Published
Past 2,500 gallons a day, one septic system is not enough.
The threshold is 2,500 gallons a day, and it is not per building
Once a project's wastewater reaches a certain daily volume it stops being an ordinary septic system in the eyes of the rules and becomes a large soil absorption system, with a different and considerably heavier set of requirements. For anyone contemplating a lodge, a cluster of rental cabins, an RV park or a multi-unit development on Island Park ground, this is the line that changes the budget.
“Large Soil Absorption System (LSAS). A subsurface sewage disposal system designed to receive two thousand five hundred (2,500) gallons of wastewater or more per day, including where the total wastewater flow from the entire proposed project exceeds two thousand five hundred (2,500) gallons per day, but is separated into absorption modules that receive less than two thousand five hundred (2,500) gallons per day.”
— IDAPA 58.01.03, Individual/Subsurface Sewage Disposal Rules, section 003.21, Definitions. Idaho Department of Environmental Quality. Rule text as amended 1 July 2025. Retrieved 1 September 2026
The clause after the word including is the whole point of the definition. You cannot get under the threshold by dividing the project into smaller pieces. If the total flow from the entire proposed project exceeds 2,500 gallons per day, it is an LSAS even when every individual absorption module sits below that figure.
That closes the obvious workaround before anyone reaches for it, and it is worth knowing early — because the design that splits the flow is exactly the design a developer arrives with when the LSAS requirements are first explained.
What the classification actually requires
| Requirement | What the rule specifies |
|---|---|
| Site investigation | Conducted by a soil scientist or hydrogeologist, determining whether the effluent will adversely impact the waters, submitted to the Director for review and approval. |
| Who designs it | All design elements and application rates developed using sound engineering practice and provided by a professional engineer licensed by the state of Idaho. |
| Module size cap | No disposal module may exceed 10,000 gallons per day. Above that, the system must be divided into modules each designed for 10,000 gallons per day or less. |
| Number of systems | A minimum of two disposal systems, each sized to accept the full daily design flow. |
| Reserve ground | A replacement area equal to the size of one disposal system must be reserved on top of that. |
| Distribution | The distribution system must be pressurized with a duplex dosing system. |
| Inspection provision | Permit application plans must include provisions for inspection by the design engineer, a designee, or the Director during construction. |
| Close-out | Within 30 days of completing installation, the design engineer provides as-built plans or a certificate of substantial compliance. |
Three systems' worth of ground for one project
The land requirement is the item that most often kills a scheme, and it is easy to underestimate when reading the rule quickly.
The rule requires a minimum of two disposal systems, each sized to accept the daily design flow — not two halves of one system, but two complete ones. It then requires a replacement area equal to the size of one disposal system to be reserved as well.
Add those together and the project has to find room for the equivalent of three full-sized disposal systems, only one of which is doing anything on a given day. On constrained ground near water, that arithmetic decides feasibility long before anyone reaches the question of cost.
There are two further constraints on where it can go. The vertical and horizontal hydraulic limits of the receiving soils must be established, and flows must not exceed those limits, so that no absorption module or replacement area is hydraulically overloaded. And no LSAS may be installed above a downslope scarp or cut unless approved by the Director — a provision with obvious relevance on the broken, benched ground common through parts of the county.
Who has to be involved, and why it is two professions
The rule names two distinct specialists for two distinct questions, and they are not interchangeable.
- A soil scientist or hydrogeologist performs the site investigation, answering whether the effluent will adversely impact the waters. That is a question about the ground and what lies under it.
- A professional engineer licensed in Idaho develops the design elements and application rates. That is a question about the system.
Neither substitutes for the other, and the site investigation is logically first: there is no point engineering a system for ground that has not been shown capable of receiving it. On a project of this size the sequence is also the cheapest way to fail early, which is the outcome to want when a scheme is not going to work.
Everything not specified in section 013 falls back to the ordinary rules — all design and installation requirements for standard systems apply to an LSAS unless the section says otherwise. The section adds obligations rather than replacing the ordinary ones.
How to test a project against this before spending money
- Estimate total daily design flow for the entire project, not per building and not per phase. The definition is explicit that the whole proposed project is what counts.
- If the total is at or above 2,500 gallons per day, treat it as an LSAS from the first sketch. Discovering the classification late is what makes it expensive.
- Do not design around the threshold by splitting flows. The definition anticipates that and closes it.
- Work out whether the site has room for two full systems plus a replacement area equal to one, and do that on a plan rather than by eye.
- Commission the site investigation before the engineering, so an unsuitable site is identified by the cheaper of the two studies.
- Check the ground for downslope scarps and cuts early, since an LSAS above one needs the Director's approval rather than being a matter of design.
- Budget for the annual and construction-phase involvement of the design engineer, including the 30-day as-built or compliance certificate that closes the installation out.
For a single cabin none of this applies, and the ordinary permit route is the one that matters. It becomes live the moment a project starts counting units — and the point at which it becomes live is well below the size most people would call a development, which is the same reason sizing a system for a rental cabin deserves an honest flow estimate rather than an optimistic one. The county has its own overlapping requirement where central service already exists, set out at when the county can make you connect.
Common questions
What makes a septic system a large soil absorption system?
A subsurface sewage disposal system designed to receive 2,500 gallons of wastewater or more per day. The definition also captures projects whose total flow exceeds 2,500 gallons per day even where it is split into modules each receiving less than that.
Can I stay under the threshold by splitting the system up?
No. The definition explicitly includes the case where total wastewater flow from the entire proposed project exceeds 2,500 gallons per day but is separated into absorption modules that each receive less. Splitting the flow does not change the classification.
How much land does an LSAS need?
A minimum of two disposal systems, each sized to accept the full daily design flow, plus a reserved replacement area equal to the size of one disposal system. In effect the project must find room for three full-sized systems.
Is there a maximum size for one module?
Yes. The maximum size of any subsurface sewage disposal module is 10,000 gallons per day. Developments with greater flow must divide the system into absorption modules each designed for 10,000 gallons per day or less.
Who has to design an LSAS?
All design elements and application rates must be developed using sound engineering practice and provided by a professional engineer licensed by the state of Idaho. Separately, a soil scientist or hydrogeologist must conduct the site investigation.
What does the site investigation have to establish?
Whether the LSAS effluent will adversely impact the waters. It is submitted to the Director for review and approval, and it answers a different question from the engineering design.
Can an LSAS go above a slope break?
No LSAS may be installed above a downslope scarp or cut unless approved by the Director. It is a matter requiring approval rather than something that can be designed around.
What kind of distribution does the rule require?
The distribution system must be pressurized with a duplex dosing system. The vertical and horizontal hydraulic limits of the receiving soils must also be established so no absorption module or replacement area is hydraulically overloaded.
What happens when installation is finished?
Within thirty days of completing system installation the design engineer must provide either as-built plans or a certificate that the system was installed in substantial compliance with the installation permit application plans.