ADU Permits & Regulations in Boise
A comprehensive guide to zoning requirements, setback rules, building permits, impact fees, utility connections, and building codes for accessory dwelling unit construction in Boise, Idaho. Understand every regulatory requirement before your ADU project begins.
Quick answer
Yes. All accessory dwelling units in Boise require a building permit regardless of size or type (attached, detached, or garage conversion). You will need to submit architectural plans, a site plan showing property lines, existing structures and setback dimensions, and obtain separate permits for electrical, plumbing, and mechanical work.
Boise's ADU standards live in the Modern Zoning Code at Section 11-03-03.2.B, which establishes where accessory dwelling units can be built, how large they can be, and the conditions property owners must meet. Understanding these zoning requirements is the first step in any ADU project.
Allowed Zones
ADUs are permitted in the Boise residential zoning districts below. Minimum lot area and maximum density standards do not apply to an ADU in any of them (Section 11-03-03.2.B(1)), so a small lot is not by itself a disqualifier — what varies by zone is the setback envelope left for the unit.
R-1A (Residential, Single-Family)
Largest-lot single-family zone, with the deepest setbacks: 10 ft interior side and 20 ft rear, which is what limits the ADU footprint here.
R-1B (Residential, Single-Family)
Medium-lot single-family zone covering much of established Boise, including parts of the North End and East End. Setbacks match R-1A at 10 ft side and 20 ft rear.
R-1C (Residential, Single-Family)
Smaller-lot single-family zone with the tightest setbacks — 5 ft interior side and 15 ft rear — which is what makes an ADU fit on a narrow lot that would otherwise look too small.
R-2 and R-3 (Residential, Medium Density)
Medium-density zones allowing duplexes and attached housing. R-2 matches R-1C's 5 ft side setback, and in R-2 and R-3 the rear setback drops to 5 ft where the rear property line abuts an alley.
MX districts, A-1 and A-2
Mixed-use districts and the open lands / agricultural zones also permit ADUs. A-1 and A-2 are less common for urban ADU projects but apply to larger parcels in the Boise foothills, where a private well or septic system changes the utility plan.
Key Restrictions
Even if your property is in an eligible zone, the following restrictions and conditions must be satisfied before an ADU can be approved. State rules come from Idaho's 2026 ADU law, Idaho Code § 67-6541, which Boise must adopt by February 1, 2027.
One ADU per lot (for now)
Boise's current code permits one accessory dwelling unit per residential lot. Idaho's 2026 ADU law (Senate Bill 1354) requires the city to allow either one internal or one detached ADU per lot by February 1, 2027, so confirm the rule in effect before planning a second unit.
No owner-occupancy requirement
Boise's Modern Zoning Code does not require the owner to live on the property (except in the Harris Ranch Specific Plan, which still requires it until the city conforms), and Idaho's 2026 ADU law bars cities over 10,000 residents from imposing an owner-occupancy requirement (Idaho Code § 67-6541). You may rent both units.
ADU size limit: 70% or 900 sq ft
An ADU cannot exceed 70% of the primary structure's total floor area or 900 square feet, whichever is smaller, and may have no more than two bedrooms (Zoning Code Section 11-03-03.2.B). For a 1,000 sq ft home, the maximum ADU would be 700 sq ft; a 1,300 sq ft or larger home reaches the 900 sq ft cap. Idaho's 2026 ADU law (Senate Bill 1354) requires Boise to raise the cap to at least 1,000 sq ft or 75% of the primary home and to allow either one internal or one detached ADU per lot by February 1, 2027 — confirm the limit in effect with Planning & Development Services.
Same limit for attached, detached, and conversions
Boise applies the 900 sq ft / 70% limit to every ADU type: interior conversions, additions to the house, detached units, and garage conversions (which may not reduce off-street parking below the lot's minimum).
No added parking requirement
Boise does not require an added off-street parking space for the ADU, and Idaho's 2026 ADU law (Senate Bill 1354) limits when cities can require one. A garage conversion may not reduce the lot's required parking for the primary dwelling, so plan replacement parking for that case.
Primary residence must exist
An ADU can only be built on a lot with an existing primary residence. You cannot build an ADU before or instead of a primary dwelling.
How to check your zoning: Visit the City of Boise's online zoning map at cityofboise.org or contact Planning and Development Services at (208) 608-7100. Provide your property address and they will confirm your zoning designation, lot dimensions, and any overlays or special conditions that may affect ADU eligibility. Iron Crest Remodel verifies all zoning requirements during our initial site assessment.
Setbacks determine how close your ADU can be built to property lines. These requirements vary by zoning district and directly affect where an ADU can be placed on your lot, how large it can be, and whether a detached structure is feasible. Setback compliance is one of the most common barriers to ADU approval. Boise sets no ADU-specific setback: an ADU must meet the dimensional standards of the underlying zoning district (Section 11-03-03.2.B(6)), with the accessory-structure exceptions in Section 11-04-03.3. Figures below are from the Boise Modern Zoning Code (Tables 11-02.3, 11-02.5, 11-02.7, 11-02.9, and 11-04.4). Confirm your parcel's zone before designing.
| Setback Type | Requirement | Notes |
|---|---|---|
| Front Setback | 15 ft (R-1A/R-1B/R-1C); 10 ft (R-2) | Set by the underlying zone, not by any ADU rule. A front-entry garage requires 20 ft in every residential district. Detached ADUs are generally placed in the rear yard, so the front setback usually applies only to attached ADUs. |
| Interior Side Setback | 10 ft (R-1A/R-1B); 5 ft (R-1C/R-2) | R-1C and R-2 require 10 ft for the third story of a building abutting a single-story building. Eaves, cornices, and roof overhangs may project up to 2 ft into any setback (Table 11-04.4). |
| Rear Setback | 20 ft (R-1A/R-1B); 15 ft (R-1C/R-2) | In R-2 and R-3 the rear setback drops to 5 ft where the rear property line abuts an alley; R-1C has no such reduction. The full alley setback exemption applies to alley-loaded parking structures only. |
| Reduced Accessory-Structure Setbacks | 3 ft side / 9 ft rear | Available in R-1A, R-1B, R-1C, R-2, and R-3 for a detached accessory structure of 121–500 sq ft and under 14 ft in height (Table 11-04.4, Section 11-04-03.3.B). Cannot conflict with an existing easement. |
| Height Limit | 2 stories max for the ADU | Section 11-03-03.2.B(3) caps an ADU at two stories; the zone's own maximum still applies (35 ft in R-1A/R-1B, 3 stories/40 ft in R-1C). There is no 25 ft ADU cap. |
| Separation from Primary | No zoning minimum | The zoning code sets no fixed separation between a detached ADU and the primary dwelling. Separation is governed by the adopted building code's fire-separation-distance provisions — confirm the requirement for your layout with the plans reviewer. |
| Minimum Lot Area | Does not apply | Minimum lot area and maximum density standards do not apply to ADUs (Section 11-03-03.2.B(1)). Boise's residential districts also set no maximum lot coverage percentage, so the setback envelope is what limits the footprint. |
How to Read Your Property's Setbacks
Your property's specific setback requirements are determined by your zoning district, lot dimensions, and any special overlays. Here is how to find them:
- Pull your property's plat map from Ada County Assessor records to confirm lot dimensions and property lines
- Identify your zoning district on the City of Boise zoning map (cityofboise.org)
- Cross-reference the setback table in Boise City Code Title 11 for your specific zone
- Check for any overlay districts (historic, floodplain, hillside) that may impose additional restrictions
- A professional land survey is recommended if your property lines are uncertain or if construction will be near the minimum setback
Common Setback Challenges
These are the setback-related issues we encounter most frequently on Boise ADU projects. Early identification prevents costly redesigns during plan review.
- Existing garage or shed already consumes buildable rear yard area
- Existing structures, driveway and easements leaving too little area inside the setback lines
- Utility easements along the rear or side property line reducing buildable area
- Irregular lot shapes making it difficult to meet setbacks on all sides simultaneously
- Corner lots with two street-side setbacks reducing the available rear yard
The ADU permit process in Boise involves multiple city departments and several stages of review. Plan review for new ADU construction is more involved than a typical remodel permit because it covers zoning compliance, structural design, and all building systems. Here is the step-by-step process from pre-application through certificate of occupancy.
Pre-Application Meeting (Recommended)
While not mandatory, a pre-application meeting with the City of Boise Planning and Development Services department is strongly recommended for ADU projects. This meeting allows you to present your concept and receive feedback on zoning compliance, setback issues, and any site-specific concerns before investing in full architectural plans.
What to bring: A site plan showing your lot, existing structures, proposed ADU location, and approximate dimensions. The planning staff will review zoning eligibility, the setback envelope, and identify any potential issues early. Pre-application meetings are scheduled through Planning and Development Services at (208) 608-7100; ask for the current lead time when you call.
Plan Submission
Submit complete construction documents to the City of Boise through the ePermit online portal. ADU plan submissions require significantly more documentation than a standard remodel permit. Incomplete submissions are the number one cause of delays.
Required documentation: Completed permit application, site plan (showing property lines, setbacks, existing structures, proposed ADU footprint, utility connections, and parking), architectural plans (floor plans, elevations, cross-sections), structural engineering drawings, energy code compliance documentation (REScheck or the performance path), and mechanical/plumbing/electrical plans. The contractor's Idaho registration number and proof of insurance are also required. Boise does not require an owner-occupancy covenant for an ADU outside the Harris Ranch Specific Plan — confirm the current submittal checklist with Planning and Development Services, since it is updated independently of this page.
Plan Review (3–6 Weeks on Our Projects)
The City of Boise conducts a multi-department plan review covering zoning compliance, structural adequacy, building code compliance, fire code, mechanical systems, plumbing, and electrical design. Each discipline reviews independently and may generate comments or correction requests.
Review timeline: The City publishes processing-time targets rather than a guaranteed turnaround. On our own Boise ADU projects, first-round review has typically run 3 to 6 weeks, with a garage conversion clearing faster than a new detached unit carrying structural engineering and a full site plan. Every round of corrections puts the drawings back in the queue, so the single biggest schedule lever is a complete first submission. Confirm the current turnaround with Planning and Development Services at (208) 608-7100 before you commit to dates.
Permit Issuance
Once all plan review comments are resolved and fees are paid, the building permit is issued. The permit card must be posted at the construction site and visible from the street. Under the adopted building code a permit becomes invalid if work does not begin within 180 days of issuance, or if work is suspended for 180 days (IRC R105.5), so an expired permit has to be renewed. For ADU projects, which often have long lead times for materials and scheduling, it is important to coordinate permit timing with your construction start date.
Construction & Inspections
Construction proceeds through a defined sequence of inspections. Each stage must pass before the next phase of work begins. Inspections are requested through the city's online portal with a minimum 24-hour notice.
Inspection sequence: Foundation/footing, framing/structural, rough electrical, rough plumbing, rough mechanical, insulation/energy code, drywall (for fire-rated assemblies), and final inspections for each trade. Each stage in that sequence is a separate inspection request, and the trade finals add more, so an ADU accumulates inspections throughout the build rather than at the end.
Certificate of Occupancy
After all final inspections pass, the City of Boise issues a Certificate of Occupancy (CO). This document confirms that the ADU was built in compliance with the approved plans and all applicable codes. The ADU cannot be legally occupied until the CO is issued. The CO is also required by lenders, insurance providers, and for property tax assessment purposes. Iron Crest Remodel ensures all inspections pass and the CO is obtained before project closeout.
Fee estimate: The building permit and plan review can be calculated exactly from the City of Boise fee schedule once the project valuation is set — about $1,220 on a $120,000 garage conversion, about $1,560 on a $175,000 one-bedroom. The agency fees that sit alongside it (ACHD, sewer, water) are set by each agency and have to be confirmed with that agency. See the breakdown in the next section.
Beyond the building permit fee, ADU construction in Boise triggers charges from several separate agencies. Only the City of Boise building permit and plan review can be calculated in advance from a published schedule; the rest are set by whichever agency levies them and change when that agency updates its schedule. We publish the arithmetic we can show and tell you who to call for the rest — a made-up total is worse than no total.
| Fee Type | Agency | How It Is Set | Notes |
|---|---|---|---|
| Building Permit | City of Boise | $913.09 + $5.17 per $1,000 over $100,000 of valuation | City of Boise building code fee schedule, Table 1-A, effective 10-1-21. Lower valuation tiers: $362.80 + $9.30 per $1,000 from $25,001–$50,000; $595.30 + $6.35 per $1,000 from $50,001–$100,000. |
| Plan Review Fee | City of Boise | +20% of the permit fee | Boise adds 20% of the permit fee for residential plan review. Other Treasure Valley cities set their own rate — Nampa charges 65% — so a percentage quoted for another city does not apply here. |
| ACHD Impact Fee | Ada County Highway District | ACHD's adopted impact fee schedule | Transportation impact fee for a new dwelling unit. ACHD sets and periodically updates the rate — confirm the current residential amount with ACHD for your address. |
| Sewer Connection | City of Boise Public Works | City of Boise sewer fee schedule | Charged for a new connection; sharing the existing lateral, where the City approves it, avoids the cost of a new one. |
| Water Connection | Veolia Water Idaho | Veolia's tariffed connection charges | Boise's drinking water is supplied by Veolia Water Idaho, not the City. The charge depends on whether a new meter and service line are required or a shared connection is approved. |
| Electrical Permit | City of Boise | Trade permit schedule | Covers all electrical work; issued and charged separately from the building permit. |
| Plumbing Permit | City of Boise | Trade permit schedule | Covers kitchen, bathroom, and laundry plumbing installations. |
| Mechanical Permit | City of Boise | Trade permit schedule | HVAC system, exhaust fans, and gas appliance installations. |
$120,000 valuation (garage conversion)
~$1,220
$913.09 + $5.17 × 20 = $1,016.49 permit fee, plus 20% residential plan review. Building permit and plan review only.
$175,000 valuation (600 sq ft, 1-bed)
~$1,560
$913.09 + $5.17 × 75 = $1,300.84 permit fee, plus 20% residential plan review. Building permit and plan review only.
$260,000 valuation (900 sq ft, 2-bed)
~$2,090
$913.09 + $5.17 × 160 = $1,740.29 permit fee, plus 20% residential plan review. Building permit and plan review only.
Note: The three figures above are the City of Boise building permit plus residential plan review, calculated from the fee schedule in effect 10-1-21 at the stated valuation. They do not include ACHD impact fees, sewer or water connection charges, or the separate trade permits — those are set by other agencies, and we will not print a number we cannot source. Iron Crest Remodel confirms each current amount with the issuing agency during the design phase and includes all permit fees in your construction contract.
Every ADU needs water, sewer and electrical service, plus gas unless the unit is all-electric. Whether you can share connections with the primary dwelling or must install separate service depends on the city's requirements, utility capacity, and the specifics of your project. Utility connections are often the most complex and costly portion of the permitting and site work for ADU construction.
Water Service
Boise's drinking water is supplied by Veolia Water Idaho, the regulated utility serving Boise and much of Ada County. An ADU may connect to the existing water service (shared meter) or require a separate meter and service line. The determination depends on the distance from the water main, the existing service size, and the ADU's demand.
- Shared connection: tap into the existing 3/4" or 1" service line — the lower-cost route, limited to the plumbing run itself.
- Separate meter: new service line from the water main, new meter box, and Veolia's connection charge for that meter size.
- Contact Veolia Water Idaho to confirm the requirements and current charges for your address.
- Hot water heater: ADU requires its own water heating system (tankless or tank).
Sewer Service
Sewer connections for ADUs in Boise are managed by the City of Boise Public Works Department. The existing sewer lateral from the primary home may be shared or a new lateral may be required.
- Shared lateral: connect ADU waste lines to the existing sewer lateral — the lower-cost route, limited to the new pipe run on your own property.
- Separate lateral: new connection to the sewer main in the street, $5,000-$15,000 including the street cut and repair, plus the City's connection fee.
- Sewer capacity verification may be required by the city, especially in older neighborhoods with aging infrastructure.
- Properties on septic systems require Idaho Department of Environmental Quality approval for ADU connections.
Electrical Service
Electrical service is one of the most important utility decisions for your ADU. The choice between a sub-panel fed from the primary home and a separate meter affects both cost and long-term operational independence.
- Sub-panel from the primary service: a sub-panel sized by the NEC 2023 load calculation for the unit, fed from the existing main panel ($1,500-$3,000), with the ADU on your bill.
- Separate meter: Idaho Power installs a new meter and service drop, $2,000-$6,000, but the tenant is then billed directly.
- Primary home panel must have adequate capacity. If the existing panel is 100 amps, an upgrade to 200 amps may be needed ($2,000-$4,000).
- Underground conduit run from primary panel or utility pole to ADU location. Trench depth: 18" minimum per NEC.
Gas, Internet & Cable
Additional utility services round out the ADU's functionality. Gas service is especially important for heating in Boise's cold winters.
- Gas: Intermountain Gas provides service in Boise. A new gas meter and service line runs $1,500-$4,000. A gas permit and pressure test are required.
- Internet: Contact Sparklight, CenturyLink/Lumen, or another Boise provider about extending service to the ADU. We run conduit to the unit during construction so a line can be pulled later without trenching the yard twice.
- Alternative: A mesh Wi-Fi system extending from the primary home can serve many ADUs without a separate internet connection.
- All-electric option: Heat pump HVAC and an electric water heater eliminate the gas service, the gas meter, and the combustion-air and venting details that come with it.
Utility coordination tip: Contact all utility providers early in the design process. Water, sewer, gas, and electrical providers each have their own application processes and timelines. Some require separate permits and inspections. Iron Crest Remodel coordinates with all utility providers as part of our ADU construction service, handling applications, scheduling, and inspections on your behalf.
ADUs must comply with the International Residential Code (IRC) as adopted by Idaho for fire and life safety. These requirements protect occupants and are closely inspected. Non-compliance is a failed inspection, which halts construction until corrections are made.
Smoke & CO Detection
Smoke alarms and carbon monoxide detectors are required in all ADUs per Idaho code and the IRC.
- Smoke alarms in every bedroom, outside each sleeping area, and on every level
- CO detectors outside each sleeping area and on every level with fuel-burning appliances
- All alarms must be hardwired with battery backup and interconnected
- Combination smoke/CO units are acceptable and simplify installation
Fire Separation
Attached ADUs require specific fire-rated separation between the ADU and the primary dwelling to prevent fire spread.
- 1-hour fire-rated wall assembly between attached ADU and primary dwelling
- 1-hour fire-rated floor/ceiling assembly if ADU is above or below primary living space
- Self-closing, 20-minute fire-rated door at connecting doorways
- Detached ADUs within 5 ft of property line may need fire-rated exterior wall on the line side
Egress & Sprinklers
Emergency egress and fire suppression requirements ensure safe evacuation and fire control for ADU occupants.
- Every bedroom must have an egress window: min 5.7 sq ft opening, max 44" sill height
- Emergency escape and rescue opening required per IRC R310
- Whether a residential sprinkler system applies to your unit is a question for the plans examiner and the Boise Fire Department — confirm it in review rather than assuming either way
- Address numbers must be visible from the street for emergency response (minimum 4" height, contrasting color)
Attached vs. detached fire code differences: Detached ADUs have simpler fire code requirements because the physical separation between structures provides inherent fire protection. Attached ADUs require more extensive fire-rated assemblies and separation details. The choice between attached and detached construction significantly affects both fire code compliance costs and construction complexity. Iron Crest Remodel evaluates both options during the design phase and recommends the most practical approach for your specific lot and budget.
New ADU construction in Idaho must comply with the International Energy Conservation Code (IECC) 2018 edition as adopted by the state, for Climate Zone 5 (Boise). A ground-up detached unit meets those requirements in full; a garage conversion has to bring the newly conditioned space up to code, which in practice means insulating an envelope that was never built to hold heat. These requirements affect insulation, windows, air sealing, HVAC efficiency, and lighting.
Insulation R-Value Minimums
Boise falls in Climate Zone 5 under the IECC, which requires the following minimum insulation values for new residential construction. ADUs must meet these standards for the building envelope.
Exterior Walls
R-20 or R-13 + R-5ciR-20 cavity insulation OR R-13 cavity plus R-5 continuous insulation on the exterior. The continuous insulation option reduces thermal bridging through studs.
Attic / Ceiling
R-49Blown-in or batt insulation in the attic space. Cathedral ceilings must meet R-49 or use a combination of cavity and rigid insulation.
Floors Over Unconditioned Space
R-30Applies to floors over crawl spaces, garages, or open areas. Proper installation and air sealing at the floor plane are critical.
Basement / Crawl Space Walls
R-15 continuous or R-19 cavityTable R402.1.2 lists Climate Zone 5 basement and crawl space walls as 15/19 — R-15 of continuous insulation, or R-19 in the cavity. Continuous insulation also helps with moisture management against a Boise foundation wall.
Slab-on-Grade Edge
R-10 for 2 ft depthApplies to slab-on-grade floors generally, not only heated slabs; a heated slab carries additional perimeter insulation under the table's own footnote. Perimeter insulation reduces heat loss through the slab edge.
Air Sealing & Performance Testing
Air sealing is one of the most impactful energy efficiency measures in new construction. The IECC 2018 requires blower door testing to verify that the building envelope meets the maximum air leakage rate.
Blower Door Test: 3 ACH50 Maximum
The completed ADU must test at or below 3 air changes per hour at 50 pascals of pressure (3 ACH50) — the Climate Zone 3–8 limit in IECC 2018 R402.4.1.2. This is verified by a blower door test conducted before the final inspection. If the test fails, additional air sealing work is required before the building passes.
Duct Leakage Testing
If the ADU uses a ducted HVAC system, duct leakage testing is required. The IECC 2018 limits duct leakage to 4 CFM25 per 100 square feet of conditioned floor area. Properly sealed duct connections and mastic at all joints are essential for passing this test. Ductless mini-split systems eliminate this requirement entirely.
HERS Rating Option
As an alternative to the prescriptive R-value path, you can use a HERS (Home Energy Rating System) rating to demonstrate energy code compliance. A HERS rater models the building's energy performance and verifies it meets the energy budget. This path offers more design flexibility and can allow trade-offs between components.
Idaho Power Efficiency Incentives
Idaho Power's Heating and Cooling Efficiency Program covers measures such as ductless and air-source heat pumps, heat pump water heaters, duct sealing, and smart thermostats — the equipment most ADUs use. Idaho Power sets the amounts and eligibility, so check idahopower.com for current programs before you order equipment. There is no Idaho Power rebate for windows, insulation, or appliances.
Not every lot or every ADU design sails through the permit process without challenges. These are the most common issues we encounter on Boise ADU projects and the strategies Iron Crest Remodel uses to resolve them.
Setback Encroachments
The proposed ADU footprint extends into a required setback zone. This is the single most common issue, especially on smaller lots or lots with existing accessory structures.
Solution: Redesign the ADU footprint to fit within setbacks. Options include reducing the footprint, adjusting the placement on the lot, removing an existing non-conforming structure to create space, or in rare cases applying for a variance through the City of Boise Board of Adjustment. Variances require demonstrating a hardship unique to the property and are not guaranteed. Iron Crest Remodel conducts a detailed site analysis before design begins to identify setback constraints early.
No Buildable Area Left
Boise's residential districts set no maximum lot coverage percentage, so the constraint is not a coverage ratio — it is the area left inside the setback lines once the existing house, garage, shed, driveway and easements are drawn on the site plan. On a small R-1C lot with a large house and a detached garage, that remaining area can be too small for a unit worth building.
Solution: Reduce the ADU footprint, use the reduced 3 ft side / 9 ft rear accessory-structure setbacks if a unit of 121–500 sq ft under 14 ft in height works for you, remove an existing shed or non-conforming structure to open the envelope, or convert an existing structure rather than building new. A garage conversion uses a footprint that already exists, which is often the most practical answer on a tight lot.
Neighbor Objections
While ADUs in Boise do not require a public hearing or neighbor notification for standard approvals, neighbors may raise concerns about privacy, parking, property values, or construction impacts.
Solution: Proactive communication. Inform neighbors before construction begins. Address privacy concerns through thoughtful window placement and landscaping screening. Minimize construction impacts with defined work hours and clean job sites. Iron Crest Remodel can assist with neighbor communication as part of the project planning process.
Utility Capacity Limitations
The existing water, sewer, or electrical service to the property is insufficient to support an additional dwelling unit. This is more common in older Boise neighborhoods with aging infrastructure.
Solution: Upgrade utility service as needed. Common upgrades include a 200-amp electrical panel upgrade, a new water service line, or a sewer lateral replacement. Each is a real line item in the budget — see the utility section above for the ranges we quote — but the upgrade also improves the primary home's service. Iron Crest Remodel coordinates with all utility providers early in the design phase to identify capacity issues before construction begins.
Historic District Restrictions
Properties in Boise's historic districts (Harrison Boulevard, North End Historic District, and others) face additional design review requirements. The Boise Historic Preservation Commission reviews exterior design, materials, and massing for compatibility with the historic neighborhood character.
Solution: Work with the Historic Preservation Commission early. Design the ADU's exterior to complement the primary home and neighborhood character (matching materials, roof pitch, window proportions). Submit design documents to the commission before or concurrently with the building permit application. The commission meets on a published schedule, so build its next available hearing date into your timeline rather than assuming a fixed number of weeks. Iron Crest Remodel has experience working within Boise's historic districts and understands the commission's design guidelines.
Common questions about ADU permits, zoning, and building regulations in Boise, Idaho.
Do I need a permit to build an ADU in Boise?
Yes. All accessory dwelling units in Boise require a building permit regardless of size or type (attached, detached, or garage conversion). You will need to submit architectural plans, a site plan showing property lines, existing structures and setback dimensions, and obtain separate permits for electrical, plumbing, and mechanical work. The City of Boise Planning and Development Services department reviews all ADU applications for compliance with zoning and building codes before issuing permits.
How long does it take to get an ADU permit in Boise?
The City of Boise publishes processing-time targets rather than a guaranteed turnaround, so no one can promise you a date. Across our own Boise ADU projects, plan review has typically run 3 to 6 weeks, varying with project complexity and the department's current workload — a garage conversion with minimal structural change reviews faster than a new detached unit with structural engineering, a site plan, and utility coordination. An incomplete or non-compliant submission restarts part of that clock, which is why we submit complete packages rather than placeholder sheets. Confirm the current turnaround with Planning and Development Services at (208) 608-7100 before you build a schedule around it.
How much do ADU permits and fees cost in Boise?
The building permit itself is valuation-based under the City of Boise fee schedule (Table 1-A, effective 10-1-21): above a $100,000 valuation it is $913.09 plus $5.17 per $1,000 over $100,000, and residential plan review adds 20 percent. That works out to roughly $1,220 on a $120,000 garage conversion and about $1,560 on a $175,000 one-bedroom. Trade permits for plumbing, electrical, and mechanical work are issued and charged separately. The larger numbers homeowners hear quoted are impact fees and utility connection charges, which are separate agency fees — ACHD for transportation impact, the City of Boise for sewer, and Veolia Water Idaho for water service. Those are set by each agency's own fee schedule and change, so confirm the current amounts with each agency rather than budgeting from a figure on a contractor's website.
Can I build an ADU on any residential lot in Boise?
Not every lot. Boise allows an ADU on most lots with an existing single-family home. Minimum lot-area standards do not apply to ADUs (Zoning Code 11-03-03.2.B(1)), but the unit must fit the zone's setbacks and dimensional standards, and the primary residence must already exist. The current code permits one ADU per lot; Idaho's 2026 ADU law (Senate Bill 1354) requires Boise to allow either one internal or one detached ADU per lot by February 1, 2027. You can verify your property's zoning designation through the City of Boise's online zoning map or by contacting Planning and Development Services at (208) 608-7100.
Does the property owner have to live on-site with an ADU?
No. Boise's Modern Zoning Code (effective December 2023) has no owner-occupancy requirement outside the Harris Ranch Specific Plan, and Idaho's 2026 ADU law (Senate Bill 1354, Idaho Code § 67-6541) bars cities over 10,000 residents from requiring one. You may live in the main house, live in the ADU, or rent both units. Short-term rental use is regulated separately, and lenders or insurers may have their own occupancy conditions, so check those before planning around rental income.
Does Iron Crest Remodel handle the full ADU permit process?
Yes. Iron Crest Remodel manages the entire ADU permit process from start to finish. This includes the pre-application meeting with the City of Boise, preparing all required architectural and site plans, submitting permit applications, paying fees (included in your project cost), coordinating with ACHD and utility providers, scheduling all required inspections during construction, and obtaining the final certificate of occupancy. You do not need to visit the permit office or manage any part of the regulatory process yourself.
Explore our complete library of ADU construction guides for Boise homeowners. Each guide covers a different aspect of planning, building, and financing your accessory dwelling unit.
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Our full ADU construction service overview for Boise homeowners
ADU Cost Guide
Detailed ADU pricing from $95,000 to $320,000 by type and size
ADU Construction Timeline
Phase-by-phase scheduling from design through move-in
ADU Materials Guide
Foundation, framing, finishes, and fixture selections
Boise ADU Permits & Zoning Resource
In-depth resource on Boise's ADU regulatory landscape
ADU Utility Hookups Guide
Water, sewer, electrical, and gas connection details
We Handle All ADU Permits for You
Iron Crest Remodel manages the entire ADU permit process from pre-application meeting through certificate of occupancy. Zoning verification, plan submission, impact fees, utility coordination, and all inspections are handled on your behalf.