Colorado · Colorado Regulation 43
CDPHE writes Regulation 43 and counties adopt local regulations that must be at least as strict, so the operative rule is almost always the county's.
Minimum tank
One to three bedroom dwelling.
Design flow
Held two days for settling.
Cost index
Regional labour and permits.
Sets the design flow and the legal minimum.
Code sizes on bedrooms, not people.
Used to size the matching drainfield. Unsure? Sandy loam is the common default.
Front Range clays with high shrink-swell potential, decomposed granite in the foothills, shallow bedrock in the mountains, and sandy ground on the eastern plains. Elevation and frost depth shape trench depth across the mountain counties.
Conventional trenches on the plains and better foothill sites. Treatment level requirements under Regulation 43 push many mountain and clay-soil sites toward advanced treatment, and engineered designs are common.
| Dwelling | Design flow | Minimum tank | With disposal |
|---|---|---|---|
| 1 bedroom | 300 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 |
Regulation 43 sets treatment level requirements (TL1 through TL3) based on site conditions, so the soil evaluation determines not just field size but the treatment technology required. A registered professional engineer's design is needed for many sites.
The state sets a floor. These Colorado counties are where the largest septic populations sit, and where local requirements most often exceed it.
| County | What makes it different |
|---|---|
| El Paso | Colorado Springs; the largest septic county, with expansive clay soils |
| Douglas | Rapid growth south of Denver on clay |
| Jefferson | Foothills; decomposed granite and shallow rock |
| Larimer | Fort Collins and mountain areas, wide soil variation |
| Weld | Agricultural plains, generally better percolation |
A three-bedroom home at 150 gallons per bedroom produces 450 gallons per day of design flow. On sandy loam that needs roughly 750 square feet of absorption area. On clay loam it rises to about 2,250 square feet, and on clay a conventional field is usually not permitted at all. Run your own soil class through the drainfield calculator.
Colorado counties frequently require inspection at property transfer, and advanced systems carry operating permits with reporting obligations. Use the pump schedule calculator to work out the interval for your tank size and household.
Regional labour and permit costs in Colorado run about 112% of the national baseline. Here is what typical installations land at for a three-bedroom home:
| System type | Typical installed cost in Colorado | When it applies |
|---|---|---|
| Conventional gravity | $10,100 – $17,900 | Good soil, adequate depth to water |
| Chamber system | $11,100 – $20,600 | Alternative to gravel trenches |
| Mound system | $19,200 – $37,600 | Shallow water table or thin soil |
| Aerobic treatment | $16,200 – $32,200 | Clay soil or small lots |
| Tank pump-out (service) | $360 – $730 | Every 3–5 years |
Replacing an existing system costs more than these figures — old-system removal and re-permitting add to every line. The replacement cost calculator covers that case, and the cost estimator adjusts all of the above for your bedroom count and soil.
Verify before you build. The figures above are state minimums compiled for planning, drawn from Colorado Regulation 43, 5 CCR 1002-43, On-Site Wastewater Treatment System Regulation. County and health district rules routinely exceed them, code text changes, and only Colorado CDPHE — OWTS Program (Reg 43) or your local health department can confirm what applies to your parcel.
Send your sizing numbers to installers who work under Colorado rules every day.