Idaho · IDAPA 58.01.03.007
Septic system requirements in Idaho
Tank sizes, drainfield rules, setbacks, permits, and real costs — pulled from Idaho's own code rather than a national average.
- 3-bedroom tank
- 1,000 gal
- Design flow
- 150 gpd/BR
- Typical install
- $8,000–$22,000
- Permit fee
- $250–$800
- Well setback
- 100 ft
- Permitted by
- County / local
Septic tank sizes required in Idaho
Idaho 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 | 900 gal | 1,000 gal |
| 2 bedrooms | 300 gpd | 900 gal | 1,000 gal |
| 3 bedrooms | 450 gpd | 1,000 gal | 1,000 gal |
| 4 bedrooms | 600 gpd | 1,000 gal | 1,250 gal |
| 5 bedrooms | 750 gpd | 1,250 gal | 1,500 gal |
| 6 bedrooms | 900 gpd | 1,500 gal | 2,000 gal |
Source: IDAPA 58.01.03.007. 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 Idaho 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 | $10,600 |
| Sandy loam | 0.5 gpd/ft² | 900 ft² | 1,800 ft² | $13,400 |
| Loam | 0.4 gpd/ft² | 1,125 ft² | 2,250 ft² | $15,200 |
| Silt loam | 0.3 gpd/ft² | 1,500 ft² | 3,000 ft² | $18,300 |
| Clay loam | 0.2 gpd/ft² | 2,250 ft² | 4,500 ft² | $24,500 |
Estimates use Idaho's cost index of 0.92× the national average. Build a line-item estimate →
Permits and who issues them
Seven regional public health districts issue permits under IDEQ rules. The district, not the county, is your permitting authority.
| Permit required | Yes |
|---|---|
| Typical fee | $250 – $800 |
| Soil evaluation | Required before permit |
| Licensed installer | Required |
| Turnaround | 2–6 weeks |
| Ongoing inspection | Not mandated statewide |
| 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 | 5 ft |
| Surface water | 50 ft |
| Building foundation | 10 ft |
Regulatory contact
Idaho Department of Environmental Quality — Septic Systems
www.deq.idaho.gov · 208-373-0502
What makes septic hard in Idaho
Basalt bedrock across the Snake River Plain sits shallow and is expensive to excavate; nitrate loading over the aquifer triggers extra treatment requirements.
Panhandle soils are glacial and highly variable — two test pits fifty feet apart can classify differently.
Rapid growth around Boise has consumed the easy lots, leaving marginal ground.
Things that catch people out
- Nitrate Priority Areas require enhanced treatment. Check whether your parcel sits in one before you budget.
- Total tank volume must be at least 115% of rated liquid capacity, which quietly rules out some imported tanks.
Idaho septic questions
What size septic tank do I need in Idaho?
A three-bedroom home in Idaho requires a minimum 1,000 gallon tank under IDAPA 58.01.03.007. 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 Idaho?
Yes. Seven regional public health districts issue permits under IDEQ rules. The district, not the county, is your permitting authority. Fees typically run $250 to $800, 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 Idaho?
A conventional system for a three-bedroom home typically runs $8,000 to $22,000 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 Idaho 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.
- IDAPA 58.01.03.007 — Septic Tanks Design and Construction Standards · verified 2026-09-18
- IDAPA 58.01.03 — Individual/Subsurface Sewage Disposal Rules · verified 2026-09-18