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2026 Cost Guide · Updated July 2026

ADU Cost in Idaho — Boise, Meridian, Nampa & Statewide

City-by-city ADU cost data across Idaho's major markets. Built from permit records, contractor data, and regional labor rates.

$150K–$290K Detached ADU range
$65K–$150K Internal ADU (lowest cost)
SB 1354 Took effect July 1, 2026 — one week ago
One ADU/lot Choose internal OR detached, not both
Idaho Northwest-modern ADU exterior
Idaho ADU kitchen interior Idaho ADU living room interior
At a Glance

Idaho ADU costs — by type.

Detached ADU
$150K – $290K
One of two "pick one" options protected under SB 1354
🏡
Attached ADU
$120K – $240K
Classified as an internal ADU option under state law
🔗
Garage Conversion
$70K – $160K
Can qualify as internal ADU if attached; check with your city
🚗
Basement / Lower-Level (Internal ADU)
$65K – $150K
Fastest, cheapest, most protected path statewide
⬇️
Modular / Prefab
$135K – $250K
Treated as detached under SB 1354's rear-yard rule
📦
Junior ADU
$40K – $90K
A form of internal ADU — the lowest-cost protected option
🚪
💡

Implementation transition: SB 1354 (2026). Effective July 1, 2026 for covered cities; cities above 10,000 residents have an implementation deadline of February 1, 2027. Permit fees and review timing remain planning estimates until confirmed for the specific property.

Calculate My ROI →
ADU Types

Which type fits your property?

01 / The "Detached" Choice
Detached ADU
$150K – $290K

A standalone structure in the rear yard. SB 1354 protects this as one of two mutually exclusive options — choosing detached means you're not separately protected for an internal unit as well.

Detached ADU Guide →
02 / Internal ADU Category
Attached ADU
$120K – $240K

An addition sharing a wall with the main house. Falls under Idaho's "internal ADU" protection alongside basement and garage conversions — the other of the two mutually exclusive protected options.

Attached ADU Guide →
03 / Most Affordable
Garage Conversion
$70K – $160K

A conversion of an attached or detached garage. Idaho Code defines internal ADUs to include units within the primary home or its attached or detached garage — check with your city on which category applies.

Garage Conversion Guide →
04 / Fastest, Cheapest
Basement / Lower-Level Conversion
$65K – $150K

A converted lower level with an independent entrance, within the existing home's footprint. The classic "internal ADU" — protected statewide, no owner-occupancy requirement as of July 1, 2026.

Basement ADU Guide →
05 / Detached Category
Modular / Prefab
$135K – $250K

A factory-built unit installed on a permanent foundation in the rear yard — treated as the "detached" option under SB 1354, not the internal option.

Prefab ADU Overview →
06 / Lowest Cost
Junior ADU
$40K – $90K

A small private suite within the existing home footprint — Idaho's cheapest internal ADU option, carrying the same statewide protection as a basement conversion.

Junior ADU Overview →
Cost by Region

Every Idaho market is different.

Boise, Idaho ADU regional photo
Boise
Ada County · currently caps ADUs at 900 sqft
$160K–$290K
City code update required by Feb. 2027 to comply with SB 1354
High
Meridian, Idaho ADU regional photo
Meridian
Ada County · Treasure Valley
$155K–$280K*
Runs comparable to or slightly above Boise
High
Nampa, Idaho ADU regional photo
Nampa
Canyon County · south Treasure Valley
$130K–$230K*
More affordable than Boise/Meridian
Moderate
Coeur d'Alene, Idaho ADU regional photo
Coeur d'Alene
Kootenai County · North Idaho
$140K–$250K*
Home district of SB 1354's primary sponsors
Moderate

*Total-cost ranges outside Boise are derived from regional construction-cost data at a representative ~600 sq ft build; verify against local bids before publishing final figures. Idaho Falls and Twin Falls are notable additional markets not pictured here — each qualifies under SB 1354's 10,000+ population threshold and is worth checking with local contractors directly.

Cost Factors

Why Idaho ADUs cost what they cost.

👷
Treasure Valley Growth Pressure
Boise and Meridian's rapid population growth has pushed construction labor and material costs upward across the Treasure Valley. Nampa and Coeur d'Alene run meaningfully more affordable than the Boise core.
Premium in Boise & Meridian
📐
One-ADU "Pick a Lane" Rule
Because SB 1354 protects only one ADU per lot — internal or detached, not both — homeowners must commit to a single strategy. Choosing wrong can mean redesigning from scratch if you later want the other type.
Upfront decision has real downstream cost
🅿️
Parking Exceptions Near Transit
Cities generally cannot require additional parking if the primary home already has off-street parking — except within a quarter-mile of a transit stop, employment area, or commercial services, a narrow exception more relevant in Boise's urban core than most of the state.
Mostly favorable statewide
⚖️
Current ADU Legal Framework
Effective July 1, 2026 for covered cities; cities above 10,000 residents have an implementation deadline of February 1, 2027.
Implementation transition · SB 1354 (2026)
Homeowners reviewing ADU permit documents in Idaho
Permits & Fees

What permitting actually costs.

Effective July 1, 2026 for covered cities; cities above 10,000 residents have an implementation deadline of February 1, 2027. The detailed figures below are representative planning ranges, not a statewide fee schedule or review-time guarantee.

📄
Building Permit Fee*
$2,500 – $8,000
Varies by city; some cities also require zoning clearance separately.
🔌
Electrical, Plumbing & HVAC Permits
$800 – $3,000
Typically separate from the base building permit.
💧
Utility Connection Fees*
$2,000 – $10,000
SB 1354 caps utility hookup fees at no more than what's charged for a single-family home.
📋
Plan Review*
$1,000 – $3,500
Structural/architectural review.
🅿️
Additional Parking (if required)
$0 – $6,000
Only applies if your home lacks existing off-street parking, or near transit/employment areas.
Verified August 5, 2026 · Implementation transition
SB 1354 (2026)

Effective July 1, 2026 for covered cities; cities above 10,000 residents have an implementation deadline of February 1, 2027.

Legal classificationImplementation transition
Governing frameworkSB 1354 (2026)
Representative permit fees$2,500 – $8,000
Representative review timeVerify locally
Local verificationRequired

Source: Idaho Legislature — SB 1354 →

Verify the property before design

These are planning benchmarks, not a permit quote, eligibility ruling, or legal opinion. Confirm zoning, building, utility, site, private-covenant, fee, and timing requirements with the current local authorities. See the 51-jurisdiction law directory and national permit guide.

Return on Investment

Idaho ROI planning inputs.

$825–$1,275
Adopted statewide 1-bedroom rent range
15%
National property-value planning input
10%–20%
Property-value uncertainty range
30%
Operating-expense planning reserve
Rent source
HUD FY2026 revised
One-bedroom FMR market basket
Exact HUD mappings
ROI-CM-1.4
Home-value source
Statewide Zillow ZHVI
June 30, 2026
One statewide input
Not a valuation
Payback period
Calculated live
Budget, ADU type, size and rent
Open calculator →
No static statewide promise
Full methodology
Transparent sources
HUD, Census PUMS, Zillow and FHFA
Review methodology →
Planning estimate, not a valuation
Save Money

7 ways to cut your
Idaho ADU cost now.

SB 1354 is in implementation transition for covered cities. These steps help homeowners select an appropriate path while confirming the current local compliance status.

Calculate My ADU ROI →
Compact detached ADU exterior with Northwest-modern landscaping
🏡
Compact Detached ADU
Idaho — statewide range
Property-Value Planning Input
15%
1
Decide internal vs. detached before you design anything
SB 1354 protects only one ADU per lot — internal or detached, not both. Committing to the wrong one first can mean paying for a redesign if you later want the other type.
Avoids a full redesign later
2
Default to internal if you don't specifically need a backyard structure
Basement, garage, and attached conversions are typically cheaper and faster to permit than new detached construction — and they've carried statewide HOA protection since 2023, longer than the detached option.
Often 40–55% cheaper than new detached construction
3
Check your city's SB 1354 compliance status before finalizing design
Cities have until February 1, 2027 to update their codes. Boise's current 900 sqft ADU cap, for example, already conflicts with the new law and will need to change — confirm your city's current rules before committing to a design.
Free — avoids designing against an outdated local rule
4
Confirm your existing off-street parking situation
Cities generally can't require additional parking for your ADU if your home already has off-street parking of its own — unless you're within a quarter-mile of transit, employment, or commercial areas.
Can eliminate a $2,000–$6,000 parking requirement
5
Review your HOA's CC&R date before assuming you're protected
Pre-existing HOA restrictions on ADUs remain enforceable under SB 1354. New restrictions require your written consent — but if your covenant already banned ADUs before the law passed, it may still apply.
Prevents a legal dispute after you've started building
6
Ask about your city's bedroom count limits
Some jurisdictions, like Ada County, limit ADUs to one bedroom, while the City of Boise allows up to two. Confirming this before designing avoids paying for a layout your jurisdiction won't approve.
Free — avoids paying for an infeasible floor plan
7
Check Idaho's new short-term rental rules if relevant
House Bill 583, also effective July 1, 2026, changes how cities can regulate and tax short-term rentals statewide. If Airbnb income is part of your plan, review how this affects your specific city before finalizing your ROI case.
Prevents building a rental strategy around outdated rules
Frequently Asked Questions

Idaho ADU answers.

Does Idaho have a statewide ADU law?
SB 1354 (2026) is classified as being in implementation transition in NationwideADU's August 5, 2026 review. Effective July 1, 2026 for covered cities; cities above 10,000 residents have an implementation deadline of February 1, 2027. Local zoning, building, utility, site, and private-covenant requirements may still apply; verify the current rules for the property before design or construction.
Can I have both an internal and a detached ADU in Idaho?
Not as a guaranteed right under SB 1354. The law protects one ADU per lot, and you choose either the internal option or the detached option — not both. Some cities may voluntarily allow more, so check your specific city's code.
Do I have to live on the property to have an ADU in Idaho?
No. SB 1354 eliminates owner-occupancy requirements at the city level entirely, building on a 2023 law (HB 166) that had already removed the HOA-level owner-occupancy requirement for internal ADUs.
Can my HOA block my Idaho ADU?
It depends on timing. HOA restrictions on ADUs that existed before SB 1354's effective date remain enforceable. New restrictions added after July 1, 2026 require the affected homeowner's express written consent — an HOA can't unilaterally impose a new ADU ban going forward.
How big does my Idaho ADU have to be allowed to be?
Cities cannot set a minimum size smaller than 1,000 sqft or 75% of your primary home's square footage, whichever is larger. On a 2,000 sqft home, your protected minimum is 1,500 sqft. This is a size floor, not a cap — cities can still allow larger ADUs.
What is the cheapest ADU to build in Idaho?
An internal ADU — within the primary home or an attached/detached garage — is typically the cheapest and fastest-permitted option, commonly $65,000–$150,000 depending on finish level.
Does Idaho require additional parking for an ADU?
Generally no, if your home already has off-street parking of its own. Cities can only require additional parking if your home has no off-street parking, or if your property sits within a quarter-mile of a transit stop, employment area, or commercial services.
What is the ROI on an ADU in Idaho?
The adopted Idaho one-bedroom planning range is $825–$1,275 per month, derived from the HUD FY2026 revised FMR market basket under ROI-CM-1.4. Payback is calculated from the homeowner's budget, ADU type, size, rent range, and a 30% operating-expense reserve. Property value uses a 15% national planning input with a 10%–20% uncertainty range; it is not a valuation.
Idaho ADU exterior at blue hour, warm interior light glowing
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Planning Disclaimer: Cost, fee, and timeline ranges are approximations for education and early planning; actual requirements and costs vary with the property, local authority, utilities, site conditions, design, and contractor. The Idaho legal classification and framework shown here were verified on August 5, 2026, but they do not determine parcel eligibility and are not legal advice. Confirm current zoning, building, utility, private-covenant, fee, and timing requirements directly with the applicable authorities before design or construction. Obtain at least three contractor bids before committing to a project budget. Last updated August 2026.