Idaho · IDAPA 58.01.03
Idaho DEQ writes the rules and seven regional public health districts issue permits and perform site evaluations.
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.
Basalt bedrock close to the surface across the Snake River Plain, gravel and cobble in river valleys, and clay in parts of the north. Very fast-draining gravel is as much a problem as slow clay, because it fails to treat effluent.
Conventional trenches where soil depth allows. Where basalt is shallow or gravel is excessively permeable, designs move to sand-lined trenches or advanced treatment.
| 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 |
Idaho's health districts perform the site evaluation and issue the permit, and district practice varies. Central district requirements around Boise differ in detail from those in the panhandle.
The state sets a floor. These Idaho counties are where the largest septic populations sit, and where local requirements most often exceed it.
| County | What makes it different |
|---|---|
| Ada | Boise metro; basalt at variable depth |
| Canyon | Agricultural land converting to housing |
| Kootenai | Coeur d'Alene area; growth pressure and aquifer protection |
| Bonner | Lakefront lots with strict setbacks |
| Bonneville | Idaho Falls; shallow basalt |
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.
No statewide interval mandated for conventional systems. Use the pump schedule calculator to work out the interval for your tank size and household.
Regional labour and permit costs in Idaho run about 102% of the national baseline. Here is what typical installations land at for a three-bedroom home:
| System type | Typical installed cost in Idaho | When it applies |
|---|---|---|
| Conventional gravity | $9,200 – $16,300 | Good soil, adequate depth to water |
| Chamber system | $10,100 – $18,700 | Alternative to gravel trenches |
| Mound system | $17,500 – $34,200 | Shallow water table or thin soil |
| Aerobic treatment | $14,700 – $29,300 | Clay soil or small lots |
| Tank pump-out (service) | $330 – $660 | 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 IDAPA 58.01.03, Individual and Subsurface Sewage Disposal Rules. County and health district rules routinely exceed them, code text changes, and only Idaho DEQ — Subsurface Sewage Disposal or your local health department can confirm what applies to your parcel.
Send your sizing numbers to installers who work under Idaho rules every day.