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Accessory Dwelling Unit (ADU) / Internal ADU (IADU) in Unincorporated Box Elder County, Utah

Required for detached ADUs (via Administrative Conditional Use Permit) and internal ADUs (permitted use) in unincorporated Box Elder County, governed by Chapter 5-6 of the Land Use Management & Development Code.

Verified 2026-07-03 · Source

When you need this permit

  • Property owner must occupy either the principal unit or the ADU/IADU as their permanent residence, with a recorded affidavit of owner occupancy
  • Minimum lot size of 20,000 sq ft in zones allowing single-family dwellings
  • ADU floor area must be less than 40% of the primary residence's square footage
  • Only one ADU or IADU permitted per lot/property
  • No separate utility meters or separate addresses for the ADU/IADU
  • Septic feasibility review and approval by the Bear River Health Department
  • Maximum height no taller in elevation than the roof of the primary structure
  • Minimum one off-street parking space
  • Occupants must be related by blood/marriage/adoption or up to two unrelated individuals living as a single housekeeping unit; no subleasing or rentals under 30 days
  • Multi-family homes, mobile homes, RVs, trailers, campers, and tents do not qualify as an ADU or IADU

Required documents

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Fee schedule

Filing fee
Not published — contact the department

Review timeline

Plan review
Not published — contact the department

How long did your Accessory Dwelling Unit (ADU) / Internal ADU (IADU) permit actually take in Unincorporated Box Elder County?

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Inspection process

  1. 1

    Conditional Use Permit Compliance Inspection

    Following issuance of the conditional use permit, the Building Official may approve a building permit application once construction plans meet the conditions/requirements established by the Planning Commission.

  2. 2

    Final / Certificate of Occupancy Inspection

    Building department representatives inspect the completed project to ensure all required improvements meet the conditional use permit conditions and Chapter 5-6 before a certificate of occupancy is issued.

See the full Unincorporated Box Elder County inspection guide — scheduling & re-inspection fees →

Tips

  • Only detached ADUs require an Administrative Conditional Use Permit (Sec 2-2-110) approved by the Zoning Administrator; Internal ADUs (IADUs) are allowed as a permitted use in any zone that allows single-family residences, per Chapter 5-6 of the Land Use Management & Development Code.
  • The ADU/IADU affidavit must be recorded against the property and re-executed by any future purchaser to continue the ADU's legal use — this runs with the title, not the occupant.
  • The minimum lot size of 20,000 sq ft (roughly half an acre) for ADUs/IADUs is significantly larger than many other Utah jurisdictions — confirm your parcel's zoning and size before applying.
  • No person may erect, alter, relocate, or construct an ADU or IADU commercially without both a valid Utah contractor's license and a valid Box Elder County business license. Contractor

Frequently asked questions

Unincorporated Box Elder County requires an accessory dwelling unit (adu) / internal adu (iadu) permit for: Property owner must occupy either the principal unit or the ADU/IADU as their permanent residence, with a recorded affidavit of owner occupancy; Minimum lot size of 20,000 sq ft in zones allowing single-family dwellings; ADU floor area must be less than 40% of the primary residence's square footage; Only one ADU or IADU permitted per lot/property; No separate utility meters or separate addresses for the ADU/IADU; Septic feasibility review and approval by the Bear River Health Department; Maximum height no taller in elevation than the roof of the primary structure; Minimum one off-street parking space; Occupants must be related by blood/marriage/adoption or up to two unrelated individuals living as a single housekeeping unit; no subleasing or rentals under 30 days; Multi-family homes, mobile homes, RVs, trailers, campers, and tents do not qualify as an ADU or IADU. If your project isn't listed, confirm with Box Elder County Building Permits / Inspection at (435) 734-2634 before starting work.

Unincorporated Box Elder County does not publish a plan-review timeline for this permit. Contact Box Elder County Building Permits / Inspection at (435) 734-2634 for current turnaround.

You'll need: Administrative Conditional Use Permit application (detached ADU) or Permitted Use application (IADU); Affidavit and Notice of Accessory Dwelling Unit (or Internal ADU); Building Permit Application. See the required-documents checklist on this page for details on each.

Unincorporated Box Elder County requires 2 inspection(s) for an accessory dwelling unit (adu) / internal adu (iadu) permit, in order: Conditional Use Permit Compliance Inspection, Final / Certificate of Occupancy Inspection. Schedule each through Box Elder County Building Permits / Inspection ((435) 734-2634).

Apply through Box Elder County Building Permits / Inspection at 1 S Main Street, Room 19, Brigham City, UT 84302. Phone: (435) 734-2634, email: cillum@boxeldercountyut.gov. Office hours: Monday through Friday, 8:00 AM to 5:00 PM (per Box Elder County Building Permits/Inspection page). Official information: https://www.boxeldercountyut.gov/195/Building-Permits-Inspection.

Unincorporated Box Elder County, Utah has adopted: 2024 International Building Code (IBC), including Appendices C and J — governs per Utah Code §15A-2-103(1)(a) (https://le.utah.gov/xcode/Title15A/Chapter2/15A-2-S103.html), in force statewide since 7/1/2026 and binding on counties under §15A-1-204. Box Elder County's own '2021 Adopted Code' document (DocumentCenter/View/173) still lists the 2021 IBC with Appendix J only (no Appendix C) — live-checked 2026-07-16, the county's own document has not been updated to the current statewide edition.; 2021 International Residential Code (IRC), including Appendix AQ — governs per Utah Code §15A-2-103(1)(b)/(c); this edition is UNCHANGED from the prior code cycle. Box Elder County's own document lists the 2021 IRC (without a distinct Appendix AQ citation) — live-checked 2026-07-16, consistent with the current statute on the base edition.; 2024 International Plumbing Code (IPC) — governs per Utah Code §15A-2-103(1)(d). Box Elder County's own document still lists the 2021 IPC — live-checked 2026-07-16, lags the current statewide edition.; 2024 International Mechanical Code (IMC) — governs per Utah Code §15A-2-103(1)(e). Box Elder County's own document still lists the 2021 IMC — live-checked 2026-07-16, lags the current statewide edition.; 2024 International Fuel Gas Code (IFGC) — governs per Utah Code §15A-2-103(1)(f). Box Elder County's own document still lists the 2021 IFGC — live-checked 2026-07-16, lags the current statewide edition.; 2023 National Electrical Code (NEC) — governs per Utah Code §15A-2-103(1)(g); this edition is UNCHANGED from the prior code cycle. Box Elder County's own '2021 Adopted Code' document lists the 2017 NEC — live-checked 2026-07-16, a genuine and long-standing discrepancy in the county's own published document (not a transcription error); confirm the currently enforced edition directly with the Chief Building Official, Codey Illum, at (435) 734-3303 before relying on this for a live electrical project.; 2024 International Energy Conservation Code (IECC) — one code, no residential/commercial split, per Utah Code §15A-2-103(1)(h). Box Elder County's own document lists the 2021 IECC (also undifferentiated by residential/commercial) — live-checked 2026-07-16, lags the current statewide edition.; 2024 International Existing Building Code (IEBC), subject to Utah Code §15A-2-104(2) (the HUD Code) — governs per Utah Code §15A-2-103(1)(i)/(j). Box Elder County's own document lists the 2021 IEBC — live-checked 2026-07-16, lags the current statewide edition.; Residential provisions of the 2024 International Swimming Pool and Spa Code (ISPSC) — governs statewide per Utah Code §15A-2-103(1)(n). Box Elder County's own '2021 Adopted Code' document does not list any ISPSC edition — live-checked 2026-07-16; added here per the statute, not omitted.; Modular Building Institute (MBI) Standards 1200 and 1205 — governs statewide per Utah Code §15A-2-103(1)(o). Not separately listed in Box Elder County's own '2021 Adopted Code' document — live-checked 2026-07-16.. Local amendments apply — see the Unincorporated Box Elder County overview page for the full list.

Yes, if your property is zoned for single-family residential use and the lot is at least 20,000 sq ft. You must obtain an Administrative Conditional Use Permit from the Zoning Administrator, meet the development standards in Chapter 5-6 of the Land Use Management & Development Code (owner occupancy, ADU floor area under 40% of the primary home, one off-street parking space, no separate utility meter/address, septic approval by Bear River Health Department), and record an owner-occupancy affidavit before applying for a building permit.

Sources & verification

Verified against official sources. Last reviewed 2026-07-03.

Fees, timelines, and adopted codes are researched from each jurisdiction's published records — see how we verify. Requirements change and vary by project, so always confirm the current details with the Unincorporated Box Elder County building department before you submit. PermitBase is an independent reference and is not affiliated with any government agency.

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Accessory Dwelling Unit (ADU) / Internal ADU (IADU) in other cities

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