New Hampshire · New Hampshire Code of Administrative Rules Env-Wq 1000
NHDES administers the program directly and approves system designs statewide, with no county layer.
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.
Glacial till over granite, with ledge at shallow depth across much of the state. Lakefront and river lots are small and old. Frost depth is significant and slopes are common.
New Hampshire relies on designers approved by the state, and imported fill systems are widespread because native soil depth over ledge is inadequate on so many lots.
| 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 |
NHDES approves the design before construction and requires a licensed designer and installer. There is no county permit — the state issues approval directly, which makes the process more uniform than in most states.
The state sets a floor. These New Hampshire counties are where the largest septic populations sit, and where local requirements most often exceed it.
| County | What makes it different |
|---|---|
| Rockingham | Seacoast growth; small lots and high water table |
| Hillsborough | Southern growth corridor on till |
| Merrimack | Concord area, mixed soils |
| Carroll | Lakes region, shoreline setbacks |
| Grafton | Rural, ledge and slope constraints |
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 mandated statewide interval, though pumping records are relevant at property sale. Use the pump schedule calculator to work out the interval for your tank size and household.
Regional labour and permit costs in New Hampshire run about 120% of the national baseline. Here is what typical installations land at for a three-bedroom home:
| System type | Typical installed cost in New Hampshire | When it applies |
|---|---|---|
| Conventional gravity | $10,800 – $19,200 | Good soil, adequate depth to water |
| Chamber system | $11,900 – $22,100 | Alternative to gravel trenches |
| Mound system | $20,500 – $40,300 | Shallow water table or thin soil |
| Aerobic treatment | $17,300 – $34,600 | Clay soil or small lots |
| Tank pump-out (service) | $380 – $780 | 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 New Hampshire Code of Administrative Rules Env-Wq 1000, Subsurface Systems. County and health district rules routinely exceed them, code text changes, and only New Hampshire DES — Subsurface Systems Bureau or your local health department can confirm what applies to your parcel.
Send your sizing numbers to installers who work under New Hampshire rules every day.