Can I build a second home or ADU on agricultural land in Bonneville or Jefferson County?

How agricultural zoning works in Bonneville and Jefferson counties

Agricultural zoning exists to keep farming viable by limiting what can be built on productive land, and each county runs its own system. In Bonneville County, the rules live in Chapter 156 of the Bonneville County Code, and the A-1 Agricultural zone is the workhorse district: the article begins at Section 156.125 with permitted uses at Section 156.126, and the zone carries its own standards for area, width, and yards (Sections 156.129 through 156.131). There is also a second agricultural-style district and an RA-2 Residential Agricultural zone, so the exact rules follow the exact zone on the parcel, never the seller's description of the place.

Jefferson County organizes things a little differently, with three agricultural districts in Chapter 112 of its code: AG-10 (Section 112-191), AG-20 (Section 112-192), and AG-40 (Section 112-193). The numbers are the minimum lot sizes. Per the dimensional table at Section 112-263, an AG-10 lot must be at least 10 acres, AG-20 at least 20 acres, and AG-40 at least 40 acres, and residential density is capped at one single-family dwelling per minimum lot size. The two counties share the same logic, but the numbers are theirs, not a statewide standard, which is the first reason a zoning answer from one county tells you nothing about the other.

The zoning ordinance, minimum lot sizes, and the comprehensive plan

Three documents shape the answer for any parcel. The zoning ordinance, Chapter 156 in Bonneville and Chapter 112 in Jefferson, is the binding one: it assigns every parcel a zone, lists what is allowed in that zone without a hearing, what needs a conditional use permit, and what is simply not allowed. Minimum lot sizes sit in the same document, and they matter twice for a second home: the parcel must be large enough to hold the dwellings it will carry under the density rules, and a lot that is already undersized may not qualify for a building permit even for the first house.

The comprehensive plan is the longer-range document that describes where the county wants growth to go, and the planning commissions and county commissioners use it as the backdrop when they weigh individual applications. A parcel's zone comes from the official zoning map, not from the county line on a real estate site, so the first step for any buyer is to confirm the zone in writing for the exact legal description, township, range, and section.

When a second dwelling or ADU is allowed, and when it needs a permit

In Jefferson County the rules are spelled out. Section 112-371 allows one accessory dwelling unit per lot or parcel where the zone district permits it, the ADU may be attached to the single-family dwelling, and in some situations an ADU can even be the first residence on a property if it meets the section's requirements. Beyond one ADU, the density table at Section 112-263 sets the ceiling at one dwelling per minimum lot size, and a footnote permits up to three single-family dwellings on a single parcel when the parcel is large enough, so a second home is possible, but the acreage has to support it.

Bonneville County takes a similar shape, but its published code is less turnkey about second dwellings on agricultural land, and I will not pretend otherwise. The ordinance provides conditional use permit procedures at Section 156.013, and the A-1 article includes standards controlling use hearings at Section 156.128, which tells you the county evaluates certain uses in agricultural zones case by case. Whether a particular parcel's second dwelling or ADU is a permitted accessory use, a conditional use, or something needing a variance depends on the zone and the parcel, and the honest answer is to confirm it with the county's Planning and Zoning office.

The general pattern, and where the two counties line up: a genuinely accessory dwelling tied to an existing residence on a properly sized lot is far more likely to be handled as a permitted use, while a freestanding second home, a dwelling on a small or nonconforming lot, or any use that stretches the zone's intent usually walks into the conditional use permit process. If a requested use cannot be made to fit even under a CUP, a variance is the harder route: variances are only granted for hardships tied to the land itself, never for the owner's convenience.

What applications and fees are typically involved

The application goes to the county's planning department, not to the state. In Bonneville County, submit through the Planning, Zoning & Building Department at 357 Constitution Way in Idaho Falls and the county's online permitting portal, with the Planning and Zoning Commission hearing the case. In Jefferson County, the Conditional Use Permit application is available from the Planning, Zoning & Building Department at 210 Courthouse Way in Rigby or from the county's permits page at jcgov.us.

Fees are set by the county commissioners in each county's fee resolution, not by the ordinance itself. Bonneville County's current fees come from the commissioners' fee resolution (2025 Resolution 25-19); Jefferson County's come from Resolution 2026-18, and its CUP application references the current fee schedule. Older Bonneville hearing notices have referenced a base filing fee plus the cost of certified letters and publication, and the current dollar figure for either county is a one-call question: Bonneville at (208) 524-7920, Jefferson at (208) 745-9220.

How the process works, step by step

The conditional use permit process follows the same arc in both counties, with the schedule set by each county's calendar. You submit a complete application with the site plan, septic and water information, and the fee; planning staff review it for compliance with the ordinance; and the case is scheduled for public notice and a hearing before the Planning and Zoning Commission, with the board of county commissioners handling appeals.

Public notice is the part buyers underestimate. Jefferson County requires newspaper notice at least 15 days before the hearing and on-site posting at least a week ahead, and its Planning and Zoning Commission meets monthly, typically the third Wednesday, so the calendar runs on roughly one-month cycles. Bonneville County sends notice by certified letter and publication, and its commission meets on Wednesday evenings. Neighbors and other interested parties get a chance to speak at the hearing, and the commission can approve, approve with conditions, or deny the application. In Jefferson County, an approved permit without a stated time limit expires after two years if the use is not established (Section 112-87), so an approval is not a blank check you can sit on forever.

Common conditions attached to approvals

Approvals rarely come with no strings. The conditions you will see most often in rural Eastern Idaho come back to the same four systems:

• Septic capacity. The property must handle another dwelling's wastewater. A second home usually means a second or expanded septic system, and the site has to pass soil and setback requirements reviewed by the Eastern Idaho Public Health District.
• Well yield or water right. A new dwelling needs a reliable water supply. If the plan calls for a new well, Idaho's domestic exemption covers up to 13,000 gallons per day and half an acre of irrigation, but areas with restricted groundwater can complicate a new well, so the water question gets verified early in the process.
• Road access. The county will want legal access to the new dwelling that meets its road standards, which can mean an easement, a shared drive, or an improvement to the approach before a building permit is issued.
• Deed restrictions and recorded conditions. Approvals are often recorded against the property, so the conditions run with the land: you might be limited to one ADU in perpetuity, or a CUP might be personal to the applicant. Read the recorded conditions before you buy, because they bind the next owner too.

Why you should verify everything before closing

The cheapest time to learn a parcel's limits is before you write the offer, and the most expensive is after you own it. A seller's "it's zoned for a second home" is not a zoning opinion, and a building that was put up without permits is a liability, not an asset. Ask for the zone in writing, check the county's records for existing permits and recorded conditional use permits, and have the county confirm whether a second dwelling or ADU is allowed on that exact parcel and what approvals it would take.

I walk buyers through this on every rural deal I work on, because the gap between "this looks like farm country" and "this parcel is legally approved for what you want to build" is where the surprises hide. If you are looking at acreage and want a second home, a shop with living quarters, or an ADU for family, bring the parcel's legal description to the conversation and we will get the zoning answers from the county before you commit. I am Melissa Holm with Hillside Realty, brokered by eXp Realty, and this is exactly the kind of thing I check on a buyer's behalf.