ADU Construction Cost & ROI in Boise
A pricing guide for Boise and Treasure Valley homeowners planning an accessory dwelling unit. Cost by type and size, component breakdowns, local regulations, permit fees, and how to underwrite the return honestly.
Quick answer
ADU construction in Boise runs $95,000 to $320,000. A garage conversion starts around $95,000, most projects land near $165,000, and a large detached unit with premium finishes reaches $320,000. A new detached one-bedroom of about 600 square feet typically falls between $145,000 and $220,000.
ADU construction costs in Boise vary dramatically based on the type of unit you build. The three primary categories each come with distinct advantages, limitations, and price ranges that reflect the scope of work involved.
Detached ADU
$145,000 - $320,000
Ground-up construction, maximum flexibility
- Standalone structure with independent foundation
- Full design flexibility for layout and orientation
- Maximum privacy for both occupants and homeowner
- New utility connections required (water, sewer, electrical)
- The configuration an appraiser most readily treats as a separate dwelling
Attached ADU
New Space, Shared Systems
$120,000 - $260,000
Addition to existing home, shared systems
- Addition built onto existing home structure
- Shared wall reduces framing and exterior costs
- Can share HVAC, water heater, or electrical panel
- Separate entrance required by Boise code
- Shorter utility runs lower connection costs
- Faster construction timeline than detached
Garage Conversion
$95,000 - $145,000
Converting existing structure, lowest entry cost
- Existing foundation and structure reduces costs
- Fastest path to a completed ADU
- Limited by existing footprint and ceiling height
- May require foundation upgrades for residential use
- Eliminates garage parking (check Boise requirements)
Every ADU project is composed of individual cost components. This breakdown reflects current Boise-area pricing and helps you understand where your investment goes. Component costs vary based on ADU type, site conditions, and finish level.
| Component | Low Range | High Range | Notes |
|---|---|---|---|
| Foundation & Site Work | $15,000 | $40,000 | Soil conditions, slope, excavation, concrete |
| Framing & Exterior | $20,000 | $50,000 | Walls, roof, siding, windows, exterior doors |
| Plumbing | $12,000 | $25,000 | New water/sewer connections, fixtures, water heater |
| Electrical | $8,000 | $20,000 | Separate meter vs. shared panel, wiring, fixtures |
| HVAC | $6,000 | $15,000 | Mini-split systems popular for Boise ADUs |
| Interior Finish | $15,000 | $40,000 | Kitchen, bath, flooring, paint, trim, doors |
| Permits & Impact Fees | $5,000 | $15,000 | Boise building, ACHD, sewer/water fees |
| Design & Engineering | $5,000 | $15,000 | Architectural plans, structural, site survey |
| Utility Connections | $8,000 | $25,000 | Water, sewer, electrical hookups to street |
| Landscaping Restoration | $3,000 | $10,000 | Grading, sod, fencing, driveway repair |
* These are installed ranges, inclusive of the work required to complete each component. Garage conversions will have lower foundation and framing costs but may require additional structural upgrades. The component ranges sum to roughly $97,000 at the low end and $255,000 at the high end, which is why the Boise ADU range we quote is $95,000 to $320,000 rather than the arithmetic sum of every column maximum — the high end of every line item does not land on the same project.
Foundation & Site Work Details
Foundation costs in Boise vary significantly based on soil conditions. Much of Boise's North End and Bench areas sit on clay-heavy soils that may require engineered fill or deeper footings. Sloped lots in the Foothills or East End can add $5,000 to $15,000 for retaining walls and additional excavation. A standard slab-on-grade foundation for a 600 square foot ADU typically costs $12,000 to $20,000, while a crawl space foundation runs $18,000 to $30,000. Site preparation includes clearing, grading, temporary utilities, and construction access, which can be complicated on tight Boise lots with limited backyard access.
Plumbing & Utility Connections
Plumbing is often the most variable cost component in ADU construction. A detached unit generally needs its own water and sewer runs, which means trenching from the ADU to the main line in the street or alley. Trenching costs depend heavily on distance: a 50-foot run costs significantly less than a 150-foot run across a large lot. Connection and system-development charges are set by fee schedule and are billed by two different agencies — Veolia Water Idaho supplies Boise drinking water, while the City of Boise handles sewer — so confirm the current amounts with each before you budget. Attached ADUs and garage conversions can sometimes tie into existing home plumbing, reducing this cost substantially.
HVAC for Boise ADUs
Ductless mini-split heat pump systems are the most popular HVAC choice for ADUs in the Boise market, and for good reason. They provide both heating and cooling in a single system, require no ductwork, and operate efficiently in Boise's climate where temperatures range from single digits in winter to over 100 degrees in summer. A single-zone mini-split system (sufficient for most ADUs under 700 square feet) costs $4,000 to $8,000 installed. Larger units or multi-zone systems for ADUs with separate bedroom areas run $8,000 to $15,000. Some homeowners opt for in-floor radiant heat paired with a mini-split for cooling, adding $3,000 to $6,000 to the HVAC budget.
Interior Finish Levels
Interior finish costs depend on whether the ADU is designed for rental income, family use, or personal retreat. A rental-grade finish with LVP flooring, laminate countertops, builder-grade cabinets, and standard fixtures runs $15,000 to $25,000 for a 600 square foot unit. A mid-range finish with quartz countertops, semi-custom cabinets, tile bathroom, and upgraded fixtures costs $25,000 to $35,000. Premium finishes with custom cabinetry, natural stone, high-end appliances, and designer fixtures can push interior costs to $35,000 to $50,000 or more. Most Boise ADU projects target the mid-range level, which attracts quality tenants while maintaining reasonable construction costs.
Boise's ADU standards (Zoning Code Section 11-03-03.2.B) establish specific requirements that directly influence design options and construction costs. Understanding these regulations before design begins prevents costly revisions and ensures a smooth permitting process.
Lot Size Requirements
Boise does not apply minimum lot-area or density standards to ADUs (Zoning Code 11-03-03.2.B(1)), but the unit still has to meet the underlying zone's setbacks and other dimensional standards. On smaller infill lots those setbacks, not a lot-size minimum, are what usually limit a detached unit and push the project toward an attached ADU or garage conversion.
Setback Requirements
A Boise ADU has no setback standard of its own — it meets the dimensional standards of the underlying zone (Zoning Code 11-03-03.2.B(6)): 5 feet at the interior side and 15 feet at the rear in R-1C, 10 feet and 20 feet in R-1A and R-1B, plus that zone's front and street-side setbacks. Boise sets no maximum lot coverage percentage in its residential districts, so the setback envelope is what limits the footprint. On narrow or shallow lots that envelope can force a smaller or differently shaped design, and corner lots carry an additional street-side setback. A custom plan drawn to a constrained envelope costs more in design time than an off-the-shelf plan. Confirm your zone's standards with Planning & Zoning at (208) 608-7100.
Size Limits
Boise caps ADU size at 900 square feet or 70% of the primary structure's total floor area, whichever is smaller (Zoning Code Section 11-03-03.2B). In practice this limits most ADUs to roughly 600 to 900 square feet. Idaho's 2026 ADU law (Senate Bill 1354) requires the city to raise that cap to at least 1,000 square feet or 75% of the primary home by February 1, 2027, so confirm the limit in effect with Planning & Development Services. This cap affects the cost-per-square-foot calculation because fixed costs like utility connections, permits, and site work represent a larger percentage of the total budget for smaller units. A 400 square foot ADU costs far more per square foot than an 800 square foot unit because these fixed costs are spread across less area.
Height Limits & Design Standards
Boise caps an ADU at two stories (Zoning Code 11-03-03.2.B(3)), inside the height maximum of the underlying zone — there is no separate 25-foot ADU height cap. The ADU's materials, colors and design motif must be compatible with and complement the principal structure (11-03-03.2.B), which means you cannot simply specify the cheapest available siding and roofing if it does not read with your home. Matching an existing material package costs more than a builder-grade one, and that delta belongs in the budget from the first drawing rather than in a change order after design review.
Owner-Occupancy & Parking
Boise's Modern Zoning Code has no owner-occupancy requirement, and Idaho's 2026 ADU law (Senate Bill 1354) prohibits cities from imposing one, so you may rent both the primary dwelling and the ADU. That widens the investment case for homeowners considering ADUs purely as rental properties. Outside the Harris Ranch Specific Plan, Boise also does not require an added off-street parking space for the ADU — Harris Ranch still requires owner-occupancy and one parking space until the city conforms its code to the state statute. In either case, a garage conversion may not reduce the lot's required parking for the primary dwelling; if it eliminates existing parking, you may need to create a new parking pad ($2,000 to $5,000).
Connection Fees & Impact Fees
Beyond building permit fees, Boise ADU projects are subject to impact and connection charges: an Ada County Highway District (ACHD) impact fee for a new dwelling unit, a City of Boise sewer connection charge, and a water system development charge from Veolia Water Idaho. Each is set by that agency's own fee schedule and changes on its own cycle, which is why the table above carries them as one combined planning range rather than three fixed figures — confirm the current amounts with ACHD, the City and Veolia before you finalize a budget. Idaho Code § 67-6541 caps ADU fees at no more than the fees for a single-family dwelling. These charges are largely fixed regardless of ADU size or finish level, which is why they weigh more heavily on a small unit.
ADU size directly impacts total construction cost, but the relationship is not linear. Smaller ADUs cost less overall but more per square foot because fixed costs like utility connections, permits, and site work are spread across fewer square feet. Here is how common ADU sizes compare in the Boise market.
| Unit Type | Size | Total Cost | Cost per Sq Ft |
|---|---|---|---|
| Studio | 400 sq ft | $120,000 - $180,000 | $300 - $450/sq ft |
| 1-Bedroom | 600 sq ft | $145,000 - $220,000 | $242 - $367/sq ft |
| 2-Bedroom | 800 sq ft | $200,000 - $300,000 | $250 - $375/sq ft |
| Large 2-Bedroom | 1,000 sq ft | $240,000 - $320,000 | $240 - $320/sq ft |
* Cost per square foot decreases with larger units because fixed costs (permits, utility connections, site work, design fees) are amortized across more living space. Garage conversions typically fall at the lower end of these ranges due to existing structure. Note that Boise currently caps an ADU at 900 square feet or 70% of the primary structure, whichever is smaller, so the 1,000-square-foot row applies only once the city raises that cap to the 1,000 sq ft / 75% floor Idaho Code § 67-6541 requires by February 1, 2027 — or on a parcel outside Boise city limits whose jurisdiction already allows it.
Why Smaller ADUs Cost More Per Square Foot
A 400 square foot studio ADU and an 800 square foot two-bedroom ADU require nearly identical investment in permit fees, utility connections, site preparation, and design work. These fixed costs can represent $30,000 to $50,000 regardless of unit size. In a 400 square foot unit, that translates to $75 to $125 per square foot before any construction begins. In an 800 square foot unit, those same fixed costs drop to $38 to $63 per square foot. This is why building slightly larger, when your lot and budget allow, often delivers better value per square foot and stronger rental income potential.
Optimal Size for Boise Rental Market
A 550 to 700 square foot one-bedroom is the configuration we build most often, because it keeps construction cost inside the $145,000 to $220,000 band while still reading as a real apartment rather than a studio. Studios appeal to a narrower tenant pool; two-bedroom units cost meaningfully more to build and do not always rent for proportionally more at ADU scale. We do not publish Boise rent averages — pull current comparables for your ZIP code from a property manager before you underwrite the project. The right size depends on your lot constraints, budget, and whether the ADU is intended for long-term rental, family use, or a combination.
An ADU is one of the few home improvements that can generate ongoing income while simultaneously increasing your property value. We do not publish Boise market rent averages, projected cap rates, or percentage property-value uplifts on this page. Those figures move, they are property-specific, and a contractor is not the right source for them. Here is what we can document instead.
A Real Boise ADU, With Its Recorded Numbers
- 600 sq ft detached one-bedroom, North End Boise, 1940s craftsman main house
- Budget range: $120,000–$150,000
- Project duration: 16 weeks
- Appraised added value: $155,000
- Rental potential recorded for this unit: $1,400–$1,600 per month
These are that project's own recorded figures, not a Boise market average, and they do not predict what a different lot will produce. Full scope, phasing, and photos: 600 sq ft Detached ADU Build in North End Boise.
How To Underwrite Your Own Project
Start with a construction number you can hold someone to — an itemized estimate for your specific lot, not a range off a website. Then get the rent side from a property manager working your ZIP code, using current comparable one-bedroom listings rather than a projection.
Subtract the four costs that gross rent hides: property management if you use it, a maintenance reserve, the insurance increment, and the property tax increase Ada County assesses once the certificate of occupancy is issued. Net rent after those is the number the investment actually turns on.
If a short-term rental is the plan, check the rules before you design. Boise repealed its short-term rental license ordinance effective May 18, 2026, but nuisance, noise, parking, safety, and occupancy rules still apply — and your HOA covenants, lender, and insurer each have their own position on it. Confirm all four in writing before you commit, and budget separately for furnishing, smart locks, dedicated internet, and sound isolation, none of which are in a standard ADU construction estimate.
Iron Crest provides the itemized construction estimate and milestone schedule. We do not provide rent projections, appraisal opinions, or investment advice.
Beyond the core construction budget, several often-overlooked costs can significantly impact your total ADU investment. Experienced builders account for these items during the planning phase, but homeowners who are budgeting independently should be aware of every potential expense.
Tree Removal & Protection
Boise's mature neighborhoods often have large trees that conflict with ADU placement. The City of Boise has a tree preservation ordinance that protects certain trees, particularly those in the public right-of-way and some larger specimens on private property. Tree removal permits cost $50 to $200, but the actual removal runs $500 to $3,000 per tree depending on size and access. Stump grinding adds $200 to $500. If a protected tree must remain, root protection zones may restrict where you can excavate, potentially forcing a different ADU footprint or foundation type. Budget $500 to $4,000 for tree-related costs if trees are present on your lot.
Soil Testing & Geotechnical
Boise's soils vary significantly across the city. Areas near the Boise River may have high water tables or sandy soils. Bench and North End neighborhoods often have expansive clay that requires engineered foundations. Foothills properties may sit on rocky or sloped terrain. A geotechnical soil report ($1,500 to $3,000) is required for most new construction in Boise and identifies any soil conditions that affect foundation design. Poor soil conditions can add $3,000 to $10,000 to foundation costs for deeper footings, pier-and-beam systems, or specialized drainage.
Utility Trenching Distance
The distance from your ADU to existing utility connections is one of the biggest cost variables. Water, sewer, and electrical trenching costs $30 to $75 per linear foot in Boise, depending on soil conditions, depth requirements, and whether the route crosses landscaping, concrete, or other obstacles. A detached ADU located 80 feet from the street main could face $6,000 to $12,000 in trenching costs alone. Routing utilities under an existing driveway or sidewalk requires boring ($1,500 to $3,000 per crossing) instead of open trenching. Locating the ADU closer to existing utility access points can save thousands.
Driveway & Access
If your ADU requires a new driveway extension or parking pad (particularly for garage conversions that eliminate existing parking), plan for $3,000 to $8,000 for concrete or $1,500 to $4,000 for gravel. ACHD has specific requirements for driveway approaches and may require permits for new or modified access points. Construction vehicle access to the building site can also be a hidden cost on tight Boise lots: if a crane, concrete pump, or excavator cannot reach the backyard, manual labor and smaller equipment increase construction time and cost.
Privacy Fencing & Screening
Privacy fencing or landscape screening between the ADU, the primary home, and neighboring properties is not required by code, but it is a common addition and one that is cheaper to plan than to retrofit around finished landscaping. A 6-foot cedar privacy fence runs $25 to $45 per linear foot installed in the Boise market. For a typical ADU perimeter of 80 to 120 linear feet, fencing costs $2,000 to $5,400. Alternatively, privacy landscaping (arborvitae, columnar trees, hedge plantings) costs $1,500 to $4,000 but takes time to reach full screening height. Some Boise neighborhoods have HOA requirements for fencing materials and heights that may increase costs.
Fire Sprinklers & Safety
Idaho exempts one- and two-family dwellings, including ADUs, from residential fire sprinkler requirements (Idaho Code § 39-4116 deletes IRC R313), so a sprinkler system is optional rather than required. If you choose to add one, a residential fire sprinkler system for a 600 to 800 square foot ADU typically costs $3,000 to $6,000 installed. Additional fire safety costs may include fire-rated wall assemblies for attached ADUs ($1,000 to $3,000), hardwired interconnected smoke and CO detectors ($500 to $1,000), and fire-rated windows if the ADU is close to property lines ($500 to $2,000 premium over standard windows).
Separate Address & Mail Service
A separate address is required for ADUs with separate utility meters. Boise assigns new addresses through the Planning and Development Services department. While the address assignment itself is typically free or minimal cost, you will need a separate mailbox ($100 to $500), potentially a separate address marker visible from the street ($50 to $200), and your utility accounts will need to be established separately. These small costs add up, and ensuring proper addressing is essential for emergency services, mail delivery, and tenant convenience.
Temporary Living Disruption
Unlike a kitchen remodel that may displace your daily routine, ADU construction primarily disrupts your yard and outdoor spaces. However, expect 2 to 6 months of construction activity, noise, contractor vehicles, material storage, and restricted yard access. If utility connections require temporary shutoffs to the main home (for water or sewer tie-ins), plan for brief disruptions. Construction access may require temporarily removing fence sections or landscaping. Budget $500 to $2,000 for temporary site fencing, portable restroom rental for contractors, and post-construction landscape restoration beyond what is included in the main budget.
Most Boise homeowners do not pay cash for ADU construction. Several financing options make ADU projects accessible, each with distinct advantages depending on your financial situation, timeline, and equity position.
Home Equity Line of Credit (HELOC)
A HELOC allows you to borrow against your home's existing equity with flexible draw periods. Lenders size the line against a combined loan-to-value limit applied to your home's appraised value less the outstanding mortgage balance, and that limit varies by lender — ask yours for the figure rather than assuming one. How much that leaves you depends on your own equity position, which your lender will size from a current appraisal.
- Draw funds as needed during construction
- Interest-only payments during draw period
- Lower closing costs than a full refinance
Cash-Out Refinance
A cash-out refinance replaces your existing mortgage with a new, larger mortgage and gives you the difference in cash. This works best when current interest rates are comparable to or lower than your existing rate — if your existing rate is well below market, a cash-out refinance repriced the whole balance, not just the new money, which is the trade-off to run past your lender first.
- Single fixed monthly payment
- Potentially lower rate than HELOC
- Full lump sum available at closing
Construction Loan
Construction loans are short-term loans specifically designed for building projects. They fund the project in stages (draws) as construction progresses and convert to a permanent mortgage or are paid off upon completion. Boise-area banks and credit unions write construction loans, but terms, draw schedules and whether a given lender will underwrite an ADU specifically vary — compare offers directly rather than assuming any one institution's program fits.
- Purpose-built for construction projects
- Pay interest only on funds drawn
- Converts to permanent financing at completion
ADU-Specific Loan Programs
A growing number of lenders now offer loan products specifically designed for ADU construction. These programs may factor projected rental income into your qualification, allowing homeowners who might not otherwise qualify to finance an ADU. Some programs also offer streamlined appraisal processes that account for the ADU's value upon completion rather than the property's current value.
- Rental income can help with qualification
- Completion-value appraisals available
- Designed for the unique ADU financing structure
Before You Finalize the Budget, Ask About Fee Programs
We do not publish a list of ADU incentives, because any program that exists is set by the City and changes on its own schedule — and Idaho Code § 67-6541 already caps ADU fees at no more than those charged for a single-family dwelling. Ask Boise Planning & Development Services at (208) 608-7100 whether any fee reduction, deferral, or affordability program applies to your project before you lock the budget, and get the answer in writing. If one applies, we will build it into the estimate; we will not assume one exists.
Common questions Boise homeowners ask about ADU construction costs.
How much does it cost to build an ADU in Boise, Idaho?
ADU construction in Boise runs $95,000 to $320,000. A garage conversion starts around $95,000, most projects land near $165,000, and a large detached unit with premium finishes reaches $320,000. A new detached one-bedroom of about 600 square feet typically falls between $145,000 and $220,000. Costs vary with ADU type, size, site conditions, utility connection requirements, and interior finish level.
What permits and fees are required for an ADU in Boise?
Boise requires a building permit for all ADU construction, plus separate trade permits for plumbing, electrical, and mechanical work. 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, with residential plan review adding 20 percent. On a $175,000 ADU that is roughly $1,560. The larger numbers homeowners hear quoted are impact fees and utility connection fees, which are separate charges from ACHD and the City of Boise; confirm the current amounts with each agency, because they are set by fee schedule and change.
How long does it take to build an ADU in Boise?
A typical detached ADU in Boise takes 4 to 7 months from permit application to move-in, depending on complexity and weather conditions. The City of Boise publishes no guaranteed plan-review turnaround; across our own Boise ADU projects it has typically run 3 to 6 weeks, and an incomplete submittal restarts part of the clock — confirm the current queue with Planning & Development Services at (208) 608-7100. Construction then runs about 3 to 5 months. Garage conversions are faster, often completing in 2 to 4 months of construction time. Winter weather can slow foundation and exterior work, so many Boise homeowners start the permitting process in late fall to begin construction in early spring.
Will building an ADU increase my property taxes in Boise?
Yes. Ada County reassesses your property after ADU construction is complete and a certificate of occupancy is issued. We do not publish an estimate of the added assessed value or the resulting tax figure — the assessment is parcel-specific and the levy rate is set annually by the taxing districts, so any number we printed would be a guess. Ask the Ada County Assessor what the reassessment is likely to look like for your parcel, and check whether the ADU affects the homeowner's exemption on your primary dwelling. Whatever that number turns out to be, subtract it from rent before you call the project cash-flow positive.
Can I finance an ADU construction project in Boise?
Yes, several financing options are available for ADU construction in Boise. The most common are home equity lines of credit (HELOCs), cash-out refinances, and construction-to-permanent loans. Interest rates, terms, and the share of construction cost a lender will advance vary by lender and by your equity position, so compare offers directly. Iron Crest Remodel is not a lender and does not arrange or recommend financing — what we provide is the itemized budget, construction documentation, and milestone draw schedule that renovation lenders ask for.
Cost is just one piece of the ADU planning puzzle. Explore our other ADU guides and resources to make fully informed decisions about your project.
ADU Construction Service
Our full ADU construction service overview
ADU ROI in Boise
What actually drives the return — rent, drag, taxes
Detached ADU Construction
Ground-up standalone units on a Boise lot
Attached ADU Additions
Additions that share a wall with the main home
Garage Conversion ADU
Lowest-cost path using an existing structure
ADU Permits Guide
Boise ADU permit process and requirements
ADU Utility Hookups
Water, sewer, and electrical connection details
Rental-Ready ADUs
Design features that maximize rental income
ADU Floor Plans
Popular layouts for Boise ADUs
Cost Guides Hub
Cost guides for all remodeling services
Get a Free Estimate
Request your personalized ADU construction quote
Financing Options
How Boise homeowners fund ADU projects (Iron Crest is not a lender)
ADU Construction reads differently in each Boise neighborhood — different housing era, different permit overlay, different finish-tier expectations. 9 neighborhood-specific guides:
Plan Your ADU Build
Every property is different. Contact Iron Crest Remodel for a free, detailed ADU construction estimate tailored to your Boise lot, your goals, and your budget.