon and Washington are not connected to municipal sewer systems. If your lot is off-grid or outside city limits, you’ll likely need an onsite wastewater treatment system—a septic system tailored to the land’s unique conditions.
Septic requirements vary depending on:
• Soil quality and depth
• Drainage speed (percolation rates)
• Groundwater separation distance
• Slope of the land
• Proximity to wells, streams, or lakes
Pro Tip: Always Verify Past Perk Test Results Before You Buy
Just because a property “perked” 10 years ago doesn’t mean it still will today. Soil conditions, groundwater levels, and regulations change over time. Always request documentation of recent percolation or soil evaluations, and if none exist—or they’re outdated—schedule a new test during the contingency period of your land purchase.
The Basic System: Septic Tank + Drainfield
The most common septic system includes:
• A septic tank that separates solids from wastewater
• A distribution box or drop box to evenly distribute liquid
• A drainfield where filtered effluent is absorbed by soil
This system only works if the soil is permeable, groundwater is low, and there’s enough space to safely disperse the wastewater.
Pressure Distribution Systems: Even Flow, Even on Slopes
When gravity alone isn’t enough—either due to a small lot or sloped terrain—a pressure distribution system may be required.
Key Features:
• A pump delivers wastewater in timed doses through pressurized pipes
• Ensures even dispersion of effluent, avoiding over-saturation
• Used on lots with uneven topography or coarse soils
Proper installation includes hydraulic calculations to ensure squirt height and flow rates meet the health department’s specs. A certified installer is typically required in both states.
Sand Filter Systems: Advanced Treatment for Challenging Soils
Bottomless sand filters are used when the natural soil doesn’t provide adequate treatment—especially when:
• The site has shallow or coarse soils
• Groundwater is too close to the surface
• A traditional drainfield is not feasible
How It Works:
Wastewater is pumped through a sand bed that filters and treats it biologically and physically before absorption into the native soil below.
Sand filters are highly effective but require:
• Careful construction
• High-quality sand and gravel
• Frequent inspections during and after installation
Capping Fill Drainfields: When Soil Needs a Boost
If the site has suitable native soil but lacks vertical separation from impermeable layers or groundwater, a capping fill system can help.
What It Is:
• Shallow drainfield trenches are dug
• A “cap” of fill soil is placed over the trenches
• The system is shaped and sloped to prevent erosion and protect groundwater
This system is typically installed in summer months and must follow strict permit guidelines around soil texture, trench depth, and backfill quality.
Alternative Treatment Technologies (ATT): Smart Solutions for Difficult Sites
ATT systems are required when none of the conventional solutions are safe or effective—often in areas near sensitive water resources or with limited space.
ATT Systems Are Required When:
• You can’t meet separation-to-groundwater standards
• Soil conditions prevent proper treatment
• Disinfection or nitrogen removal is required
ATT System Components May Include:
• Aerobic treatment units (ATUs)
• UV disinfection or chlorination chambers
• Advanced monitoring and alarms
ATTs often allow for smaller drainfields, but they do require:
• DEQ-certified design and installation
• A maintenance contract for the life of the system
• Annual reports and inspections
Permits, Inspections & Maintenance in Oregon and Washington
Regardless of the system you install, both Oregon and Washington counties require:
• Site evaluation and soil testing
• Permitting from the local environmental health authority
• Inspections during and after installation
• Ongoing maintenance contracts for certain systems (ATTs and pressure systems)
Regular septic tank pumping (every 3–5 years) is required for all systems, and inspection ports must be accessible for county staff.
Explore Oregon DEQ’s Onsite Wastewater Resources or contact your Washington County Health Department for local standards.
Pro Tip: Budget for the Right System—Not Just the Cheapest One
Don’t assume a standard system will be approved. In areas with steep slopes, small lots, or poor soils, you may need a pressure distribution, sand filter, or even an ATT system, which can cost significantly more. Planning early for the right system—rather than the cheapest—can prevent budget blowouts and build delays.
Key Takeaways: Choose the Right Septic System for Your Lot
Not all land is ready for building. But with the right septic design, most properties can be developed safely and sustainably. Here’s how to get started:
✔ Evaluate the land early using a SiteFacts report
✔ Understand local regulations in your county
✔ Choose the system that matches your site’s challenges—whether it’s pressure distribution, sand filter, or ATT
✔ Budget for maintenance and inspections to avoid fines or system failure
