ADU Construction Timeline in Boise
A realistic, phase-by-phase look at how long ADU construction takes in Boise and the Treasure Valley. From initial design through certificate of occupancy, know what to expect at every stage of your accessory dwelling unit project.
Quick answer
A detached ADU in Boise typically takes 4 to 8 months from initial design through certificate of occupancy. Design and permitting account for roughly 1.5 to 2.5 months of that, and active construction 3 to 5 months, depending on size, complexity, foundation type, and weather.
ADU construction timelines vary dramatically based on the type of structure you are building. A garage conversion that reuses an existing shell is fundamentally different from a ground-up detached ADU that requires excavation, foundation, and full construction. Here is how each type breaks down in the Boise market.
Garage Conversion
3 - 5 Months Total
The fastest path to a livable ADU. The existing garage shell — foundation, walls, and roof — eliminates the most time-intensive construction phases. Work focuses on interior buildout: adding insulation to meet current energy code, installing drywall, running new electrical circuits, plumbing for a kitchen and bathroom, HVAC installation, flooring, and finish work. A new exterior door and window modifications are typically required. The City of Boise reviews garage conversions faster than new construction because the structural shell already exists.
Attached ADU
4 - 7 Months Total
An attached ADU is built as an addition to the primary residence — sharing at least one wall with the existing home. This reduces some site work and utility connection costs but adds complexity at the connection point between old and new construction. Foundation work, framing, and roofing must integrate seamlessly with the existing structure. Matching exterior materials and rooflines to the primary home requires careful planning. Design and permitting take longer because structural engineering for the tie-in connection is required, and the City of Boise reviews the impact on the existing structure.
Detached ADU
4 - 8 Months Total
A detached ADU is a standalone structure built from the ground up on your property. This is the most complex ADU type and involves every phase of new home construction: site preparation, excavation, foundation, framing, roofing, exterior finishes, utility connections, and full interior buildout. Detached ADUs require separate utility runs from the property line or main house, ACHD right-of-way review for driveway access, and often a more extensive City of Boise site plan review. What you get for that is the most design flexibility, full separation between the two households, and a unit that can be metered and entered entirely independently of the main house.
A detached ADU is the most involved project type and moves through seven distinct phases. Understanding each phase helps you plan around disruptions, anticipate inspection milestones, and track progress accurately. Here is the exact sequence Iron Crest Remodel follows for ground-up ADU construction in Boise.
Phase 1: Design & Engineering
Months 1 - 2Every ADU project begins with a thorough site assessment and design development process. We evaluate your lot for buildable area, setback requirements (the City of Boise requires minimum setbacks from property lines, existing structures, and easements), utility connection feasibility, and access for construction equipment. The design phase produces architectural drawings, structural engineering calculations, and a complete set of construction documents that will be submitted to the City of Boise for plan review. Engineering is critical for Boise's seismic zone requirements and snow load calculations specific to the Treasure Valley.
- Site assessment and lot survey
- Architectural design and floor plan development
- Structural engineering (seismic and snow load)
- Energy code compliance calculations (IECC)
- Material selections and finish specifications
- Construction document preparation for City submittal
Phase 2: Permits, Site Prep & Utilities
Months 2 - 3Once construction documents are complete, we submit to the City of Boise Planning and Development Services for plan review. Concurrently, we initiate utility coordination — Boise City Public Works for the sewer connection, Veolia Water Idaho for water service, Idaho Power for electrical service planning, and Intermountain Gas for gas line routing if the unit is not all-electric. Dig Line (Idaho 811) locates are scheduled for the public utilities; note that 811 does not mark privately owned laterals and irrigation lines on your side of the meter, so those get located separately before we trench. Site preparation begins once permits are issued: clearing the build area, establishing construction access, installing temporary fencing, and setting up the erosion and sediment controls required by the site's stormwater permit. Excavation for the foundation follows immediately.
- City of Boise plan review submission (3-6 weeks on our projects)
- ACHD right-of-way and access review
- Sewer and water connection applications
- Idaho 811 utility locates
- Site clearing and excavation
- Erosion control and temporary fencing
Phase 3: Foundation, Framing & Roofing
Months 3 - 4Foundation work begins with footings excavated to the frost line depth required by Boise building code (typically 24 inches in the Treasure Valley). Depending on the design, the foundation may be a monolithic slab, stem wall with crawl space, or full basement. Rebar is placed, anchor bolts are set, and concrete is poured. The concrete then has to gain enough strength to carry framing loads — about a week in normal conditions, and longer when it is cold, because fresh concrete has to be kept from freezing while it sets. The foundation inspection is one of the most critical checkpoints in the project. Once framing starts, the structure takes shape quickly: walls are raised, roof trusses or rafters are set, roof sheathing and underlayment are installed, and the building is dried in. A framing inspection by the City of Boise is required before proceeding to the next phase.
- Footing excavation and rebar placement
- Concrete pour and strength gain before framing
- Foundation inspection (City of Boise)
- Wall framing and sheathing
- Roof truss installation and sheathing
- Framing inspection and dry-in
Phase 4: Rough Plumbing, Electrical & HVAC
Months 4 - 5With the structure dried in, the mechanical rough-ins run together. Supply and drain lines go in for the kitchen, bathroom, and laundry (if included), and the water heater location is roughed in — a tankless unit saves floor area in a small footprint. Circuits are pulled from the new panel: kitchen appliance circuits, GFCI protection everywhere NEC 2023 210.8 calls for it — in a kitchen that is every 125–250 V receptacle, not just the ones over the counter — lighting circuits, interconnected smoke and CO detector wiring, and exterior circuits. HVAC is ductwork, or mini-split line sets for a ductless system — the common choice in an ADU this size because there is no duct chase to build and nothing to leak into an unconditioned space. Each trade must pass its own rough inspection by the City of Boise before walls can be closed up.
- Supply and drain plumbing rough-in
- Electrical panel and circuit wiring
- HVAC ductwork or mini-split line sets
- Gas line installation (if applicable)
- Low-voltage wiring (data, cable, security)
- Rough mechanical inspections (all trades)
Phase 5: Insulation, Drywall & Exterior
Months 5 - 6After all rough inspections pass, insulation is installed. Boise sits in Climate Zone 5, where the 2018 IECC prescriptive table calls for R-20 cavity insulation in wood-frame walls (or R-13 plus R-5 continuous), R-49 at the ceiling, and R-30 in floors over unconditioned space such as a crawl space. Spray foam, fiberglass batts, or blown-in cellulose are common choices. An insulation inspection verifies coverage and R-values before drywall is hung. Drywall is installed, taped, mudded (three coats), and sanded. Priming and painting follow. Simultaneously on the exterior, siding is installed (fiber cement, wood, or engineered wood are popular in Boise neighborhoods), exterior trim is finished, gutters are hung, and exterior paint or stain is applied. Windows and exterior doors are installed and flashed during framing but receive final trim and weathersealing during this phase.
- Wall, ceiling, and floor insulation
- Insulation inspection (energy code compliance)
- Drywall hang, tape, mud, and sand
- Interior priming and painting
- Exterior siding and trim installation
- Gutters, downspouts, and exterior paint
Phase 6: Interior Finishes, Kitchen & Bathroom
Months 6 - 7This is where the ADU transforms from a construction site into a livable space. Cabinets are installed in the kitchen and bathroom, followed by countertop templating, fabrication, and installation. Flooring goes in throughout — LVP is the most popular choice in Boise ADUs for its durability, water resistance, and ease of maintenance. Tile is installed in the bathroom (shower surround, floor, and backsplash). Plumbing fixtures are set: kitchen sink and faucet, bathroom vanity, toilet, and shower/tub fixtures. Electrical finish work includes light fixtures, switches, outlets, and cover plates. Interior doors are hung, trim and base molding are installed, and all hardware (door handles, cabinet pulls, towel bars) is mounted. Appliances are delivered and installed: range, refrigerator, dishwasher, microwave, washer and dryer connections (if included).
- Kitchen and bathroom cabinet installation
- Countertop templating, fabrication, and install
- Flooring installation (LVP, tile, or hardwood)
- Bathroom tile and shower surround
- Plumbing and electrical fixture installation
- Appliance delivery and hookup
Phase 7: Final Inspections & Certificate of Occupancy
Months 7 - 8The final phase involves completing all remaining punch list items and scheduling the battery of final inspections required by the City of Boise. Final inspections include building, electrical, plumbing, mechanical, and fire safety. Utility connections are finalized: Veolia Water Idaho activates the water meter, the sewer connection is completed and inspected by Boise City Public Works, Idaho Power connects permanent electrical service (the project runs on a temporary power pole during construction), and Intermountain Gas activates the gas meter. Once all inspections pass, the City of Boise issues the Certificate of Occupancy (CO) — the legal document that authorizes the ADU to be occupied as a dwelling unit. Final landscaping, driveway or pathway work, and site cleanup complete the project. We conduct a detailed walkthrough with you, review all systems, provide warranty documentation, and hand over keys.
- Final building, electrical, plumbing inspections
- Water meter activation (Veolia Water Idaho)
- Permanent electrical service (Idaho Power)
- Certificate of Occupancy issued by City of Boise
- Final landscaping and site restoration
- Owner walkthrough and warranty handoff
Permitting is one of the most unpredictable variables in any ADU project. The City of Boise has become increasingly supportive of ADU construction, but the review process still requires patience and thorough preparation. Here is what to expect from the permitting process in the Boise area.
City of Boise Plan Review
The City of Boise Planning and Development Services department reviews all ADU construction plans for compliance with zoning, building code (IRC), energy code (IECC), fire code, and accessibility requirements. The City publishes processing-time targets rather than a guaranteed turnaround; on our own Boise ADU projects, plan review has run 3 to 6 weeks from the date of a complete submission, with a garage conversion carrying minimal structural change at the faster end. Plans that require revisions are returned with correction comments and go back into the queue for another round. Iron Crest Remodel submits construction-ready documents with all required details to minimize revision requests.
Ada County vs. City of Boise Permits
If your property is within Boise city limits, your permits go through the City of Boise Planning and Development Services. Properties in unincorporated Ada County are reviewed by Ada County Development Services, which has its own review timeline (often slightly shorter but with different zoning requirements). Some Boise-area communities like Eagle, Meridian, and Star have their own permitting departments with varying ADU regulations. It is critical to confirm your jurisdiction before beginning the design process, as ADU size limits and setback requirements still vary between jurisdictions until every city conforms to Idaho's 2026 ADU law (Senate Bill 1354) by February 1, 2027.
ACHD Access Review
The Ada County Highway District (ACHD) reviews any project that may affect the public right-of-way. For ADU projects, ACHD reviews driveway access, curb cuts, and potential impacts on street parking and traffic flow. If your ADU requires a new driveway approach or modification to an existing one, ACHD must approve the design. This review runs concurrently with City plan review; ask ACHD for its current turnaround when you file, since it sets its own schedule. Properties on ACHD-maintained roads (which includes most streets in the Boise metro area) are subject to this review regardless of whether the property is in the City of Boise or unincorporated Ada County.
Sewer & Water Connection Timing
Utility connections are among the longest lead-time items in ADU construction. Sewer is handled by Boise City Public Works; drinking water is supplied by Veolia Water Idaho, the regulated utility serving Boise and much of Ada County, so the water application goes to Veolia rather than the City. Sewer connection permits are obtained separately. If the existing sewer lateral has sufficient capacity, connecting the ADU to the same lateral is usually approved. If a new lateral is needed, excavation in the public right-of-way requires coordination with ACHD, which adds its own scheduling step. Each of these agencies sets its own lead time, so we ask each for its current one rather than assuming. We initiate utility applications as early as possible — often during the design phase — to prevent utility connections from becoming the critical path delay.
Understanding common delay factors helps you plan proactively. While no construction project is entirely predictable, an experienced ADU contractor anticipates these bottlenecks and builds buffer time into the schedule. Here are the most common delay sources specific to Boise ADU projects.
Permit Revisions & Resubmissions
Incomplete or non-compliant plan submissions are the most common cause of permit delays. Every revision cycle puts the drawings back in the queue behind whatever was filed while you were correcting them. Common rejection reasons include insufficient structural details, missing energy code calculations, setbacks that do not close, and incomplete site plans. The City of Boise reviewers are thorough — they check every detail against the 2018 IRC as adopted by Idaho and the local amendments. Iron Crest Remodel maintains a detailed pre-submission checklist from our own Boise ADU submittals to get first-pass approval whenever possible.
Utility Hookup Scheduling
Utility connections operate on the utility's schedule, not the contractor's. A new water service runs through Veolia Water Idaho, sewer through Boise City Public Works, and a service drop or transformer through Idaho Power — and each is busiest in the same summer months as everyone else's project. Intermountain Gas line extensions depend on the distance from existing infrastructure. We ask each utility for its current lead time at application rather than working from a figure that was true last season. We submit utility applications during the design phase — months before connections are needed — to ensure these long-lead items do not hold up the final phases of construction.
Concrete Curing in Cold Weather
Boise winters bring temperatures that regularly drop below freezing from November through February, and Boise averages 87 freeze-thaw cycles a year. Fresh concrete has to be kept warm enough to keep gaining strength and protected from freezing while it sets. Cold-weather concrete work requires heated enclosures, insulated blankets, and sometimes accelerating admixtures — all of which add cost, and the slab still takes longer to reach framing strength than the same pour would in May. This is why we recommend scheduling foundation pours between April and October.
Material Lead Times
Critical materials can have extended lead times that impact the construction schedule. The long poles are usually windows, custom cabinets, specialty doors, HVAC equipment and metal roofing panels — the items that are built to order rather than pulled from a shelf. Lead times move with the supplier and the season, so we confirm each one with the vendor at the time we order rather than quoting you a number from last year. We order long-lead items during the permitting phase so materials arrive on-site when needed, and we discuss acceptable substitutions upfront as a contingency.
Inspection Scheduling
ADU construction requires numerous inspections at specific milestones: foundation, framing, rough electrical, rough plumbing, rough mechanical, insulation, and final inspections for each trade. The City of Boise inspection division schedules inspections based on request date and inspector availability, and requests are made through the city's portal with at least 24 hours' notice. Waits run longer during the summer building season than in winter, and a failed inspection means corrections plus a second trip before the next phase can start. We schedule inspections proactively and ensure all work is inspection-ready before requesting each milestone inspection.
Site Access & Soil Conditions
Boise properties vary significantly in soil conditions and site accessibility. Some areas near the Boise River have high water tables that require dewatering during excavation. Properties in the Boise foothills may encounter rocky soil or caliche — a cemented calcium-carbonate hardpan — that requires heavier excavation equipment and more time. Tight backyard access that prevents standard equipment from reaching the build site may require smaller equipment or hand-digging, which extends the foundation phase. A thorough geotechnical assessment during the design phase helps identify these issues before construction begins.
Boise's four-season climate has a direct impact on ADU construction scheduling. Different phases of the project are affected differently by weather and seasonal conditions. Strategic project timing can shave weeks off your overall timeline and reduce weather-related costs.
Foundation Work: Best April Through October
Concrete work is the most weather-sensitive phase of ADU construction. Boise temperatures from April through October consistently support proper concrete curing without the need for heated enclosures or insulated blankets. Footings in Ada County are set below the 24-inch frost depth, and frozen ground in winter makes that excavation harder. Pouring in warm months lets the concrete reach framing strength on a normal schedule and eliminates the cost premium for winter concrete protection. If your project requires a winter pour, expect a line item for cold-weather protection — enclosures, blankets, and the extra days before the slab can be loaded — priced against the actual measures your pour needs.
Framing: Year-Round With Crew Scheduling
Framing can proceed in nearly any Boise weather condition. Lumber is not significantly affected by cold temperatures, and framing in winter conditions is routine here. The primary winter consideration is reduced daylight — Boise gets about nine hours between sunrise and sunset around the December solstice against roughly fifteen in late June — which shortens the workable day. Snow can stop work for a day at a time. Rain is more disruptive than cold — framing lumber that gets repeatedly soaked can swell and require drying before sheathing and insulation. We protect framed structures with tarps and prioritize getting the roof sheathed and dried in quickly.
Exterior Finishes: Best April Through November
Exterior siding installation, painting, and staining require temperatures consistently above 40 to 50 degrees Fahrenheit for proper adhesion and curing. In Boise, this window runs from approximately mid-March through late November, though October and November can be unpredictable. Fiber cement siding can be installed in cooler temperatures than wood siding because it does not require field painting (it comes pre-primed or pre-finished). Exterior painting is the most temperature-sensitive exterior task — most exterior paints require a minimum of 50 degrees during application and for 4 hours after. Scheduling exterior finishes for completion before Thanksgiving is a reliable target for Boise ADU projects.
Interior Work: Year-Round
Once the structure is dried in (roof on, windows installed, exterior weather barrier complete), all interior work proceeds regardless of exterior conditions. Temporary heat is provided during winter months using propane heaters or the permanent HVAC system (once installed). Insulation, drywall, painting, flooring, cabinetry, plumbing fixtures, electrical fixtures, and all finish work are entirely weather-independent. This is why starting design and permitting in late fall or winter is the optimal strategy: you complete the planning phases during months when outdoor construction is least efficient, break ground in spring when conditions are ideal for site work and foundation, and have the structure dried in before the following winter so interior work continues uninterrupted through the cold months.
The Ideal Boise ADU Timeline Strategy
Based on years of ADU construction experience in the Boise market, the most efficient project timeline follows this seasonal pattern:
- November - January: Design development, engineering, material selections, and construction document preparation. Use the winter months for all planning and decision-making.
- February - March: Permit submission, utility applications, and material ordering. Plans are in review while you wait for spring.
- April - May: Permits issued, site prep, excavation, and foundation pour in ideal concrete weather.
- June - August: Framing, roofing, rough mechanicals, and exterior work during the longest, driest days.
- September - November: Interior finishes, final inspections, utility connections, and certificate of occupancy. Move-in ready before winter.
Common questions about ADU construction timelines and scheduling in the Boise area.
How long does it take to build a detached ADU in Boise?
A detached ADU in Boise typically takes 4 to 8 months from initial design through certificate of occupancy. Design and permitting account for roughly 1.5 to 2.5 months of that, and active construction 3 to 5 months, depending on size, complexity, foundation type, and weather. Our 600 sq ft North End detached ADU ran 16 weeks from design start to completion — that project's own recorded duration, not a promise for another lot. Starting design in late fall or winter positions you for a spring groundbreak, which is the most efficient seasonal strategy in the Treasure Valley.
How long does the City of Boise take to approve ADU permits?
The City of Boise publishes processing-time targets rather than a guaranteed turnaround, so no one can promise you a plan review date. On our own Boise ADU projects, plan review has typically run 3 to 6 weeks for a complete submission, with a garage conversion clearing faster than a new detached structure carrying structural engineering and a full site plan. Every round of corrections puts the drawings back in the queue. Confirm the current turnaround with Planning and Development Services at (208) 608-7100 before you build a schedule around it.
What is the fastest type of ADU to build in Boise?
A garage conversion is the fastest ADU type in Boise, typically completing in 3 to 5 months total including design and permits. Because the shell structure already exists, you skip foundation work, framing, roofing, and exterior siding entirely. The project focuses on interior buildout: insulation, drywall, electrical, plumbing, HVAC, flooring, kitchen, and bathroom installation. This makes garage conversions both faster and more affordable than ground-up construction.
Can I build an ADU in Boise during winter?
Yes, but with strategic planning. Interior work (electrical, plumbing, insulation, drywall, finishes) can proceed year-round in Boise. However, foundation work and concrete pouring are best scheduled between April and October when temperatures consistently stay above 40 degrees Fahrenheit. Framing can proceed in winter with proper crew scheduling. Exterior siding and painting are best completed before November. Starting design and permitting in winter for a spring groundbreak is the ideal Boise ADU strategy.
What causes the most delays in Boise ADU projects?
The three most common ADU delay factors in Boise are permit revisions, utility scheduling, and cold-weather concrete. Every round of plan corrections puts the drawings back in the review queue. Utility work runs on the utility's calendar, not ours — Veolia Water Idaho for a water service, the City of Boise for sewer, and Idaho Power for a service drop or transformer — so we file those applications during the design phase and ask each one for its current lead time rather than assuming last year's. Material lead times for windows, HVAC equipment, and specialty finishes can also move the schedule; we confirm them with the supplier at order time. Working with a contractor who anticipates these bottlenecks is the best delay prevention there is.
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