Published
Twenty percent: the slope number that decides whether your lot gets a standard drainfield
One line, and it decides a large part of your budget
Buyers looking at ground here ask about soil, water table and setbacks. Slope is the condition that gets checked first and discussed least, and it is a single sentence in the state rule:
The natural slope of the site will not exceed twenty percent (20%).
— IDAPA 58.01.03.008.01.a, site suitability for a standard subsurface disposal facility (adminrules.idaho.gov, retrieved 2026-08-17; the subsection carries a 7-1-25 effective marker)
It sits at the head of Section 008, which is titled "Standard Subsurface Disposal Facility Design and Construction" — so it is not a rule about septic systems generally. It is the gate on the standard system, which is the cheap one.
The word doing the work is natural. It is the slope of the site as it lies, not the slope after a machine has been over it. You do not get under the number by regrading the drainfield area.
What twenty percent actually is
Twenty percent means twenty feet of fall for every hundred feet of horizontal run. That is a common misunderstanding worth clearing up, because it is not twenty degrees — it is a good deal less steep than that sounds.
| Fall over 100 feet | Slope | Standard drainfield? |
|---|---|---|
| 5 feet | 5% | Slope is not the constraint |
| 10 feet | 10% | Slope is not the constraint |
| 20 feet | 20% | At the published limit |
| 25 feet | 25% | Over the limit for a standard system |
| 40 feet | 40% | Well over; expect an alternative design |
On the ground, a 20% slope is a bank you would notice walking up and would not want to mow. Plenty of lots here sit under it comfortably. The ones that do not are usually the ones with the view.
Slope is one condition of several, and they are cumulative
Passing the slope test does not deliver a standard system by itself. Section 008 lists the site conditions together, and the drainfield area must meet all of them:
- Slope — the natural slope will not exceed twenty percent.
- Suitable soil types present at depths corresponding with the sidewalls of the proposed drainfield.
- Effective soil depth — suitable soil between the bottom of the proposed drainfield and any limiting layer. On volcanic ground this is frequently the binding one, and it is covered on the alternative systems page.
A lot can be flat and still fail, and it can be steep and fail twice. The reason the slope figure is worth knowing separately is that it is the only one of the three you can estimate yourself before paying for anything.
What happens above twenty percent
Nothing about a steep lot makes it unbuildable. It moves the design out of the standard category and into the alternatives, and that has three consequences worth pricing before you buy:
- A different design. Steep-slope systems are among the basic alternatives; sand mounds and pressure distribution are complex ones. Which you need is a function of the whole site, not the slope alone.
- A differently permitted installer. Complex alternative systems require a complex installer's permit, which is a smaller field of contractors carrying a larger bond. The bond page covers what that difference is worth.
- A longer and more expensive path to a permit, because the design work is real design work rather than a standard layout.
The mistake to avoid is assuming a steep lot simply costs more for the same system. It usually is not the same system.
How to check slope before you spend anything
- Find the part of the lot the drainfield would occupy, not the building site. They are frequently different pieces of ground, and the test applies where the drainfield goes.
- Estimate the fall across that area — a hand level and a tape, contour lines on a county or USGS map, or a phone inclinometer will all get you close enough to know whether twenty percent is even in question.
- Look at the area as it lies, ignoring any previous grading, cuts or fill you can see.
- Book the site evaluation if the answer is anywhere near the line. The site evaluation page covers what the test involves — it is the only reading that counts, and it also settles soil and effective depth.
- Do it before closing, not after. A slope reading is cheap; discovering the drainfield class after you own the ground is not.
Re-verify before relying on any figure here. This page was written from the current text of IDAPA 58.01.03 retrieved on 2026-08-17. The rule is free to read at adminrules.idaho.gov, and Eastern Idaho Public Health is the health district that permits septic systems in Fremont County.
Common questions
How steep is too steep for a standard septic drainfield in Idaho?
The rule's site-suitability condition is that the natural slope of the site will not exceed twenty percent. Above that, a standard subsurface disposal facility is not the design, and an alternative system is.
Is twenty percent the same as twenty degrees?
No. Twenty percent means twenty feet of fall over a hundred feet of horizontal run, which is noticeably less steep than twenty degrees. It is a slope you would notice walking up but would not call a cliff.
Can I regrade the ground to get under the limit?
The rule says natural slope, which is the slope of the site as it lies rather than after earthwork. Treat regrading the drainfield area as a question for Eastern Idaho Public Health rather than an assumption.
Does the slope of my building site matter, or the drainfield?
The condition applies to the area where the drainfield will be constructed. On many lots that is a different piece of ground from the house site, so measure the right one.
Does passing the slope test mean I get a standard system?
No. Slope is one of the site conditions listed together in the rule. Suitable soil types at the drainfield sidewall depths, and effective soil depth above any limiting layer, must also be satisfied.
What happens if my lot is steeper than twenty percent?
The design moves to an alternative system. Steep-slope systems sit among the basic alternatives, while sand mounds and pressure distribution are complex alternatives. Which applies depends on the whole site evaluation, not the slope alone.
Does a steep lot just cost more for the same system?
Usually not — it is generally a different system, often needing a differently permitted installer and real design work rather than a standard layout. Price it as a different job, not a surcharge.
Can I measure the slope myself?
Well enough to know whether it is close to the line. A hand level and tape, contour lines on a map, or a phone inclinometer will all give a usable estimate. The site evaluation is the reading that actually decides it.
When should I check this?
Before closing on the ground. A slope estimate costs nothing, and it is the one site-suitability condition you can assess without hiring anyone.