Everything homeowners need to know about building an accessory dwelling unit in Boise - size limits, setbacks, permits, and the brand-new Idaho state law that just overhauled the rules for every fast-growing city in the state.
Last verified: August 2026
7/10 · Increasingly ADU-friendly
1,000 sq ft*
Max ADU size
25 ft*
Max height
Not required*
Owner-occupancy
By-right
Permit type
This page summarizes Boise's zoning code alongside Idaho's new statewide ADU law (Senate Bill 1354, codified at Idaho Code 67-6541) for general reference - it is not legal advice. The state law took effect July 1, 2026 and cities have until February 1, 2027 to formally update their own codes, so some local rules are in transition. Always confirm current requirements with Boise Planning & Development Services before designing or applying for permits. See sources below.
Zoning & size requirements
What you're allowed to build
ADU type
Max size
Max height
Notes
Detached (new build or garage conversion)
Boise's published code still lists 700 sq ft or 10% of lot area, whichever is smaller - but state law now sets a floor of 1,000 sq ft or 75% of the primary home's floor area, whichever is larger
25 ft, or the height of the primary dwelling, whichever is less
Must always be smaller than the primary dwelling on the lot
Sized within the primary structure, subject to the same new state-law size floor
Same as primary structure
Common approach for basement or above-garage conversions
Boise's ADU rules are mid-update
Idaho's SB 1354 bars cities over 10,000 residents from banning ADUs, mandating owner-occupancy, or capping size below 1,000 sq ft (or 75% of the primary home, whichever is larger). It took effect July 1, 2026, but Boise's own zoning code text - written before the law passed - hasn't been formally rewritten yet; the city has until February 1, 2027 to bring its land use code into line. In the meantime, the state floor is the legally enforceable standard even where Boise's published code still shows older, smaller numbers. Confirm current interpretation with Planning & Development Services before you design. Allowed zoning districts: most single- and two-family residential zones citywide, per Boise's 2023 zoning code rewrite.
Additional standards
Setbacks, parking & other rules
Setbacks
5 ft from side and rear property lines for most detached ADUs; reduced to 3 ft side / 9 ft rear if the unit is under 500 sq ft and under 14 ft tall. Front setback matches the primary dwelling's zone standard.
Parking
No additional off-street parking required for one-bedroom ADUs; larger units may need standard parking depending on the zoning district
Units per lot
1 ADU permitted in addition to the primary dwelling, now guaranteed statewide for cities over 10,000 residents under SB 1354
Owner-occupancy
Boise's code has historically required the owner to live in either the primary home or the ADU. SB 1354 bars cities from imposing that condition as of July 1, 2026, but Boise's own code text hasn't been formally amended yet - confirm current enforcement before you rely on this.
Building code
Must meet the 2018 International Residential Code as adopted by the city
Impact fees
State law requires ADU impact fees to be assessed at a reduced portion of the standard single-family rate, not the full rate
Ordinance history
How Boise's ADU rules got here
2019Local ADU standards loosened. Boise City Council adopted ORD-27-19 on June 11, 2019, increasing the maximum ADU size, allowing a second bedroom, and removing the parking requirement for one-bedroom ADUs.
2023First full zoning code rewrite since 1966. Boise's new zoning code carried ADU provisions forward and expanded the city's catalog of pre-approved ADU plan sets designed to speed up permitting.
2023Idaho's first (narrow) state ADU protection. House Bill 166 barred homeowners associations from strictly prohibiting "internal" accessory dwelling units statewide - but left city zoning power over ADUs otherwise untouched.
2026Idaho passes a sweeping statewide ADU mandate. Governor Brad Little signed Senate Bill 1354 on March 31, 2026 (effective July 1, 2026), barring cities over 10,000 residents from banning ADUs, eliminating mandatory owner-occupancy, and setting a 1,000-sq-ft-or-75%-of-home size floor. Cities must formally update their codes by February 1, 2027.
Worth knowing: Boise's median home sale price roughly doubled over the decade to 2024, climbing from around $200,000 to over $500,000 as the metro added more than 150,000 residents, many of them transplants from California, Washington, and Oregon. That in-migration-driven price growth is a big part of why the legislature and the city have both moved to make ADUs easier to build - and why Boise built out a catalog of pre-approved plans rather than leaving every applicant to design from scratch.
Permit process
How Boise ADU permitting works
1
Early Assistance meeting
Talk with Boise Planning & Development Services about your specific lot before you spend money on design.
2
Pick a plan or design custom
Choose from the city's catalog of pre-approved ADU plan sets - ranging from a 280 sq ft studio up to a 695 sq ft two-bedroom - to skip design review, or submit your own custom plans.
3
Submit a Zoning Certificate application
Apply through the city's online Permitting and Licensing system. Qualifying ADU applications are reviewed administratively, with no public hearing, consistent with SB 1354's by-right mandate.
4
Building permit
Once zoning clears, apply for your building permit under the city's adopted 2018 International Residential Code.
5
Inspections & final approval
Schedule the standard framing, plumbing, electrical, and final inspections before you can occupy the unit.
Fees, review timelines, and exact plan-set availability change - confirm current figures with Planning & Development Services before you budget.
Budgeting
What it costs to build an ADU in Boise
Garage conversion
Roughly $65,000-$150,000, generally the least expensive path since the shell already exists
Basement conversion
Roughly $95,000-$190,000, depending on existing ceiling height and utility access
New detached ADU
Roughly $140,000-$260,000 for a code-compliant, freestanding unit; a pre-approved plan set can trim design costs
These are general ranges from local contractor estimates and regional reporting, not quotes. Boise's construction market is more moderate than coastal cities, but rapid population growth has kept skilled-labor availability tight in recent years - get multiple local bids before committing to a budget.
Before you apply
Tips for a smooth approval
Check the pre-approved plan catalog first. Picking a design that's already been vetted by the city can save real time and design fees compared to a fully custom submission.
Don't assume the old owner-occupancy rule still applies. State law now bars it, but Boise's code text hasn't formally caught up - confirm current guidance directly with Planning & Development Services.
Start with an Early Assistance meeting. It's a low-cost way to catch zoning issues before you've paid for full design work.
Understand the size floor, not a fixed size. The new 1,000-sq-ft-or-75%-of-home standard is a minimum protection against overly restrictive local caps - your exact maximum still depends on your home's footprint.
Line up contractors early. Boise's building trades have stayed busy through years of regional growth, so lead times can run longer than the estimate you get on a first call.
FAQ
Common questions
Do I need to live on the property to build an ADU in Boise?
Boise's own zoning code has historically required the owner to live in either the primary home or the ADU. Idaho's new statewide law (SB 1354) bars cities from requiring owner-occupancy as of July 1, 2026, but Boise's published code text hasn't been formally updated yet - confirm current enforcement with Planning & Development Services before you plan around either answer.
Does Idaho have a statewide ADU law?
Yes, as of 2026. Senate Bill 1354, signed by Governor Brad Little on March 31, 2026 and effective July 1, 2026, bars cities with more than 10,000 residents from banning ADUs, mandating owner-occupancy, or capping ADU size below 1,000 sq ft (or 75% of the primary home, whichever is larger). Before that, Idaho's only statewide ADU provision was a narrower 2023 law barring HOAs from prohibiting internal ADUs.
How many ADUs can I build on one lot?
One ADU per lot in addition to the primary dwelling, which is now guaranteed statewide for cities over 10,000 residents under SB 1354.
Do I need extra parking for an ADU?
No additional off-street parking is required for one-bedroom ADUs. Larger units may need standard parking depending on the zoning district - confirm with Planning & Development Services.
What's the maximum size for an ADU in Boise?
Boise's published code still lists 700 sq ft or 10% of lot area, whichever is smaller, but the new state law (SB 1354) sets a floor of 1,000 sq ft or 75% of the primary home's floor area, whichever is larger, that the city cannot go below as of July 1, 2026. Boise's code text is expected to be formally updated by February 1, 2027.
How long does the permit process take?
Because ADU applications in cities over 10,000 residents are reviewed administratively with no public hearing under SB 1354, timelines are generally faster than a discretionary approval - but exact review times vary with application completeness. Confirm current timelines with Planning & Development Services.
How much does an ADU cost to build in Boise?
Garage conversions tend to run roughly $65,000-$150,000, basement conversions $95,000-$190,000, and new detached units $140,000-$260,000, based on local contractor estimates and regional reporting - not counting permit fees. Choosing one of the city's pre-approved plan sets can reduce design costs.
What are Boise's pre-approved ADU plans?
The city offers a catalog of pre-vetted ADU plan sets, ranging from a 280 sq ft studio up to a 695 sq ft two-bedroom unit, that applicants can choose based on lot size and zoning to bypass standard design review. Plans can't be altered without a separate permit.
Why did Idaho suddenly pass a statewide ADU law?
Rapid in-migration pushed Boise-area home prices sharply higher over the past decade, and lawmakers pointed to overly restrictive city rules - like mandatory owner-occupancy and small size caps - as part of what was keeping ADU construction low despite the demand for smaller, more affordable housing options.
