ADU Rules in Boise, ID (2026)
ADU rules, permit requirements, size limits, and fees for Boise, ID. Source-cited from official municipal code.
Yes, ADUs are allowed in Boise, ID.
Boise, ID permits accessory dwelling units (ADUs) on residential properties. ADUs — also called granny flats, in-law units, backyard cottages, or secondary suites — are secondary housing units on a single-family or multifamily lot. State and local law governs the approval process. In Boise, the Boise Planning and Development Services — Building Division handles ADU permit applications. Below you will find the current size limits, setbacks, fees, and timeline for building an ADU in Boise.
ADU Size Limits
ADU Size Limits in Boise: Detached ADU: up to 1000 sq ft (max height: 25 feet). Attached ADU: No greater than 50% of primary dwelling floor area. Boise City Code Title 11 limits detached ADUs to 1,000 sqft or the floor area of the principal structure, whichever is less. Boise does not use the JADU designation from California law. Attached ADUs are limited to 50% of primary dwelling floor area.
| Type | Max Size | Max Height |
|---|---|---|
| Detached ADU | 1000 sq ft | 25 feet |
| Attached ADU | No greater than 50% of primary dwelling floor area | Per zone height limit |
Permit Fees
ADU Permit Fees in Boise: Plan check fee: $300-$1,500. Building permit fee: Based on construction valuation — typically $800-$3,500. Impact fees: Water/sewer impact fees may apply — Boise charges connection fees. School fees: None or N/A. Total estimated permit cost: $1,500-$6,000 total permit fees for typical ADU. Boise Public Works water/sewer impact fees for ADU connections. Fee schedule: https://www.cityofboise.org/departments/planning-and-development-services/fees/
| Fee Type | Amount |
|---|---|
| Plan Check | $300-$1,500 |
| Building Permit | Based on construction valuation — typically $800-$3,500 |
| Impact Fees | Water/sewer impact fees may apply — Boise charges connection fees |
| School Fees | None |
| Total Estimate | $1,500-$6,000 total permit fees for typical ADU |
Permit Timeline
Boise ADU Permit Timeline: Boise Planning and Development Services processes building permits. Standard ADU permit: 4-8 weeks plan review. Construction typically 3-9 months. Apply through the Boise Planning and Development Services — Building Division online permit portal. Phone: (208) 384-3830.
Setback Requirements
ADU Setback Requirements in Boise: Front setback: Same as zone standard. Side setback: 5 feet. Rear setback: 10 feet. ADUs must comply with Boise zone setback standards. Minimum 5 ft side and 10 ft rear setbacks for detached ADUs. Accessory structures in rear yard may have reduced setbacks of 3 ft from interior lot lines. Boise City Code Title 11.
Parking Requirements
Parking Requirements: Boise requires 1 parking space per ADU unit in most residential zones. No additional parking required for ADUs within one-quarter mile of transit.
Restrictions & Requirements
- ADU must comply with Boise City Code Title 11 setback and lot coverage standards
- ADU cannot be sold separately from principal dwelling
- Short-term rental of ADU requires Boise STR license
- ADU must have permanent foundation
- Exterior design of ADU should be compatible with primary structure
How Many ADUs Are Allowed?
Number of ADUs Allowed in Boise: Single-family lots: 1 ADU per residential lot in R-1A, R-1B, R-1C zones. Multifamily properties: ADUs permitted in R-2 and R-3 zones per Boise City Code. Boise allows 1 accessory dwelling unit per single-family residential lot. R-2 and R-3 zones allow additional dwelling units by right.
Single-Family Lots
1 ADU per residential lot in R-1A, R-1B, R-1C zones
Multifamily Properties
ADUs permitted in R-2 and R-3 zones per Boise City Code
Historic District Considerations
Historic District Requirements: Boise has 4 historic districts where additional design review may be required for ADU construction. Design review approval is required before obtaining a building permit for properties in historic overlay zones. Boise has National Register Historic Districts including Harrison Boulevard, Warm Springs Avenue, North End, and parts of Old Boise. Local historic district overlay may require Certificate of Appropriateness for exterior modifications. Boise City Historic Preservation Commission administers review. Look up historic district boundaries at: https://www.cityofboise.org/departments/planning-and-development-services/planning/historic-preservation/
Frequently Asked Questions
Can I build an ADU in Boise?
Yes. Boise, ID allows accessory dwelling units. On a single-family lot you can build 1 ADU per residential lot in R-1A, R-1B, R-1C zones.
How big can an ADU be in Boise?
The maximum size for a detached ADU in Boise is 1,000 square feet.
How long does it take to get an ADU permit in Boise?
The typical ADU permit timeline in Boise is Boise Planning and Development Services processes building permits. Standard ADU permit: 4-8 weeks plan review. Construction typically 3-9 months.. Total estimated permit fees are $1,500-$6,000 total permit fees for typical ADU.
Do I need extra parking for an ADU in Boise?
No, Boise does not require additional parking for ADUs.
Do I have to live on-site to have an ADU in Boise?
No, owner-occupancy is not required for ADUs in Boise.
Source: Boise City Code Title 11, Section 11-04-07 — Accessory Dwelling Units. Last verified April 5, 2026. View source