Colorado · Regulation 43 · CDPHE Water Quality Control Division
Colorado calls septic systems OWTS — On-site Wastewater Treatment Systems. The state sets minimum standards through Regulation 43, updated in 2025-2026, but every permit comes from your county health department. Above 8,000 feet, engineered design is required — and mountain counties enforce some of the strictest OWTS rules in the country.
Minimum tank
Based on bedroom count and design flow. County may require larger based on local soil and site conditions.
Governing standard
CDPHE Water Quality Control Division. Updated March 2025 — counties implementing through 2026.
Above 8,000 ft
All OWTS above 8,000 feet elevation must have a stamped engineering design. No prescriptive designs.
Front Range: clay and sandy loam common. Mountain counties: shallow soils over granite or sandstone. Eastern plains: deep silty loam. Your county health department soil evaluation determines the actual classification.
| Dwelling | Design flow | Minimum tank | With garbage disposal |
|---|---|---|---|
| 1 bedroom | 150 gpd | 1,000 gal | 1,250 gal |
| 2 bedrooms | 300 gpd | 1,000 gal | 1,250 gal |
| 3 bedrooms | 450 gpd | 1,000 gal | 1,250 gal |
| 4 bedrooms | 600 gpd | 1,250 gal | 1,500 gal |
| 5 bedrooms | 750 gpd | 1,500 gal | 1,750 gal |
| 6 bedrooms | 900 gpd | 2,000 gal | 2,250 gal |
Above 8,000 feet elevation, the engineer determines tank size based on site-specific conditions — the standard table above is a starting point only. Cold temperatures at high elevation slow biological digestion, and engineers routinely specify larger tanks for mountain properties.
Next step: Use the installation cost estimator to see total Colorado OWTS costs, or check perc test costs in Colorado — soil evaluations run $300 to $900 on the Front Range, more in mountain counties.
2025-2026 update
Colorado adopted a major Regulation 43 revision in March 2025. Counties are implementing updated local rules through 2026 — if you are planning a new system, confirm which version of the rules applies in your county.
Soil testing — now the foundation
Updated Regulation 43 makes soil morphology evaluation the foundation of all OWTS design. Generic prescriptive sizing based on bedroom count alone is no longer sufficient in most Colorado counties. A certified professional must evaluate the actual soil profile — texture, structure, depth to restrictive layers, and seasonal high water table — before any design is prepared. This change affects both new installations and system replacements.
Operating permits — new requirement
Effective June 2026, systems that include higher-level treatment units — nitrogen-reducing systems (NDDS) and similar technologies — require an operating permit in addition to the construction permit. Systems with only pumps or lift stations are exempt. This primarily affects mountain counties where nitrogen management near sensitive water bodies is required, but also applies to certain Front Range counties near reservoirs.
County implementation
CDPHE's updated Regulation 43 is the minimum — counties adopt local rules that are as strict or stricter. Gilpin County implemented updated rules in March 2026. Elbert County updated June 2026. Summit, Eagle, and Pitkin counties have had stricter-than-state rules for years and are updating further. Check with your specific county health department for the version of Regulation 43 that applies to your permit.
Setbacks — updated distances
Colorado requires a minimum 100-foot setback from private wells — stricter than many states. Setbacks from streams, lakes, and reservoirs are increased under the updated regulation, particularly in mountain counties near ski resort communities and sensitive watersheds. Lot size and the ability to meet setbacks is often the binding constraint on small mountain properties, independent of soil conditions.
Elevation matters
No other state has elevation as a formal design trigger in its regulations. Colorado's terrain creates three distinct OWTS environments.
Below 6,000 feet
The Denver metro area, Colorado Springs, Pueblo, and the eastern plains sit below 6,000 feet. Soil conditions vary — clay-heavy soils dominate much of the Front Range, while the plains have deeper silty loam. Conventional gravity OWTS are common where soil percolation allows. Jefferson, Douglas, El Paso, and Larimer counties are the most active permitting counties in this zone. Frost depth is a consideration but not the dominant design constraint.
6,000 to 8,000 feet
The foothills west of Denver — Gilpin, Clear Creek, Teller, and parts of Jefferson and Park counties — sit in the transition zone. Soils are shallower and rockier than the Front Range. Frost depth increases significantly. Conventional systems are still possible in favorable locations, but engineered designs are more common. Many properties in this zone have granite bedrock close to the surface, limiting trench depth and pushing costs upward.
Above 8,000 feet
Summit, Eagle, Pitkin, Grand, and high-elevation portions of other counties require engineered designs for all OWTS. Shallow soils over granite, frost depths of 4 to 6 feet, short construction seasons, and proximity to sensitive mountain watersheds combine to make standard prescriptive approaches inadequate. Summit County (Breckenridge, Keystone) and Pitkin County (Aspen) have some of the strictest OWTS requirements in Colorado — and the highest installation costs.
By county
Colorado has 64 counties, each with its own OWTS program built on Regulation 43. These are the counties where local requirements most commonly exceed the state standard or where site conditions create the biggest design challenges.
| County | Permitting authority | What makes it different | Key local requirement |
|---|---|---|---|
| Summit County | Summit County Public Health | Breckenridge, Keystone, Dillon — high elevation, sensitive watersheds | Engineered design required, nitrogen management near reservoirs, strict setbacks |
| Eagle County | Eagle County Public Health | Vail, Beaver Creek — mountain terrain, Eagle River watershed | Engineered design above 8,000 ft, enhanced treatment near Eagle River |
| Pitkin County | Pitkin County Environmental Health | Aspen — shallow soils, Roaring Fork River watershed, high land values | Among strictest in Colorado — enhanced treatment commonly required |
| El Paso County | El Paso County Public Health | Colorado Springs — largest county by population using OWTS, clay soils | Transfer inspection required, specific clay soil design standards |
| Jefferson County | Jefferson County Public Health | Foothills west of Denver — granite bedrock, varied soil depth | Soil evaluation required, inspection before backfill strictly enforced |
| Gilpin County | Gilpin County Public Health | Central City, Black Hawk — updated Regulation 43 March 2026 | New 2026 local OWTS rules — confirm current requirements before applying |
| Elbert County | Elbert County Public Health | Rural eastern Front Range — operating permit requirement June 2026 | Operating permits required for treatment unit systems from June 2026 |
| Larimer County | Larimer County Health | Fort Collins, Loveland, Estes Park — varied terrain, Poudre River | Enhanced setbacks near Cache la Poudre River and Horsetooth Reservoir |
Installation cost
Elevation and county are the two biggest cost drivers in Colorado — a three-bedroom system can range from $8,000 on the plains to $40,000+ in a mountain county.
| Region | System type | Typical cost | Main driver |
|---|---|---|---|
| Eastern plains Elbert, Lincoln, Kit Carson |
Conventional gravity | $7,000 – $13,000 | Deep silty loam, large lots, competitive market |
| Front Range Jefferson, Douglas, Larimer |
Conventional or pressure dosed | $9,000 – $16,000 | Clay soils, urban access, inspection requirements |
| Colorado Springs area El Paso County |
Conventional or engineered | $10,000 – $18,000 | Clay and variable soils, transfer inspection requirement |
| Foothills Gilpin, Clear Creek, Teller |
Engineered, mound common | $14,000 – $25,000 | Shallow bedrock, frost depth, granite excavation |
| Mountain counties Summit, Eagle, Grand |
Engineered, advanced treatment | $20,000 – $40,000 | High elevation, short season, strict county rules |
| Aspen / Pitkin County | Engineered, enhanced treatment | $25,000 – $50,000+ | Strictest county rules, Roaring Fork watershed, high costs |
| Pump-out service | — | $250 – $500 | Tank size, access, county location |
Get a precise estimate: Use the Colorado cost estimator for a breakdown by bedroom count, soil type, and system type. For replacement costs, use the replacement calculator.
Colorado permitting runs entirely through county health departments — CDPHE sets the rules, counties issue permits. The process is similar across counties but local requirements and timelines vary.
Under updated Regulation 43, soil evaluation is the first and most critical step. A certified professional evaluates the site: soil texture, structure, depth to bedrock or seasonal high water table, and percolation characteristics. In mountain counties above 8,000 feet, this step also confirms whether any conventional system is feasible. Cost: $300 to $900 on the Front Range, $800 to $2,500 in mountain counties where engineered evaluations are required.
Based on the soil evaluation, a design is prepared. Below 8,000 feet with favorable soil, a standard prescriptive design following Regulation 43 tables is often acceptable. Above 8,000 feet, or on sites with challenging soil conditions, a stamped engineering design is required. Mountain county designs require a Registered Professional Engineer — not just a certified installer or evaluator.
Submit the soil evaluation, system design, and site plan to your county health department with the permit fee. Permit fees range from $300 to $1,000 depending on county. Review times vary — Front Range counties typically take two to four weeks; mountain counties during peak season can run longer. Gilpin and Elbert counties updated their local rules in 2026 — confirm the current application requirements before submitting.
Colorado requires a licensed OWTS installer. Search the CDPHE database for state-certified installers in your area. The installer must follow the approved design — field changes require county approval before proceeding.
The county health department must inspect the open system before backfill. Contact your county to schedule — in mountain counties, inspector availability during the short construction season can create scheduling delays. Backfilling without inspection requires excavating at the owner's expense.
Colorado requires separate licenses for different OWTS roles. Search the CDPHE online database for state-certified septic system installers and pumpers. Mountain counties requiring engineered designs effectively mandate a Registered Professional Engineer for design work — verify what your specific county requires before hiring.
El Paso County — which includes Colorado Springs — is the most populous Colorado county relying significantly on OWTS. Clay soils dominate much of the area, which affects drainfield sizing. El Paso County Public Health requires a Use Permit inspection for property transfers involving OWTS — buyers and sellers should plan for this requirement before closing. Colorado Springs contractors are generally experienced with the local clay soil conditions and El Paso County permit process.
Colorado calls the drainfield a Soil Treatment Area (STA). For a three-bedroom home producing 450 gallons per day:
| Soil type | Application rate | STA area needed | Colorado context |
|---|---|---|---|
| Sandy loam (eastern plains) | 0.60 gpd/sq ft | 750 sq ft | Common in eastern Colorado |
| Loam (Front Range) | 0.45 gpd/sq ft | 1,000 sq ft | Typical suburban Front Range |
| Clay loam (Front Range clay) | 0.20 gpd/sq ft | 2,250 sq ft | Heavy clay — Jefferson, Douglas |
| Mountain engineered | Determined by engineer | Varies | Above 8,000 ft — no standard table |
Use the drainfield calculator for your specific soil type and bedroom count. Mountain sites above 8,000 feet require an engineer to determine STA sizing based on actual soil conditions.
Verify before you build. Requirements compiled from Colorado Regulation 43 and county OWTS programs for planning purposes. County health departments set the practical standard for your parcel — and 2026 updates mean requirements may have changed recently. Confirm all requirements with your county before designing or ordering equipment. CDPHE OWTS program: cdphe.colorado.gov — OWTS.
Colorado questions
Next steps
Cost estimator
Get a detailed Colorado OWTS cost estimate by bedroom count, soil type, and system type.
Soil evaluation cost
Front Range evaluations: $300–$900. Mountain counties: $800–$2,500. See how Colorado compares.
Pump schedule
Find your service interval by household size and tank capacity.
Replacement cost
Replacement costs more than new construction — get a Colorado-specific estimate.
STA size
Calculate Soil Treatment Area size by soil type — important for clay-heavy Front Range sites.
Local quotes
Get free quotes from licensed Colorado OWTS contractors in your county.
Official resources
CDPHE — statewide
Regulation 43 — On-site Wastewater Treatment Systems
State standards, certified installer database
Large system (>2,000 gpd) design review
4300 Cherry Creek Dr S, Denver CO 80246
County health departments
All OWTS permits are issued by county health departments — not CDPHE. Contact your county for permit applications, soil evaluation requirements, fees, and local rules that exceed Regulation 43.
Nearby states