Detached ADU
A standalone backyard cottage on its own foundation. Highest privacy, highest rent ceiling, longest build. Most of Boise's pre-approved plans are detached.
Boise ADU Guide · 2026
Complete Guide to Building an ADU in Boise
Updated: May 2026
Boise raised the ADU size cap from 700 to 900 sq ft in 2023 and dropped owner-occupancy and parking minimums at the same time — then released six pre-approved plans in March 2026 that permit in weeks instead of months. This guide covers the current rules, the six plans, real cost ranges, financing paths, and how Boise owners actually use the units.
Table of contents
What's in this guide
Eleven sections covering the entire path from zoning question to keys in your hand. Each link jumps straight to the section.
What an ADU actually is
An accessory dwelling unit (ADU) is a complete, independent second home on a lot that already has a primary single-family house. It has its own kitchen, its own bathroom, its own sleeping area, and its own entrance. In Boise, only one ADU is allowed per lot, and the primary house has to exist first — you can't build an ADU on raw land.
Four common shapes show up across Boise builds. Pick based on your lot, your budget, and what you're trying to do with the unit.
Attached / above-garage ADU
Shares one or more walls with the primary home — most commonly a garage conversion or an above-garage unit. Cheaper utility runs and lower foundation cost than a detached build. Capped at 33% of the primary dwelling's total living area in Boise.
Garage conversion
Converts an existing attached or detached garage into a dwelling. Cheapest path in most cases ($80k–$150k per Iron Crest), but you lose the garage.
Backyard cottage
The Boise-friendly term for a small detached ADU on its own footprint. Same code, same permit path — different marketing word.
Across all four shapes, the same Boise rules apply — 900 sq ft max, 2-bedroom max, one ADU per lot, and the unit has to look like single-family residential construction from the street. The big functional difference is who handles the foundation, the utility runs, and the loss (or preservation) of your garage. We walk through each option against your specific lot in the free lot check.
Boise zoning & the 2023 rewrite
Boise's Modern Zoning Code took effect December 1, 2023 and made three changes that materially expanded what's possible with ADUs. Anything you read from before that date — and a surprising amount of third-party content still online — is using stale rules. The current rules in plain English:
Size cap raised to 900 sq ft
Up from the previous 700 sq ft cap. The actual rule is 900 sq ft OR 50% of the primary dwelling's living area, whichever is less. Maximum 2 bedrooms.
Owner-occupancy removed
You no longer have to live on the property. Investors can buy a Boise property with an ADU and rent both the primary house and the ADU as separate income streams.
Parking minimum removed
The mandatory off-street parking space for ADUs was eliminated. Most owners still build one in for tenant convenience and resale, but it's no longer a code trigger.
The effect was immediate. Per BoiseDev's analysis of city permit data, ADU permit applications jumped from roughly 3 per month in the year before the rewrite to 27 per month in the months after — a 9× surge. The city averaged 61 ADUs per year from 2019 through 2023; 130 ADUs were built or approved in 2024 alone, with 82 more in 2025.
Allowed zones, setbacks, height, and lot coverage
ADUs are allowed in all residential zoning districts in Boise — R-1A, R-1B, R-1C, R-2, and the rest. R-1B is the most common residential designation for Boise ADU builds. Setbacks vary by zone: R-1C uses a 5 ft side and rear setback; R-1A and R-1B require a 13 ft interior side setback. Front setbacks generally run 20–25 ft. A detached ADU must sit at least 6 ft from the primary dwelling.
Maximum ADU height is 25 ft or the primary home's height, whichever is less. Lot coverage caps depend on zone — R-1B is 40%, R-1C is 50%. Historic overlays (the North End, Hyde Park) add design review through the Historic Preservation Commission. Hillside and floodplain overlays trigger their own reviews. If your lot sits under any of these, plan for an extra cycle.
The deep dives live in dedicated articles — Boise ADU laws in 2026, ADU setbacks, the 900 sq ft size limit, and the full feasibility framework for figuring out whether your specific lot qualifies before you spend a dollar on design.
The 6 pre-approved Boise ADU plans
On March 17, 2026 the City of Boise published a catalog of free, pre-approved ADU plans designed by Parke Architecture (Eamonn Parke, AIA). Six plans are downloadable today, plus two mirrored variants — Sandpiper B and Osprey B — listed as "Coming Soon." Using one of these plans saves roughly 4–8 weeks of permit review and roughly $10,000 in architectural design fees, because the city has already vetted the structural, energy, and life-safety details.
You can build any of the six on a qualifying Boise lot. The full catalog and the city's permit guide live on the floor plans hub. Walk-through summaries below — click any name for the full plan page with floor layout, photos, and turnkey pricing.
The Goldfinch
From $115kType A · 280 sq ft · Studio · 1 bath
The smallest pre-approved Boise ADU plan — a 280 sq ft studio that fits the tightest lots and the cleanest budgets.
The Waxwing
From $135kType B · 396 sq ft · Studio · 1 bath
A 396 sq ft studio with more breathing room — full living, kitchen, and bath in a thoughtfully laid-out single space.
The Kingfisher
From $170kType C · 491 sq ft · 1 bed · 1 bath
Boise's pre-approved 491 sq ft one-bedroom — separate bedroom, full kitchen, full living room, ready to permit.
The Kestrel
From $245kType D · 695 sq ft · 2 bed · 1 bath
The largest pre-approved Boise plan — a 695 sq ft two-bedroom that lives like a small house.
The Sandpiper
From $145kType G-1 · 396 sq ft · Studio · 1 bath
A 396 sq ft studio paired with a single-car garage — pre-approved, dual-purpose, builds your storage and your rental in one footprint.
The Osprey
From $140kType G-2 · 376 sq ft · Studio · 1 bath
Sister plan to the Sandpiper — 376 sq ft studio with garage, a different layout, same pre-approved status.
For the comparison framework — pre-approved vs custom, when each wins, what you give up — see pre-approved vs custom ADU plans and the dedicated pre-approved plans landing page.
Cost — pricing by plan and finish
Turnkey Boise ADU pricing in 2026 starts at $115k for the smallest pre-approved studio at Essential finish and tops out around $345k for the largest 2BR at Premium finish. Three finish tiers drive most of the spread.
Essential
$340/sq ft
Durable, builder-grade finishes throughout. LVP floors, laminate counters, basic appliance package. Best fit for long-term rental where rent doesn't reward upgrades.
Standard
$405/sq ft
Quartz counters, LVP floors, mid-range fixtures, upgraded appliance package. The most common pick — best ROI for most Boise rental builds.
Premium
$480/sq ft
Tile baths, designer cabinets, hardwood floors, upgraded windows. Right call for owner-occupied, multigen, or aging-parents builds where you live with the finish daily.
Turnkey range by pre-approved plan (Standard finish)
| Plan | Size | Beds | Standard finish |
|---|---|---|---|
| The Goldfinch | 280 sq ft | Studio | $102k–$127k |
| The Waxwing | 396 sq ft | Studio | $144k–$180k |
| The Kingfisher | 491 sq ft | 1BR | $179k–$223k |
| The Kestrel | 695 sq ft | 2BR | $253k–$315k |
| The Sandpiper | 396 sq ft | Studio | $144k–$180k |
| The Osprey | 376 sq ft | Studio | $137k–$171k |
Full Essential / Standard / Premium breakdown on the Boise ADU cost page. Run your own numbers with the cost calculator. For investment math — gross yield, cash-on-cash, payback — use the ADU ROI calculator.
Iron Crest published cost ranges (third-party reference)
Iron Crest Remodel — the licensed builder partner on BoiseADU projects — publishes their own ADU pricing on ironcrestremodel.com. Useful as an independent reality check:
- Garage conversion: $80,000 – $150,000
- Studio ADU (400–500 sq ft): $120,000 – $200,000
- 1-bedroom ADU (500–800 sq ft): $180,000 – $280,000
- 2-bedroom ADU (800–1,000 sq ft): $250,000 – $350,000+
- Utility connections: $15,000 – $40,000 (sewer $8,000–$15,000, electrical $3,000–$6,000)
Note: the 2BR band straddles the 900 sq ft Boise cap. On a detached new-build the legal max is 900 sq ft; the upper end of that band applies to attached or above-garage configurations where the 50% primary-dwelling rule allows more.
Financing your Boise ADU
Four common financing paths, ordered roughly by cost of capital for a Boise homeowner with existing equity. Rates current as of May 2026 — verify with the lender before committing.
HELOC (home equity line)
6.25% APR (ICCU, May 2026)
Cheapest path if you have existing equity. Variable rate, interest-only payments during the draw period. Idaho Central Credit Union is the lowest publicly listed Idaho HELOC rate; Mountain America Credit Union starts at 7.25%.
Construction loan
6.75% (MACU, May 2026)
Disbursed in draws as the build hits milestones. Converts to a permanent mortgage on completion. Best fit when you don't have enough equity for a HELOC. Zions Bank and WaFd both offer single-close construction-to-permanent loans (rates by quote).
Cash-out refinance
~7.0%–7.2% (Idaho avg)
Rolls the ADU into a new primary mortgage. Resets your existing mortgage rate — only worth doing if your current rate is at or above today's market. Full underwrite, full closing costs.
Renovation loan
Varies by program
FHA 203(k) and Fannie HomeStyle let you finance the build inside the purchase or refi mortgage. Useful when you're buying a home with the intent to add an ADU; less commonly used by existing owners.
The Bankrate Idaho average for a 30-year fixed purchase mortgage sits at 6.88% and a 30-year refinance at 6.69% for the same week. Cash-out refis typically run 0.25–0.50% above standard refi pricing. ICCU's HELOC at 6.25% is the lowest publicly listed rate for an Idaho borrower at time of capture.
Full lender comparison, qualification thresholds, and payment-shock math live on the financing page.
Rates current as of May 2026; verify with lender. BoiseADU is not a mortgage broker and does not originate loans.
Permits + build timeline
The single biggest timeline lever in a Boise ADU project is whether you use a pre-approved plan or go custom. Pre-approved permits in 3–5 weeks; custom takes 7–12 weeks through Boise Planning and Development Services. That gap is roughly 4–8 weeks of calendar time and roughly $10,000 in architectural fees — the explicit savings the city designed the pre-approved program around.
Once you sign a build contract, the work moves through five stages. Total contract-to-keys runs 5–8 months for a pre-approved plan and 7–9 months for a custom design on a typical Boise lot.
1. Free lot check + consultation
Confirm zoning, setbacks, utility access, and the plan that fits the buildable envelope. Typically 1–2 weeks from inquiry to signed scope.
2. Design & engineering
Pre-approved plans skip most of this. Custom design adds 4–8 weeks for architectural and structural work. Engineering for soils and snow load happens here for both paths.
3. Permit submittal & approval
Pre-approved plans permit in 3–5 weeks. Custom plans take 7–12 weeks through Boise PDS. Plan check ($1,500–$3,000) plus impact and connection fees ($8,000–$15,000) land in this stage.
4. Construction
Site work, foundation, framing, MEP rough-in, insulation, drywall, finishes. A detached pre-approved plan typically frames-to-finish in 4–6 months on a clean lot.
5. Final inspection & move-in
City sign-offs, certificate of occupancy, utility activation, and walkthrough. Most owners are turning a key 5–8 months from contract signing.
The detailed permit walkthrough — submittals, plan check, common corrections — is in the Boise ADU permit process article. Where timelines actually slip is covered in how long does it take to build an ADU in Boise. Utility-side issues (sewer laterals, service upgrades, connection fees) are the most common surprise — covered in Boise ADU utilities. The full BoiseADU build process lives on the process page.
How Boise owners actually use ADUs
Five patterns cover most of what we see across Treasure Valley ADU projects. Pick the use case before you pick the plan — what you're optimizing for changes which configuration wins.
Long-term rental income
The most common use. A 491 sq ft Kingfisher 1BR rents in line with Boise 1BR medians ($1,479–$1,503 per Zillow / Apartment List). Iron Crest's published estimate is conservative at $1,100–$1,500. Owner-occupancy is no longer required, so investors can buy a property and rent both units.
Aging parents
Boise assisted living runs $3,125–$4,488/month per Genworth 2024 — $37,500–$53,856/year. A $180,000 1BR ADU offsets roughly 3.5–4.8 years of facility cost, and the asset stays with the property afterward. Single-floor pre-approved plans like the Kingfisher and Kestrel work well for accessible layouts.
Short-term rental (Airbnb)
Boise allows STRs but requires a separate STR permit, and the city tracks them. Run the numbers honestly — STR gross is higher, but cleaning, vacancy, platform fees, and management cut deep. Many owners net more on long-term tenants once they price all the costs.
Multigenerational housing
Grown kids, in-laws, returning adult children. The Kestrel (695 sq ft, 2BR) is the only true two-bedroom in the pre-approved set and the standard pick for a multigen ADU. Attached configurations save on utility runs when you're keeping it in the family.
House hacking
Live in one unit, rent the other, let the tenant cover most of the mortgage. The owner-occupancy removal in the 2023 rewrite means you don't have to live on-site forever — start as a house hack, move out, run both units as rentals later.
Picking a builder
ADU work isn't standard custom-home work and it isn't standard remodeling — it's a small full-build on a constrained lot with shared utilities. Five things to check before signing with anyone:
- Current Idaho RCE license. Verify at dopl.idaho.gov, don't take the website's word for it.
- $1M+ general liability insurance. Ask for a current certificate naming the project address.
- Written workmanship warranty. A real warranty document, not a sentence in the proposal. 1 year is minimum, 5 years is the bar.
- Fixed-price contract. Cost-plus on an ADU is how budgets blow up. Insist on fixed price with documented allowances.
- Actual ADU experience. Not "we do additions." Ask for ADU project addresses you can drive past.
BoiseADU partners with Iron Crest Remodel (Idaho contractor license RCE-6681702) for design and construction. Iron Crest carries $2M general liability insurance, a 5-year workmanship warranty, and operates out of 950 W Bannock St in downtown Boise. Full builder context on the Boise ADU builder page and the About page.
Cities we serve
We build across the Treasure Valley. Each city has its own wrinkles — Garden City and the smaller jurisdictions can run different impact fee schedules, and a few have their own ADU ordinances on top of state code. City-specific guides:
Boise ADU — frequently asked
The questions BoiseADU and Iron Crest field most often. Cross-referenced against the City of Boise ADU program and the 2023 Modern Zoning Code.
How much does it cost to build an ADU in Boise?
A pre-approved Boise ADU costs $115k–$345k turnkey in 2026 depending on plan size and finish. The 280 sq ft Goldfinch studio starts at $115k; the 695 sq ft Kestrel 2BR at Premium finish runs to $345k. Most homeowners land in the $170k–$245k zone with a 1-bedroom Kingfisher at Standard finish. Custom (non-pre-approved) ADUs typically run $200k–$450k. Numbers include site work, foundation, framing, MEP, insulation, drywall, finishes, permits, and impact fees.
How big can an ADU be in Boise in 2026?
The maximum ADU size in Boise is 900 sq ft, raised from 700 in the 2023 Modern Zoning Code. The actual cap is 900 sq ft OR 50% of the primary dwelling's living area, whichever is less. Attached ADUs are also capped at 33% of the primary's total living area. Maximum 2 bedrooms.
Do I have to live in the main house to have an ADU in Boise?
No. The 2023 Modern Zoning Code removed Boise's owner-occupancy requirement. An investor can buy a property with an ADU and rent both the primary home and the ADU as separate income streams. This is one of the biggest changes from the pre-2023 rules.
How long does the Boise ADU permit process take?
Pre-approved plans permit in 3–5 weeks. Custom ADUs typically take 7–12 weeks through Boise Planning and Development Services. Total contract-to-keys is 5–8 months for a pre-approved plan and 7–9 months for custom on a typical Boise lot.
Can I Airbnb my Boise ADU?
Yes, but you need a separate short-term rental permit from the City of Boise, and STR activity is tracked. Run the math honestly — STR gross income beats long-term, but cleaning, vacancy, platform fees, and management overhead cut deep. Many Boise ADU owners end up netting more on long-term tenants.
What are the pre-approved ADU plans in Boise?
Boise launched a pre-approved ADU plan catalog on March 17, 2026, designed by Parke Architecture. Six plans are downloadable today: the Goldfinch (280 sq ft studio), Waxwing (396 sq ft studio), Kingfisher (491 sq ft 1BR), Kestrel (695 sq ft 2BR), Sandpiper (396 sq ft studio + garage), and Osprey (376 sq ft studio + garage). Two mirrored variants (Sandpiper B, Osprey B) are listed as coming soon. Using a pre-approved plan saves roughly 4–8 weeks of permitting and about $10,000 in custom design fees.
What's the typical ROI on a Boise ADU rental?
Iron Crest publishes a 75–130% 10-year effective ROI for a 1BR ADU, combining rental income and property value lift. Gross rental yield on a pre-approved 1BR Kingfisher pencils in the 9–12% range against Boise 1BR median rents. Cash-on-cash return depends on financing — a HELOC-funded build at 6.25% pencils very differently from a cash build.
Who builds ADUs in Boise?
BoiseADU partners with Iron Crest Remodel (Idaho contractor license RCE-6681702) for design-build delivery. Iron Crest carries $2M general liability insurance and a 5-year workmanship warranty, operates out of 950 W Bannock St in downtown Boise, and serves the full Treasure Valley. The right builder for any ADU project carries a current Idaho RCE license, ADU-specific experience, fixed-price contracts, and a written workmanship warranty.
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