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Accessory Dwelling Unit (ADU) Permit in Unincorporated Cassia County, Idaho

Required before constructing or modifying an Accessory Dwelling Unit (ADU) in unincorporated Cassia County. Governed by County Code Title 9, Chapter 9 (9-9-4 W). ADUs must be 300–1,000 sq ft, limited to one per lot, placed on a permanent foundation, connected to the Principal Dwelling Unit's utilities, and require a recorded Declaration of Restrictions filed with the County Recorder before the permit is issued.

Verified 2026-07-08 · Source

When you need this permit

  • Property must have an existing or proposed Principal Dwelling Unit (PDU); no ADU permitted without a PDU on the lot
  • Lot must meet minimum lot size of one (1) acre
  • ADU must be 300–1,000 sq ft (excluding garage and non-living areas), and only one ADU is permitted per residentially zoned lot
  • ADU must be on a permanent foundation and declared real property under Cassia County Code
  • ADU must connect to the Principal Dwelling Unit's utilities (water, waste) — no separate well or independent utility service permitted; septic/waste handling for the ADU must be pre-approved by South Central Public Health District
  • Recorded Declaration of Restrictions must be filed with the County Recorder before the ADU permit is issued, confirming the ADU will not be sold separately from the PDU and is capped at 1,000 sq ft
  • Site plan/plot map, septic permit copy, drive approach permit copy, irrigation/fire district letters, and utility services acknowledgement required
  • Occupancy permit must be issued by the Building Department prior to occupancy of the ADU
  • Agricultural Waiver required for AR-zoned parcels per County Code 9-7-2(C)

Required documents

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Fee schedule

Building permit fee (ADU, new construction)Same fee structure as the standard residential building permit; Cassia County Administrative Fees schedule
Calculated based on square footage using the IBC 2003 Valuation Table 1
Placement Fee — Single Section (if ADU is a manufactured home)Without permanent foundation; Cassia County Administrative Fees schedule / ADU Permit Application
$150.00
Placement Fee — Multi-Section (if ADU is a manufactured home)Without permanent foundation; Cassia County Administrative Fees schedule / ADU Permit Application
$200.00

Review timeline

Plan review
Not published — contact the department

How long did your Accessory Dwelling Unit (ADU) Permit permit actually take in Unincorporated Cassia County?

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Inspection process

  1. 1

    Site/Setbacks

    Verifies compliance with road and interior lot line setbacks

  2. 2

    Footings / Foundation

    Required since ADUs must be on a permanent foundation

  3. 3

    Rough Framing

    After State Electrical/Plumbing/HVAC rough inspections (DOPL) are complete

  4. 4

    Final / Occupancy

    Occupancy permit must be issued by the Building Department prior to occupancy of the ADU per County Code 9-9-4 W(3)(d)

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

Tips

  • A recorded Declaration of Restrictions must be filed with the County Recorder BEFORE the ADU permit is issued — this is a distinct extra step not required for a standard residential permit.
  • Only one ADU is allowed per lot, and only if a Principal Dwelling Unit already exists (or is being built) on that same lot.
  • The ADU cannot have its own separate well or independent utility connection — it must tie into the Principal Dwelling Unit's utilities.
  • Electrical, plumbing, and HVAC permits for the ADU are issued by Idaho DOPL (call 1-208-332-4700), not Cassia County.

Frequently asked questions

Unincorporated Cassia County requires an accessory dwelling unit (adu) permit for: Property must have an existing or proposed Principal Dwelling Unit (PDU); no ADU permitted without a PDU on the lot; Lot must meet minimum lot size of one (1) acre; ADU must be 300–1,000 sq ft (excluding garage and non-living areas), and only one ADU is permitted per residentially zoned lot; ADU must be on a permanent foundation and declared real property under Cassia County Code; ADU must connect to the Principal Dwelling Unit's utilities (water, waste) — no separate well or independent utility service permitted; septic/waste handling for the ADU must be pre-approved by South Central Public Health District; Recorded Declaration of Restrictions must be filed with the County Recorder before the ADU permit is issued, confirming the ADU will not be sold separately from the PDU and is capped at 1,000 sq ft; Site plan/plot map, septic permit copy, drive approach permit copy, irrigation/fire district letters, and utility services acknowledgement required; Occupancy permit must be issued by the Building Department prior to occupancy of the ADU; Agricultural Waiver required for AR-zoned parcels per County Code 9-7-2(C). If your project isn't listed, confirm with Cassia County Zoning and Building Department at 208-878-7302 before starting work.

In Unincorporated Cassia County, the published Building permit fee (ADU, new construction) is: Calculated based on square footage using the IBC 2003 Valuation Table 1. Additional published fees: Placement Fee — Single Section (if ADU is a manufactured home) — $150.00; Placement Fee — Multi-Section (if ADU is a manufactured home) — $200.00. These are the jurisdiction's own published figures — see the fee schedule table and source citations on this page.

Unincorporated Cassia County does not publish a plan-review timeline for this permit. Contact Cassia County Zoning and Building Department at 208-878-7302 for current turnaround.

You'll need: Cassia County Accessory Dwelling Unit (ADU) Permit Application Packet (2-4-25); Recorded Declaration of Restrictions; Septic Permit (copy); Plot Map / Site Plan. See the required-documents checklist on this page for details on each.

Unincorporated Cassia County requires 4 inspection(s) for an accessory dwelling unit (adu) permit, in order: Site/Setbacks, Footings / Foundation, Rough Framing, Final / Occupancy. Schedule each through Cassia County Zoning and Building Department (208-878-7302).

Apply through Cassia County Zoning and Building Department at Cassia County Courthouse, 1459 Overland Ave., Room 210, Burley, ID 83318. Phone: 208-878-7302, email: bldgdept@cassia.gov. Office hours: Mon–Fri 8:30 AM – 5:00 PM. Official information: https://www.cassia.gov/permit-information.

No. The recorded Declaration of Restrictions required before permit issuance specifically prohibits selling the ADU separately from the Principal Dwelling Unit — they must remain under common ownership. Source: Cassia County Code 9-9-4 W(8)(a), ADU Permit Application Packet (2-4-25).

Between 300 and 1,000 square feet, excluding garage area and non-living spaces like workshops or greenhouses. Only one ADU is permitted per residentially zoned lot, and the lot must be at least one acre. Source: Cassia County Code 9-9-4 W(4)(a)(c).

Sources & verification

Verified against official sources. Last reviewed 2026-07-08.

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 Cassia 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) Permit in other cities

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