Colorado · 5 CCR 1002-43 §43.9
Septic system requirements in Colorado
Tank sizes, drainfield rules, setbacks, permits, and real costs — pulled from Colorado's own code rather than a national average.
- 3-bedroom tank
- 1,000 gal
- Design flow
- 150 gpd/BR
- Typical install
- $12,000–$32,000
- Permit fee
- $300–$1,500
- Well setback
- 100 ft
- Permitted by
- County / local
Septic tank sizes required in Colorado
Colorado sizes systems on a design flow of 150 gallons per day per bedroom. The tank must meet both the bedroom table below and a 2× daily-flow detention requirement — whichever produces the larger number governs.
| Bedrooms | Design flow | Code minimum | Recommended |
|---|---|---|---|
| 1 bedroom | 150 gpd | 1,000 gal | 1,000 gal |
| 2 bedrooms | 300 gpd | 1,000 gal | 1,000 gal |
| 3 bedrooms | 450 gpd | 1,000 gal | 1,000 gal |
| 4 bedrooms | 600 gpd | 1,250 gal | 1,250 gal |
| 5 bedrooms | 750 gpd | 1,500 gal | 1,500 gal |
| 6 bedrooms | 900 gpd | 1,750 gal | 2,000 gal |
Source: 5 CCR 1002-43 §43.9. The recommended column rounds the governing requirement up to a tank size manufacturers actually stock.Run your own numbers →
Drainfield size and cost by soil type
For a three-bedroom home in Colorado at 450 gpd. This single table explains most of the variation between quotes — the area scales inversely with how fast your soil accepts water, and the drainfield is the largest line item in the job.
| Soil | Loading rate | Absorption area | With reserve | Typical total install |
|---|---|---|---|---|
| Medium sand | 0.8 gpd/ft² | 563 ft² | 1,126 ft² | $12,100 |
| Sandy loam | 0.5 gpd/ft² | 900 ft² | 1,800 ft² | $15,200 |
| Loam | 0.4 gpd/ft² | 1,125 ft² | 2,250 ft² | $17,200 |
| Silt loam | 0.3 gpd/ft² | 1,500 ft² | 3,000 ft² | $20,700 |
| Clay loam | 0.2 gpd/ft² | 2,250 ft² | 4,500 ft² | $27,600 |
Estimates use Colorado's cost index of 1.02× the national average. Build a line-item estimate →
Permits and who issues them
Regulation 43 sets statewide minimums; county and district public health agencies issue permits and may adopt stricter local rules.
| Permit required | Yes |
|---|---|
| Typical fee | $300 – $1,500 |
| Soil evaluation | Required before permit |
| Licensed installer | Required |
| Turnaround | 2–6 weeks |
| Ongoing inspection | Transfer-of-title inspection required by many counties |
| Reserve area | 100% replacement area required |
Minimum setback distances
Measured horizontally from the drainfield unless noted. Local rules are frequently stricter, and on a small lot these numbers — not the soil — often decide the layout.
| Private water well | 100 ft |
|---|---|
| Well (septic tank) | 50 ft |
| Property line | 10 ft |
| Surface water | 50 ft |
| Building foundation | 10 ft |
Regulatory contact
Colorado Department of Public Health and Environment — OWTS
cdphe.colorado.gov · 303-692-3500
What makes septic hard in Colorado
Mountain sites combine shallow bedrock, steep slopes, and short frost-free seasons — the three things that most reliably force an engineered design.
Expansive bentonite clays on the Front Range swell and shrink enough to crack tanks and distribution lines.
Water rights law limits what you can do with greywater in Colorado far more than in neighbouring states.
Things that catch people out
- Regulation 43 requires a minimum 25 ft² of liquid surface area in the tank, which rules out some tall narrow tanks sold elsewhere.
- Most counties require a transfer-of-title inspection. It is the single most common surprise in a Colorado mountain-property closing.
Colorado septic questions
What size septic tank do I need in Colorado?
A three-bedroom home in Colorado requires a minimum 1,000 gallon tank under 5 CCR 1002-43 §43.9. Design flow is 150 gallons per day per bedroom, and the tank must also hold 2 times daily flow, so the practical recommendation is 1,000 gallons.
Do I need a permit for a septic system in Colorado?
Yes. Regulation 43 sets statewide minimums; county and district public health agencies issue permits and may adopt stricter local rules. Fees typically run $300 to $1,500, with a turnaround of about 2–6 weeks. A soil evaluation or percolation test is required before a permit is issued.
How much does a septic system cost in Colorado?
A conventional system for a three-bedroom home typically runs $12,000 to $32,000 installed. Soil quality moves this more than anything else — on tight clay requiring a mound, budget two to three times the low end.
How far must a Colorado drainfield be from a well?
100 feet from a private water well, with the septic tank itself allowed at 50 feet. Local rules can be stricter.
Sources
Every figure on this page traces to one of these documents.
- 5 CCR 1002-43 §43.9 — Regulation 43 — Design Criteria, Components · verified 2026-09-18
- 5 CCR 1002-43 — On-Site Wastewater Treatment System Regulation · verified 2026-09-18