By Kjel Jones, independent Island Park building researcher · licensed Idaho real estate agent, LPT Realty

Published 2026-08-28

Sixty-five percent of the parcel: the open space rule behind every rural plat

The rule nobody mentions until the plat is drawn

Ask what you can do with rural acreage in Fremont County and you will be told about setbacks and minimum lot size. Both are real. But there is a requirement sitting on top of them that decides far more about what a piece of ground can become, and it is expressed as a percentage of the whole parcel: open space.

Every rural zone in the county's Development Code requires that a large share of gross acreage be established as open space, and the share is bigger than most people guess. In the Rural Base zone it is sixty-five percent.

The four rural zones, side by side

The pattern is the useful part. As the required open space falls, the permitted density rises. The code is trading one against the other, deliberately.

All three of the base rural zones carry the same 25 percent density bonus where central water and sewer systems serve the development. That is the single largest lever in the table, and it is the reason the sewer question is never only about plumbing — it is a density question too, which is worth reading alongside the mandatory connection rule.

What open space actually has to be

Open space in this code is not simply leftover land. Two of the zones put conditions on how much of it can be actively used, and all of them require the applicant to say what the open space is for.

• Rural Conservation: at least 2 percent of the required open space must be suitable for active recreational use, and no more than 10 percent may be used for that purpose.
• Rural Living: at least 5 percent of the required open space must be suitable for active recreational use, and no more than 15 percent may be used for that purpose.
• Every rural zone: the proposed and permitted uses of the open space areas must be identified by the applicant in an open space management plan.
• Rural Mixed Use overlay: at least 20 percent of gross acreage goes to open space and public amenities, a much lower bar than the base zones because the overlay is doing a different job.

The floor-and-ceiling shape is easy to misread. The code is saying that some of the open space has to be genuinely usable, and that most of it must not be. It is not a park requirement and it is not a wasteland requirement; it is both at once, in fixed proportions.

The exception that costs you a septic system, permanently

One provision in this part of the code is a genuine trade, and it is written in a way that is easy to skim past.

“In all rural zones, where a subdivision will create a lot for an exclusively non-residential use, an exception to the minimum lot size may be granted if the applicant agrees to place a plat note and a deed restriction barring the location of any septic system on that lot in the future.”

— Fremont County Development Code, 2011 Edition (Ordinance 2011-04), minimum lot size exception for non-residential uses, as amended by Ordinance 2014-03. Retrieved 19 August 2026

Read what is being exchanged. You may go below the one-acre minimum, and the price is a permanent bar on ever putting a septic system on that lot, recorded in two places — a note on the plat and a restriction on the deed. Not a bar on building. A bar on the one thing that makes a building habitable without a sewer connection.

On ground with no central sewer, that is effectively a decision that the lot will never hold a dwelling. It is the right trade for a well house, a pump station or a storage building. It is a catastrophic one to accept by accident on a lot somebody later hopes to build a cabin on, and it does not go away when the property changes hands.

How this changes the arithmetic on acreage

The practical effect is that gross acreage overstates what you can do with a parcel, sometimes badly.

1. Start with gross acreage — the number on the listing.
2. Subtract the open space share for the zone. In Rural Base that is 65 percent of the whole parcel before anything else is considered.
3. Apply the maximum density. Twenty units per hundred acres in Rural Base is one dwelling per five acres, whatever the minimum lot size says.
4. Check whether central water and sewer are available, because the 25 percent bonus is the difference between twenty and twenty-five units on that same hundred acres.
5. Take out the water setbacks and buffers, which on this ground are substantial and are covered on the shoreline page.
6. What remains is the developable area. Compare it with what you were told the parcel could hold.

Note the interaction between density and minimum lot size, because it trips people. A one-acre minimum lot does not mean a hundred-acre parcel yields a hundred lots. In Rural Base the density cap says twenty, and the minimum lot size only sets the smallest any one of those twenty may be.

Why the county wrote it this way

The stated objectives are worth knowing, because they tell you how an application will be read. Across the rural zones the code says it is protecting agricultural and open lands, protecting fish, wildlife and recreation resources, conserving open rural and scenic non-urban landscapes, and encouraging seasonal and limited recreational and very low density residential development consistent with the conservation values of the locality.

That last phrase describes most of what gets built in Island Park. The county is not hostile to cabins; the code is written with them in mind. But it is written to keep the land around them open, and the open space percentage is the mechanism. Anyone hoping to maximise lot count on rural ground is arguing against the stated purpose of the zone, not just against a number.

What to establish before you buy acreage

• Which rural zone the parcel is actually in, since the open space share and the density cap both follow from it.
• Whether central water and sewer are available or plausibly will be, because the 25 percent bonus rides on it.
• Whether any lot in an existing plat carries the non-residential exception, and therefore a recorded bar on any septic system.
• What the existing open space management plan commits the land to, if the parcel is inside an approved subdivision.
• How much of the parcel is already spoken for by water buffers, steep ground or floodplain before the open space percentage is even applied.
• Whether the density figure you have been quoted is units per hundred acres or minimum lot size, because they are different constraints and the tighter one governs.

Common questions

How much of a rural parcel has to be left as open space?

It depends on the zone. Rural Base requires at least 65 percent of gross acreage, Rural Conservation 60 percent and Rural Living 50 percent. The Rural Village District is higher again at 80 percent, and the Rural Mixed Use overlay is much lower at 20 percent of gross acreage for open space and public amenities.

How many dwellings can a hundred acres hold?

In Rural Base the maximum density is 20 units per 100 acres, in Rural Conservation 30, and in Rural Living 40. All three carry a 25 percent density bonus where central water and sewer systems serve the development.

If the minimum lot is one acre, why can't a hundred acres make a hundred lots?

Because minimum lot size and maximum density are separate constraints and the tighter one governs. One acre is the smallest any single lot may be; the density cap limits how many lots there can be in total. In Rural Base that cap is twenty per hundred acres.

Does open space have to be usable?

Partly, and the code sets both a floor and a ceiling. In Rural Conservation at least 2 percent of the required open space must be suitable for active recreational use and no more than 10 percent may be used for it. In Rural Living the equivalent figures are 5 and 15 percent.

What is an open space management plan?

The document in which the applicant identifies the proposed and permitted uses of the open space areas. The code requires it across the rural zones, so open space is something you have to describe and commit to rather than simply leave over.

What is the non-residential minimum lot size exception?

In all rural zones, where a subdivision creates a lot for an exclusively non-residential use, the county may grant an exception to the minimum lot size if the applicant agrees to a plat note and a deed restriction barring the location of any septic system on that lot in future.

Can that septic restriction be removed later?

It is recorded as both a plat note and a deed restriction, so it runs with the land rather than with the owner who agreed to it. On ground without a central sewer, a lot carrying that restriction is effectively one that cannot hold a dwelling.

Does the density bonus apply to any central system?

The code states the bonus for central water and sewer systems, at 25 percent, in each of the three rural base zones. Whether a particular system qualifies is a question for the Planning and Building Department, and it is worth settling before the density is relied on in a purchase.