Maybe you want to house a parent without giving up your privacy. Maybe you’re eyeing rental income on the home you already love. Or maybe you just want a proper guest suite that doesn’t double as a storage room. Whatever the reason, adding an accessory dwelling unit is one of the most strategic things a Treasure Valley homeowner can do in 2026.
The question everyone gets stuck on: what does it actually cost? Numbers online vary wildly, and that confusion is common. According to Freddie Mac, 71% of consumers aren’t familiar with ADUs at all. And a new Idaho state law, SB 1354, signed March 31, 2026, is about to make ADUs easier to build than ever before. This guide cuts through the noise with real Boise-specific numbers and everything you need to plan confidently.
Key Takeaways
- ADU costs in Boise range from $80,000 (garage conversion) to $300,000 (detached unit), depending on type and finish level (based on Treasure Valley projects, 2026).
- Idaho SB 1354, effective July 1, 2026, requires all cities over 10,000 residents to permit at least one ADU per single-family lot — with no owner-occupancy restrictions.
- Permit fees alone in Boise run $11,500–$28,000 all-in (City of Boise Development Services, 2026).
- A 1-bedroom Boise ADU can generate $1,200–$1,700 per month in long-term rental income.
- ADUs increase home value 25–34% in strong rental markets (Freddie Mac, January 2023).
How Much Does an ADU Cost in Boise in 2026?
In Boise and the broader Treasure Valley, ADU costs run from $80,000 for a garage conversion to $300,000 for a fully detached unit, based on projects built across the region. Nationally, the average ADU lands around $180,000 (Mesocore, November 2025). The wide range reflects real variables: project type, lot conditions, finish quality, and where in the Treasure Valley you build.
The range isn’t just noise. A garage conversion and a brand-new detached cottage are completely different scopes of work, even if they result in a similar square footage. Understanding where you land before you talk to a contractor saves you from sticker shock — and from underbuilding something you’ll wish you’d done right.
Here’s the Boise-specific breakdown at a glance, for units between 400 and 900 square feet (based on Treasure Valley projects, 2026):
| ADU Type | Boise Cost Range |
| Garage Conversion | $80,000 – $180,000 |
| Attached Addition | $120,000 – $250,000 |
| Above-Garage ADU | $130,000 – $250,000 |
| Detached ADU | $150,000 – $300,000 |
These are build costs. Permits, design fees, and site prep are on top — and we’ll break those out shortly.
How Does ADU Type Affect Cost in the Treasure Valley?
ADU type is the single biggest cost driver in any project. The structure you start with, or choose to build from scratch, determines your foundation costs, utility extension needs, permitting category, and timeline. Here’s how each type pencils out for Treasure Valley homeowners.
Garage Conversions: $80,000–$180,000
Garage conversions are the most popular entry point, and for good reason. The slab is already there. The walls are standing. You’re adding insulation, framing interior walls, running plumbing and electrical, and finishing the space — not building from scratch. Nationally, garage conversions run $60,000–$120,000 (Mesocore, November 2025), but Boise’s labor market and material costs push that range to $80,000–$180,000 (based on Treasure Valley projects, 2026).
This makes sense when your garage is already attached, has decent ceiling height, and sits on a lot where a detached structure would require significant site work. The trade-off: you lose the garage.
Interior and Basement Conversions: $50,000–$120,000 (National Range)
If your home has an existing basement or a large bonus room, converting it into a legal ADU is often the most cost-contained path. Nationally, these run $50,000–$120,000 (Mesocore, November 2025). Local costs in Boise will vary based on what’s already finished, whether a separate entrance exists, and how much plumbing and egress work is needed. Don’t assume a finished basement is close to ready — code compliance for a legal ADU involves a different standard than a rec room.
Attached ADUs: $120,000–$250,000
An attached ADU shares at least one wall with the primary home. That shared wall simplifies the plumbing run considerably, and the ADU ties into existing mechanical systems more easily. In Boise, expect $120,000–$250,000 for this type (based on Treasure Valley projects, 2026). These are effectively additions with their own entrance, kitchen, and bath — and they’re well-suited to multigenerational living where some connection between units is welcome.
For a comparison on other types of home additions in the same cost range, see second story addition costs in Boise and Meridian. If a smaller-footprint expansion fits your situation better, a bump-out addition is worth considering first.
Detached ADUs: $150,000–$300,000
The detached ADU — a fully separate structure on the same lot — commands the highest cost and the highest income potential. Nationally, per-square-foot costs run $200–$400 (SelfStorage.com, April 2026). In Boise, that translates to $150,000–$300,000 for a finished unit (based on Treasure Valley projects, 2026). You’re pouring a new foundation, running all-new utilities, and building a complete home from the ground up. But you’re also creating something with maximum privacy, maximum rental appeal, and maximum long-term value.
What Drives ADU Cost Beyond the Build?
Permit fees alone in Boise run $11,500–$28,000 all-in, based on the City of Boise’s published fee schedule. That number surprises many homeowners, who budget for the structure but forget the fees sitting in front of it. The permit costs are real, unavoidable, and worth planning for before you fall in love with a floor plan.
Here’s how the fee components break down:
| Fee Category | Boise Range |
| Plan Check | $1,500 – $3,000 |
| Building Permit | $1,500 – $3,500 |
| Impact Fees | $5,000 – $10,000 |
| Utility Connections | $2,000 – $8,000 |
| Total | $11,500 – $28,000 |
(City of Boise Development Services, 2026)
Beyond permits, here are the other cost categories that shape the final number.
Site Work and Utility Extensions
Site work for a detached ADU, including grading, utilities, and foundation preparation, runs $15,000–$40,000 (Mesocore, November 2025). If your lot has challenging grade, mature trees to work around, or a long run from the street to the new structure, this line item climbs fast. An experienced site assessment early in the process protects your budget later.
Design and Architecture Fees
Design and architecture typically runs 10–15% of total project cost (Mesocore, November 2025). For a $200,000 detached ADU, that’s $20,000–$30,000. It’s real money, but it’s also where you avoid the costly change orders that happen when a project starts construction before the design is fully resolved.
Boise offers a shortcut worth knowing about: the city provides eight free pre-approved plan sets named after local birds (Goldfinch, Waxwing, Kingfisher, Kestrel, Sandpiper, Osprey, and others), ranging from 280 to 695 square feet. Using one of these saves 4–7 weeks compared to a custom design, and eliminates most of the plan-check back-and-forth. It’s not the right fit for every lot or every client, but for a clean rectangular backyard with straightforward utility access, it’s a genuinely good option.
Labor: The Largest Single Line Item
Labor accounts for roughly 40% of total ADU project cost (Mesocore, November 2025). In a market like Boise, where demand for skilled trades remains high, labor costs have stayed elevated. That’s embedded across every phase: framing, plumbing, electrical, HVAC, and finish work.
Interior Finishes
Interior finishes run $40,000–$80,000 for a typical detached ADU (Mesocore, November 2025). This is the category where most homeowners have the most control. Choosing builder-grade tile versus custom stone, or stock cabinetry versus semi-custom, can move this number meaningfully. The risk is cutting quality in ways that affect rental appeal or long-term durability.
The single most common budget surprise we see on Boise ADU projects isn’t the permitting fees — homeowners have started to expect those. It’s the MEP rough-in: running new water, sewer, and electrical from the main home to a detached structure across a finished yard. Once you add trenching through existing landscaping, the connection costs can run $15,000–$25,000 before a single wall goes up. Getting a utility-run estimate in the first site visit changes the whole project math.
What Are the ADU Rules in Boise, Meridian, and Eagle?
Idaho SB 1354, signed March 31, 2026 and effective July 1, 2026, is the most significant ADU law change in Idaho history. Every city over 10,000 residents, including Boise, Meridian, Nampa, and Caldwell, must permit at least one ADU per single-family lot as-of-right, with no owner-occupancy restrictions and a minimum allowed size of 1,000 square feet (Steel Structures America, May 2026). Cities have until February 1, 2027 to update their local codes to comply.
Why does this matter right now? Because homeowners who start planning today can build under the current rules and position themselves for the full flexibility the new law provides before most of their neighbors even know it exists.
Here’s where the rules stand in each jurisdiction today.
Boise
ADUs are permitted in all residential zones in Boise. The current maximum size is 900 square feet or 50% of the primary dwelling’s living area, whichever is less. The minimum is 200 square feet, with a two-bedroom cap. Detached ADUs have a 25-foot height limit, with rear setbacks of 5 feet for single-story and 10 feet for two-story structures. Dedicated parking is no longer required, thanks to Boise’s 2023 code update.
Owner-occupancy rules currently require that either the primary home or the ADU be owner-occupied. SB 1354 removes this requirement when the new state law fully takes effect. Permit timelines run 3–5 weeks with a pre-approved city plan, or 7–12 weeks for a custom design.
For questions, contact Boise Development Services directly: zoninginfo@cityofboise.org or (208) 608-7100.
Meridian
Meridian mirrors Boise’s core rules: maximum 900 square feet or 50% of primary floor area, two-bedroom cap. One wrinkle: if your address says “Meridian” but your parcel sits in unincorporated Ada County, your permit goes through Ada County Development Services, not the City of Meridian. These are genuinely different processes with different requirements. Always verify your parcel’s jurisdiction before starting an application. Contact Meridian Community Development at (208) 884-5533.
Eagle
Eagle requires one off-street parking space for an ADU. It also enforces architectural design standards: your ADU must comply with one of nine architectural themes in the Eagle Architecture and Site Design Book. Owner-occupancy is required for either the primary home or the ADU, by the owner or immediate family. Contact Planner Rick Butler at (208) 489-8817.
Ada County (Unincorporated)
If your property sits in unincorporated Ada County, the rules are tighter: maximum size is 20% of lot area, the ADU is limited to one bedroom, and owner-occupancy is required. The restriction is recorded with the County Recorder and runs with the land. Source: adacounty.id.gov/developmentservices.
What SB 1354 Changes Starting July 2026
Starting July 1, 2026, cities over 10,000 residents can no longer impose owner-occupancy requirements or restrict ADUs below 1,000 square feet (or 75% of the primary dwelling, whichever is larger). All approvals must be administrative and as-of-right, meaning no discretionary review or neighborhood hearings (Steel Structures America, May 2026). This is a meaningful change for Eagle and Ada County homeowners who currently face the most restrictive local rules.
What made Boise’s ADU environment attractive before this law? The numbers: ADU applications in Boise jumped 138% in the 3.5 months after the city’s December 2023 zoning code update, from 40 applications in the prior 11 months to 95 in the following 3.5 months (KTVB/Idaho Press, 2024). SB 1354 extends that momentum statewide.
Sam, our project manager, puts the timing reality plainly: “The biggest variable we see in Treasure Valley ADU permits isn’t the city review itself — it’s utility connection coordination. In Boise, a pre-approved plan can clear plan check in 3–4 weeks, but scheduling the actual utility tie-in with the relevant district adds weeks on the back end that homeowners don’t anticipate.” In Eagle, the architectural review adds another cycle that Boise doesn’t require. Starting the permit application while Erin is still finalizing design plans is usually the right move for keeping the overall timeline tight.
For homeowners also considering structural additions — like building up rather than out — see our overview of home addition options for different home types.
What Rental Income Can a Boise ADU Generate?
A 1-bedroom ADU in Boise can generate $1,200–$1,700 per month in long-term rental income, against a Boise median apartment rent of $1,513 per month (RentCafe, 2026). That puts a well-built ADU squarely in the current Boise rental market, not below it.
Bright modern ADU interior with natural light streaming through the window
Long-Term Rental Income Scenarios
Here’s how Boise ADU rental income stacks up by unit size (based on Treasure Valley projects, 2026):
| Unit Type | Monthly Rent Range |
| Studio | $900 – $1,300/month |
| 1-Bedroom | $1,200 – $1,700/month |
| 2-Bedroom | $1,500 – $2,200/month |
Nationally, detached ADUs generate around $39,000 per year in rental income, attached ADUs around $27,600, and garage conversions around $24,000 (Mesocore, November 2025). Boise’s rental market is strong enough to support the higher end of those ranges.
Short-Term Rental Potential
If you’re open to short-term or Airbnb rentals, Boise ADUs can generate $2,000–$4,000 per month seasonally (based on Treasure Valley projects, 2026). The Treasure Valley’s growth as a destination, combined with steady event-driven demand, makes short-term rental a real option for well-located properties.
How the Numbers Work: A Simple Payback Model
A detached ADU at $225,000 total cost, generating $1,500 per month in rent, produces $18,000 per year. At that rate, the unit reaches simple payback in roughly 12.5 years on rental income alone.
The home value math often closes the gap faster. ADU-equipped homes have been shown to sell at a 35% premium, and ADUs increase overall home value 25–34% in strong rental markets (Freddie Mac, January 2023). For a $600,000 Treasure Valley home, a 25% value increase adds $150,000. That changes the payback picture significantly.
Is an ADU the right move compared to just buying a bigger house? That’s worth thinking through carefully. See our full analysis of remodeling vs. buying a bigger home for a detailed comparison.
Ready to Plan Your ADU?
CCH Design & Remodel has helped homeowners across Meridian, Boise, Eagle, Nampa, and the broader Treasure Valley turn ideas like these into real spaces. Our in-house designer Erin and project manager Sam have seen what works on Treasure Valley lots — and what to avoid before the permit application goes in. If you’re thinking about an ADU and want a straight conversation about what it would take on your specific property, reach out for a free consultation.
Frequently Asked Questions
How much does an ADU cost in Boise in 2026?
In Boise, ADU costs range from $80,000 for a garage conversion to $300,000 for a fully detached unit, for 400–900 square foot structures (based on Treasure Valley projects, 2026). The national average across all ADU types is approximately $180,000 (Mesocore, November 2025). Permit fees, design, and site work add $11,500–$60,000 on top of build costs.
What’s the cheapest type of ADU to build?
Garage conversions are consistently the most cost-contained ADU type. In Boise, they run $80,000–$180,000 because the slab, framing, and roof are already in place (based on Treasure Valley projects, 2026). Nationally, garage conversions range from $60,000–$120,000 (Mesocore, November 2025). Interior or basement conversions can also be cost-effective when an existing structure is already close to code-compliant.
Do I need a permit for an ADU in Boise?
Yes, all ADUs in Boise require a building permit. The all-in permit cost runs $11,500–$28,000, covering plan check, building permit, impact fees, and utility connections (City of Boise Development Services, 2026). Using one of Boise’s eight free pre-approved plan sets (named after local birds) can shorten the plan-check period by 4–7 weeks compared to a custom design.
What does Idaho’s new 2026 ADU law (SB 1354) mean for homeowners?
Idaho SB 1354, signed March 31, 2026 and effective July 1, 2026, requires all cities over 10,000 residents to permit at least one ADU per single-family lot with no owner-occupancy restrictions (Steel Structures America, May 2026). ADUs cannot be restricted below 1,000 square feet, and approvals must be administrative, with no discretionary review. Cities have until February 1, 2027 to update local codes.
How long does it take to build an ADU in Boise?
Permitting alone runs 3–5 weeks using Boise’s pre-approved plan sets, or 7–12 weeks for a custom design. Construction time depends heavily on ADU type: garage conversions typically take 3–5 months, while new detached ADUs run 6–10 months from permit approval to occupancy. Utility connection scheduling and material lead times are the most common sources of delay in the Treasure Valley.
Will an ADU increase my home’s value?
Yes, consistently. ADU-equipped homes sell at a 35% premium, and ADUs increase home value 25–34% in strong rental markets (Freddie Mac, January 2023). In a market like Boise, where rental demand has driven a 138% jump in ADU applications after the 2023 code update (KTVB/Idaho Press, 2024), that value premium reflects real buyer appetite.
Can I build an ADU if I live in Meridian or Eagle?
Yes, both cities permit ADUs. Meridian allows ADUs up to 900 square feet (or 50% of primary floor area) with a two-bedroom maximum, and is worth confirming your parcel is inside city limits versus unincorporated Ada County. Eagle requires one parking space per ADU and compliance with nine architectural style standards. Both cities will need to comply with SB 1354 by February 2027. Contact Meridian Community Development at (208) 884-5533 or Eagle’s Planning Department at (208) 489-8817.
What’s the difference between a detached ADU and a garage conversion?
A detached ADU is a fully separate structure, built from a new foundation. It’s the most expensive type ($150,000–$300,000 in Boise) but offers maximum privacy and rental income potential (based on Treasure Valley projects, 2026). A garage conversion repurposes an existing structure and typically costs less ($80,000–$180,000 in Boise), but it means losing your garage. The right choice depends on your lot, your goals, and your timeline.
Sources
1. Mesocore. “35 ADU Cost Statistics.” November 2025. Retrieved May 2026. https://www.mesocore.com/blog/adu-cost-statistics
2. SelfStorage.com. “ADU Construction Cost.” April 2026. Retrieved May 2026. https://www.selfstorage.com/blog/adu-construction-cost/
3. City of Boise Development Services. “Planning & Development Fee Schedule.” 2026. Retrieved May 2026. https://www.cityofboise.org/departments/planning-and-development-services/
4. City of Boise. “Pre-Approved ADU Plans.” 2026. Retrieved May 2026. https://www.cityofboise.org/departments/planning-and-development-services/planning/boise-zoning-code/pre-approved-adu-plans/
5. Freddie Mac. “ADUs Can Increase Housing Stock.” January 2023. Retrieved May 2026. https://www.freddiemac.com/research/consumer-research/20230124-adus-can-increase-housing-stock-most-are-unfamiliar
6. Steel Structures America. “Idaho ADU Law SB 1354.” May 2026. Retrieved May 2026. https://www.steelstructuresamerica.com/idaho-adu-law/
7. KTVB / Idaho Press. “Dwelling Applications Jump After Boise Code Rewrite.” 2024. Retrieved May 2026. https://www.ktvb.com/article/news/local/idaho-press/dwelling-applications-jump-boise-code-rewrite/277-f917abe4-f677-423f-9673-dc8f20630115
8. RentCafe. “Average Rent and Market Trends — Boise City, Ada County, Idaho.” 2026. Retrieved May 2026. https://www.rentcafe.com/average-rent-market-trends/us/id/ada-county/boise-city/
9. Shovels.ai. “ADU Boom: 2.8M Permits.” October 2025. Retrieved May 2026. https://www.shovels.ai/blog/adu-boom-2.8M-permits/
