Published
One house on any lot. A second one needs 1.5 acres and a good deal more.
The reassuring rule first
Corridor acreage comes with a lot of talk about density, and it worries buyers who only want one house. So it is worth putting the code's own sentence up front, because it settles the common case cleanly.
“One primary dwelling unit may be built on any lot of any size, if all requirements of the Eastern Idaho Public Health District and the permit procedures pertaining to land division are fulfilled.”
— Fremont County Development Code, 2011 Edition (Ordinance 2011-04), section 5.54.010. Retrieved 19 August 2026
Any lot, any size. The density ratios in the zoning tables govern how many dwellings can be created across a piece of ground when it is divided; they do not stop a single existing lot from carrying a single house. What that sentence does attach are two conditions, and both are real: the health district's requirements, which in practice means the ground has to take a septic system, and the permit procedures pertaining to land division, which means the lot has to be a legitimately created lot.
The second dwelling is a different question entirely
Guest houses, a place for parents, a unit over the shop — the second dwelling is one of the most common corridor requests, and the code treats it as a wholly separate matter with its own threshold.
| Requirement for a secondary dwelling unit | What the code says |
|---|---|
| Minimum lot size | The lot must have a minimum of 1.5 acres |
| Ownership | Allowed under certain conditions, the code naming perpetual common ownership — the two dwellings are not to be split apart later, and the lot may not be divided to separate them |
| Zoning | The applicable zoning must permit an accessory secondary dwelling unit; the 1.5 acres alone is not enough |
| Water and septic | Shared wells and/or drain fields may be required to meet Eastern Idaho Public Health District requirements |
The ownership condition is the one that surprises people. A secondary dwelling under this provision is not a way of creating a second saleable property. It is an additional unit on a holding that stays in common ownership, and anyone whose plan is to build one and sell it separately later is describing a land division, which is a different application entirely.
The shared services condition is the one that costs money. Where the health district requires a shared well or a shared drainfield, the second dwelling is not simply a second building; it is a rework of the site's water and wastewater design. That is a conversation to have before the first system is installed rather than after, and it changes what the pre-purchase well and septic checks ought to cover if a second unit is ever contemplated.
Gross density and net density are not the same number
This distinction is where a lot of disappointment comes from on acreage, and the code is unusually frank about it.
Gross dwelling unit density is the ratio in the zoning tables — units allowed per hundred acres in the rural zones, applied to a project of any size. Net density is what is left after the ordinance's constraints and the open space requirement have been applied to a particular site.
The code then says the quiet part out loud: after applying all constraints, it is possible that a particular site does not allow for the gross density of the zone in which it is located, and state health regulations may separately prevent a site from reaching maximum density.
In plain terms: the zoning number is a ceiling, not an entitlement. A parcel can be zoned for a density its ground will not physically support, and the ordinance anticipates exactly that. Anyone quoting you a lot yield off the zoning table alone is quoting the ceiling.
Proving there is water
Which leads to the constraint that bites hardest on the corridor. Where a development relies on individual water supplies rather than a central system, the code requires evidence — and it says what will count.
- Central supply: where required or selected, subdivisions must provide a central domestic water supply system meeting state design and construction requirements.
- Individual supplies: evidence must be provided that an adequate quantity and quality of water is available for the proposed application.
- What counts as evidence: the code names documented experience with existing wells at geologically similar, neighbouring sites, or records of on-site well tests.
That first option is genuinely useful and underused. You do not always need to drill to make the case; documented experience from geologically similar neighbouring ground can carry it. On a corridor where neighbours have drilled and the driller's records exist, the evidence may already be in the world — it just has to be collected and documented rather than assumed.
The phrase “quantity and quality” is worth noticing too. Producing water is not the whole test. Water that arrives in adequate volume but fails on quality has not satisfied the requirement, and that is a testing question with its own cost.
If you are assembling the neighbouring-wells case, these are the things worth collecting, because a file of them is far more persuasive than a recollection:
- Driller's reports for nearby wells, which record the depth drilled, what was passed through, and what the well produced on completion.
- How far away each well is, and whether the ground between is genuinely similar — the code's word is geologically similar, not merely nearby.
- Depth to water as recorded, and whether it has moved since.
- Any water quality results the neighbours already hold, since quality is half the test and volume alone does not satisfy it.
- Whether any nearby well has failed or been deepened, which is the fact most likely to be omitted and most useful to know.
How the pieces fit together on a real purchase
- Is it a legitimate lot? The one-dwelling-on-any-lot rule is conditioned on the permit procedures pertaining to land division having been fulfilled. A parcel created informally is a problem to solve before anything else.
- Will the ground take a septic system? The health district's requirements are the other condition on that same sentence, and they are the usual reason a lot cannot carry a house.
- Is there water, and can it be evidenced? Either neighbouring well records or on-site tests, on both quantity and quality.
- Is a second dwelling ever contemplated? If so, check the 1.5 acres, check the zoning permits an accessory secondary dwelling unit, and design the well and septic for two units from the start.
- If the plan involves dividing the ground, treat the zoning density as a ceiling and get the net figure worked out against the actual constraints before relying on a lot count.
- Write down which of the five checks above are answered and which are assumed, with dates, because the ones carried forward on somebody's say-so are reliably the ones that turn out to be wrong.
None of this makes corridor acreage difficult. One house on one lot is the ordinary case and the code accommodates it plainly. It is the second dwelling and the future subdivision that carry the conditions, and both are much cheaper to plan for at purchase than to retrofit — the same pattern as everything else on the list of things that vary lot to lot.
Common questions
Can I build one house on a small rural lot?
The code states that one primary dwelling unit may be built on any lot of any size, provided all requirements of the Eastern Idaho Public Health District and the permit procedures pertaining to land division are fulfilled. The zoning density ratios govern division of ground rather than a single house on an existing lot.
How big does a lot have to be for a second dwelling?
A minimum of 1.5 acres, and that is necessary rather than sufficient. The code also requires certain conditions including perpetual common ownership, and the applicable zoning must permit an accessory secondary dwelling unit.
Can I sell the second dwelling separately?
Not under this provision. The code names perpetual common ownership among the conditions, so a secondary dwelling unit is an additional unit on a holding that stays together. Creating a separately saleable property is a land division and a different application.
Will a second dwelling change my well and septic?
It can. The code states that shared wells and drain fields may be required to meet Eastern Idaho Public Health District requirements. If a second unit is ever likely, it is far cheaper to design the water and wastewater systems for it from the start.
What is the difference between gross and net density?
Gross density is the ratio in the zoning tables. Net density is what remains after the ordinance's constraints and the applicable open space requirement are applied to a specific site. The code acknowledges that a site may not allow the gross density of its zone.
So the zoning density is not a guarantee?
No. The code explicitly says that after applying all constraints it is possible a particular site does not allow for the gross density of its zone, and that state health regulations may separately prevent maximum density. It is a ceiling rather than an entitlement.
How do I prove there is enough water?
The code requires evidence that an adequate quantity and quality of water is available, and names two acceptable forms: documented experience with existing wells at geologically similar neighbouring sites, or records of on-site well tests.
Do I have to drill before I can prove it?
Not necessarily. Documented experience from existing wells at geologically similar neighbouring sites is named as acceptable evidence, so on ground where neighbours have drilled and records exist the case may be assembled from what is already known.
Is water quality part of the test?
Yes. The requirement is for adequate quantity and quality. A well producing plenty of water that fails on quality has not satisfied the standard, and testing for that is a separate cost worth anticipating.