ADU Construction Checklist for Boise Homeowners
A comprehensive, step-by-step planning guide covering every phase of ADU construction in Boise — from zoning verification and permit applications to utility connections, rental preparation, and certificate of occupancy.
Quick answer
Iron Crest's ADU work in the Boise area runs $95,000 to $320,000 overall, averaging $165,000. Within that, a garage conversion to a finished ADU runs $95,000 to $145,000, because the existing slab, walls and roof reduce the material and labor required — though bringing a garage shell up to dwelling standards for insulation, egress and moisture control is more work than homeowners expect.
Building an accessory dwelling unit in Boise is one of the most rewarding — and most complex — residential construction projects a homeowner can undertake. Unlike a standard room addition or remodel, an ADU is a fully independent living space with its own kitchen, bathroom, sleeping area, and entrance. That means every major trade is involved: excavation, foundation, framing, roofing, plumbing, electrical, HVAC, insulation, drywall, flooring, cabinetry, and finish carpentry. Add in Boise-specific zoning regulations, utility tap fees, and energy code compliance, and the project demands careful coordination from start to finish.
This checklist is built from Iron Crest's own ADU work in the Treasure Valley, including a 600 sq ft detached one-bedroom unit in Boise's North End that ran 16 weeks of construction. It covers every phase from initial lot analysis through certificate of occupancy, with Boise-specific guidance on permits, zoning setbacks and utility connections that generic ADU guides miss entirely.
Whether you are planning a garage conversion at $95,000 to $145,000, an attached or basement ADU at $120,000 to $260,000, or a new detached ADU at $145,000 to $220,000, this checklist scales to your project type. Use it as your roadmap from day one and share it with your contractor, architect, and lender.
Before you sketch a single floor plan or request a contractor estimate, you need to confirm that your property qualifies for an ADU under current City of Boise regulations. Skipping this step can lead to thousands of dollars in wasted design fees and permit denials.
Phase 1: Lot Analysis & Feasibility
Pull your property survey or plat map from Ada County records to confirm exact lot dimensions, easements, and setback lines
Check your zone and lot layout — Boise does not apply minimum lot-area standards to ADUs, but the unit must fit the zone's setbacks and dimensional standards
Identify your zoning district on the City of Boise zoning map — ADUs are allowed in most residential zones under the Modern Zoning Code, and your zone (R-1A, R-1B, R-1C, R-2 or R-3) is what sets the setbacks and height maximum your ADU has to fit inside
Check for HOA covenants, conditions, and restrictions (CC&Rs) that may prohibit or restrict ADU construction on your property
Assess utility capacity: confirm your electrical panel has capacity for a sub-panel or separate service, verify sewer line condition and capacity, and check water pressure at the meter
Evaluate site access — detached ADUs need construction vehicle access to the backyard for excavation equipment, concrete trucks, and material delivery
Phase 2: City of Boise ADU Regulations
Boise's ADU regulations are more detailed than many homeowners realize. Understanding these requirements before you begin design saves time and prevents costly plan revisions.
| Requirement | Boise Standard |
|---|---|
| Maximum ADU Size | 900 sq ft or 70% of the primary structure, whichever is smaller; max two bedrooms (Zoning Code 11-03-03.2.B(2); state floor rises to 1,000 sq ft or 75% by Feb 1, 2027 under Idaho Code § 67-6541) |
| Minimum Lot Size | Does not apply — minimum lot area and maximum density standards are waived for ADUs (11-03-03.2.B(1)) |
| Side/Rear Setbacks | The underlying zone’s (11-03-03.2.B(6)): 5 ft side / 15 ft rear in R-1C; 10 ft / 20 ft in R-1A and R-1B. A detached unit of 121–500 sq ft under 14 ft may use 3 ft / 9 ft (Table 11-04.4) |
| Maximum Height | Two stories (11-03-03.2.B(3)), within the zone maximum — 35 ft in R-1A/R-1B, 3 stories / 40 ft in R-1C. Idaho Code § 67-6541(2)(f) bars a limit lower than the existing house |
| Parking | No added space required for the ADU; a garage conversion may not reduce the lot's required parking |
| Owner Occupancy | Not required (Modern Zoning Code; prohibited by Idaho SB 1354) |
| Separate Entrance | Required; cannot share entrance with primary dwelling |
Regulations are subject to change. Always verify current requirements with the City of Boise Planning and Development Services before finalizing your ADU design.
Choosing the right ADU type for your property depends on lot size, existing structures, budget, and intended use. Each type has distinct advantages, limitations, and cost profiles that affect everything from permitting complexity to rental income potential.
| ADU Type | Cost Range | Timeline | Best For |
|---|---|---|---|
| Detached New Build (approx. 600 sq ft) | $145K–$220K | 5–9 months, design to CO | Maximum privacy and design freedom; windows on all four sides |
| Garage Conversion | $95K–$145K | Shorter — no new foundation or shell | Existing slab, walls and roof reduce scope; you give up the garage |
| Basement Conversion | $120K–$260K (attached ADU band) | Shorter — no new foundation or shell | Uses existing footprint; egress, ceiling height and moisture control govern feasibility |
| Above-Garage ADU | $120K–$260K (attached ADU band) | Between a conversion and a new build | Keeps the backyard and the garage; needs structural reinforcement and separate stair access |
Design Phase Checklist
Hire an architect or designer experienced with Boise ADU projects to develop floor plans that maximize livability within the 900-square-foot maximum. Plans must comply with the 2018 IECC as adopted in Idaho; Boise sits in Climate Zone 5, which sets R-49 in ceilings and R-20, or R-13 plus R-5 continuous, in exterior walls, along with window U-factor limits. Structural design follows the 2018 IRC with Idaho amendments and the 2023 NEC for electrical work, on Boise's design criteria of 20 psf ground snow, 90 mph wind, seismic category C and a 24-inch frost depth. Submit plans for architectural review if your property is in a Boise historic district or overlay zone. Design for universal accessibility when possible — 36-inch doorways, curbless shower entries, and lever-style hardware add minimal cost during construction but significant value for aging in place or for accessible rental marketing.
ADU construction is a significant financial commitment, but proper budget planning and an understanding of available financing options make the investment manageable. Unlike a kitchen or bathroom remodel, an ADU generates rental income that offsets construction costs over time.
What Your ADU Budget Has to Cover
We do not publish a percentage split for an ADU budget, because the proportions swing hard with site conditions — a lot needing 90 feet of sewer trench spends money that a lot with a connection at the property line never does. What we can give you is the complete list of line items your budget must account for, so nothing arrives as a surprise. Ask any estimate you receive to name every one of these.
| Line Item | What Makes It Swing |
|---|---|
| Architecture & engineering | Whether the design is drawn from scratch or adapted; whether a structural engineer is needed for a garage or above-garage build |
| Permits & plan review | Calculated on project valuation from the Boise fee schedule, plus separate trade permits for plumbing, electrical and mechanical |
| Site work & foundation | Equipment access to the rear yard, soil conditions, slab vs. frost-wall crawlspace, 24-inch frost depth |
| Utility connections | Trench distance for the sewer lateral and water line, whether the main panel has capacity for a subpanel or a separate meter is needed, gas service extension |
| Framing, roofing & exterior shell | Square footage, roof geometry, siding material, and matching the main house for Boise's design compatibility standard |
| Plumbing, electrical & HVAC rough-in | Fixture count, whether the unit is separately metered, and the heating and cooling approach |
| Insulation & drywall | 2018 IECC Climate Zone 5 values — R-49 ceilings, R-20 or R-13 + R-5 continuous walls |
| Interior finishes, kitchen & bath | Cabinetry, counters, flooring, tile and appliance selections — the part homeowners control most directly |
| Contingency reserve | We recommend 10 to 15 percent for buried utilities, soil conditions and structural surprises; unused contingency stays with you |
Financing options include a home equity line of credit, a cash-out refinance, and construction-to-permanent loans, which disburse in draws as work progresses so interest accrues only on what has been drawn. Some homeowners use a HELOC during construction and refinance once the ADU is complete and appraised. Whether completed-ADU rental income can be counted toward your debt-to-income ratio is a lender-by-lender question — ask yours directly before you assume it.
ADU permitting in Boise is more involved than standard residential construction because the project creates a new dwelling unit on an existing residential lot. Multiple departments review the application, and the inspection schedule is more rigorous than a typical remodel.
Permit Application Process
Submit a complete permit application to the City of Boise Planning and Development Services including architectural plans, structural engineering calculations, an energy code compliance report, a site plan with setback dimensions, and a utility connection plan. The City publishes no guaranteed plan-review turnaround; across our own Boise ADU projects review has typically run 3 to 6 weeks, varying with complexity and current department workload. The building permit fee itself is calculated from the City of Boise fee schedule (Table 1-A, effective 10-1-21) on the project valuation: valuations over $100,000 are charged $913.09 plus $5.17 per $1,000 over $100,000, plus 20 percent for residential plan review — about $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 are issued separately, and sewer and water connection fees are set by Boise Public Works rather than by the building fee schedule, so get those amounts from Public Works for your address. Plan for the full inspection sequence during construction: foundation, framing, rough plumbing, rough electrical, rough mechanical, insulation, drywall, and final. Building Division (208) 608-7070; Planning & Zoning (208) 608-7100.
Construction Sequence (5–9 Months, Design to CO)
Construction follows a strict sequence: site preparation and excavation, foundation pour and cure, framing and sheathing, roofing, window and door installation, rough-in for plumbing, electrical, and HVAC, insulation and vapor barrier, drywall hang and finish, interior trim and cabinetry, flooring, paint, fixture installation, and final utility connections. For detached ADUs, weather matters in Boise: concrete needs sustained temperatures above 40 degrees Fahrenheit to cure properly under ACI 306 cold-weather practice, and Boise's 30-year January mean low at Boise Air Terminal is 25.9°F, so a December-through-February pour needs heated enclosures, blankets or hot-water mixing, all of which add cost and time. Iron Crest sequences outdoor phases into the workable weather windows wherever the schedule allows.
Utility Connections
ADUs require dedicated utility connections that go beyond what a standard addition needs, and the trench distance between the main house connections and the ADU is one of the biggest single swings in an ADU budget. Sewer connections require a permit from Boise Public Works, which sets the connection fees — get the current amounts for your address from Public Works rather than from a contractor's estimate of them. Drinking water is supplied by Veolia Water Idaho and sewer service by the City of Boise, so the two connections are arranged with two different providers. Water service may require a separate meter or a shared meter with a backflow preventer. Electrical service options are a subpanel fed from your main panel, which is the lower-cost route where the panel has capacity, or a separate meter from Idaho Power, which is required if the ADU is to be separately billed to a tenant. Gas service for heating or cooking requires coordination with Intermountain Gas for a new service line or an extended branch. Ask for each connection to be priced as its own line item, with the trench route shown on the site plan.
Once construction is complete, several critical steps remain before your ADU is ready for occupancy or rental. These post-construction tasks protect your investment, ensure legal compliance, and maximize your rental income potential.
Certificate of Occupancy
The final inspection by the City of Boise must pass before anyone can legally occupy the ADU. The inspector verifies all permitted work meets Idaho building code, fire safety standards, and energy code requirements. Once approved, you receive a Certificate of Occupancy that authorizes residential use of the unit.
Insurance Updates
Contact your insurance agent to add the ADU to your coverage. Most carriers require an endorsement or a separate landlord policy for a rental ADU. Coverage should address the structure, liability for tenant injuries, and loss of rental income. We do not publish a premium figure — it depends on the carrier, the replacement cost of the structure, your deductible and whether the unit is rented, so get the quote in writing before construction starts.
Tax Assessment Changes
Ada County will revalue your parcel to reflect the added improvement, usually in the assessment cycle following the certificate of occupancy. The size of the increase depends on the assessed value the Assessor places on the unit and on the levy rates set annually by your taxing districts, neither of which a contractor can forecast. Call the Ada County Assessor with your parcel number and ask how the improvement will be classified.
Rental Preparation
We do not publish Boise rent averages — the honest source for a rent number on your property is current comparable listings from a property manager working your ZIP code. Ask what comparable units have actually leased at in the last ninety days. Then prepare a lease that complies with Idaho landlord-tenant law, set a security deposit policy within statutory limits, and establish a system for rent collection, maintenance requests and annual inspections.
How much does it cost to build an ADU in Boise?
Iron Crest's ADU work in the Boise area runs $95,000 to $320,000 overall, averaging $165,000. Within that, a garage conversion to a finished ADU runs $95,000 to $145,000, because the existing slab, walls and roof reduce the material and labor required — though bringing a garage shell up to dwelling standards for insulation, egress and moisture control is more work than homeowners expect. A new detached ADU of about 600 square feet with one bedroom runs $145,000 to $220,000. An attached ADU addition, which includes above-garage and basement-conversion work, runs $120,000 to $260,000 depending on how much of the existing structure has to be opened up and whether the main house's electrical and plumbing can carry the added load. These ranges include design, engineering, City of Boise permit fees and the utility connection work; we recommend carrying a 10 to 15 percent contingency beyond them for buried utilities, soil conditions and structural surprises. Iron Crest provides line-item estimates rather than a per-square-foot multiplier, so you can see which part of the range your lot lands in and why.
Do I need a permit to build an ADU in Boise?
Yes, and the process is more involved than a standard home addition because you are creating a new dwelling unit. You will need a building permit from the City of Boise Planning and Development Services, submitted with architectural plans, a site plan showing setbacks, and an energy code compliance report. Trade permits for electrical, plumbing and mechanical work are issued separately. If the ADU connects to city sewer and water, Boise Public Works handles the connection permits and sets those fees — confirm the current amounts with Public Works, because they depend on the connection method and are not something a contractor should quote from memory. The City of Boise does not publish a guaranteed plan-review turnaround, so any specific week count is an estimate rather than a commitment; across our own Boise ADU projects plan review has typically run 3 to 6 weeks, varying with project complexity and current department workload. Confirm the current queue with Planning and Development Services at (208) 608-7100. Iron Crest manages the permit application and inspection sequence as part of the project.
What are Boise's zoning requirements for ADUs?
The City of Boise allows accessory dwelling units in most residential zones. Under Boise Zoning Code 11-03-03.2.B, minimum lot-area and density standards do not apply to ADUs; an ADU may not exceed 900 square feet or 70 percent of the primary structure's floor area, whichever is smaller, with no more than two bedrooms and no more than two stories; it must meet the underlying zone's setbacks and other dimensional standards; and its design must be compatible with the principal structure. Idaho's 2026 ADU law (Idaho Code § 67-6541) requires Boise to allow at least 1,000 square feet or 75% of the primary home by February 1, 2027 — confirm the limit in effect with Planning & Development Services. Boise's Modern Zoning Code does not require an added parking space or owner-occupancy for an ADU outside the Harris Ranch Specific Plan, though a garage conversion may not reduce the lot's required off-street parking. These regulations are changing, so verify current requirements with the City of Boise before beginning your project.
How long does it take to build an ADU in Boise?
On our Boise ADU projects, a new detached unit runs about 5 to 9 months from feasibility and design through certificate of occupancy. That breaks into feasibility and design at 2 to 4 weeks, plan review at 3 to 6 weeks, site prep and foundation at 2 to 4 weeks, framing and rough-in at 4 to 6 weeks, insulation and drywall at 2 to 3 weeks, interior finishes at 4 to 6 weeks, and final inspections and the certificate of occupancy at 1 to 2 weeks. One recorded example: the 600 square foot detached one-bedroom ADU we built in Boise's North End ran 16 weeks of construction, which is that project's own figure rather than a general promise. Conversions of an existing structure — a garage or a basement — skip the foundation and shell phases and therefore finish sooner than a new detached build, but the interior sequence is identical and the inspection schedule is the same. Weather matters for detached builds: concrete needs sustained temperatures above 40 degrees Fahrenheit to cure properly under ACI 306 cold-weather practice, so a winter foundation pour needs heated enclosures or blankets and takes longer than a spring pour.
Can I rent out my ADU in Boise, and where do I get a realistic rent figure?
Yes, a permitted ADU in Boise can be rented on a long-term lease. We do not publish Boise rent averages, because Treasure Valley rents move enough that any figure printed on a contractor's website would be stale, and we have no rent survey behind one. The honest source for a rent number on your property is current comparable listings from a property manager working your ZIP code — ask what comparable studio or one-bedroom units have actually leased at in the last ninety days, not what they were listed at. For context, one real project: the 600 square foot detached one-bedroom we built in Boise's North End had rental potential recorded at $1,400 to $1,600 a month for that unit, on that lot, which is that project's own figure and not a market average. Short-term rentals are regulated separately by the City of Boise and carry lodging tax obligations, so confirm the current requirements with the City before planning around nightly rates. As a landlord you are responsible under Idaho law for maintaining the unit in habitable condition, using a compliant lease, handling security deposits within Idaho's statutory framework, and carrying landlord liability coverage.
This checklist is one piece of a comprehensive ADU planning toolkit. Dive deeper into specific topics with our supporting guides built for Boise homeowners.
The following government agencies, industry organizations, and official resources provide additional information relevant to your remodeling project.
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