Building an ADU in Boise
City code and Idaho law · Last reviewed October 9, 2026
Boise allows an accessory dwelling unit (ADU) with a single-family home in most residential and several mixed-use zones. Under the current zoning code an ADU can be up to 900 square feet or 70% of the main home's floor area, whichever is smaller, with no more than two bedrooms and two stories, and no extra parking is required. The city offers free pre-approved ADU plans. Idaho's 2026 ADU law says cities of more than 10,000 people must amend their rules by February 1, 2027, including to prohibit size caps below 1,000 square feet or 75% of the main home. [1][2][3][4]
| Where allowed | R-1A, R-1B, R-1C, R-2, R-3, MX-1, MX-2, MX-3, MX-U, A-1 and A-2 zones, with a single-family home [1][2] |
|---|---|
| How many | Up to two accessory units per lot, in any mix of ADUs and tiny homes on wheels [1] |
| Max size (current code) | 900 sq ft or 70% of the main building's floor area, whichever is smaller; max 2 bedrooms [1] |
| Height | No more than two stories [1] |
| Setbacks | The underlying zone's dimensional standards apply [1][2] |
| Parking | No additional spaces required for an ADU [2] |
| Density | Maximum density standards don't apply to ADUs [1] |
| Planning application fee (pre-approved ADU) | $231, plus a historic district fee if it applies [2] |
| City impact fees, unit of 700 sq ft or less | $1,384.84 to $2,992.72 depending on planning area, plus a 1% surcharge (schedule effective October 1, 2025) [8] |
| Pre-approved plans | Free catalog of plan sets from 280 to 695 sq ft [3][7] |
| State law deadline | Idaho Code 67-6541 gives cities of more than 10,000 people until February 1, 2027 to amend their ADU rules [4] |
On this page
What you can build here
Boise adopted its first new zoning code since 1966 in 2023, and the current version reflects changes adopted through 2025 and 2026. ADUs are allowed "only as an accessory use to a single-family dwelling (attached or detached)" in the R-1A, R-1B, R-1C, R-2, R-3, MX-1, MX-2, MX-3, MX-U, A-1 and A-2 zones.
You can create an ADU by converting space inside the house, a basement or an attic; adding onto the house or a detached accessory building; building a new house with an ADU; building a detached ADU; or converting a detached accessory building or attached garage. A lot may have up to two accessory units in any mix of ADUs and tiny homes on wheels.
Boise reports 130 ADUs built or approved in 2024 and 82 in 2025, up from an average of 61 a year from 2019 to 2023. [1][2][5][6][7]
Size, height and setbacks
Size. The ADU "shall not be larger than 900 square feet or 70 percent of the total floor area of the primary building, whichever is smaller, and shall not have more than two bedrooms."
Height. No more than two stories.
Setbacks and other dimensions. The ADU must meet the dimensional standards of your zone and the building code, with the exceptions for accessory structures in Section 11-04-03.3. The city's packet notes that pre-approved plans under 500 square feet may qualify for a 9-foot rear setback in residential zones; confirm at concept review.
Design. The ADU must be compatible with the neighborhood and the main house in height, bulk, materials, colors and style. Only one entrance may face the front of the house unless more already exist. A continuous sidewalk at least 4 feet wide must run from the ADU's front door to the street. [1][2]
Idaho's 2026 law and Boise's rules
Idaho Code 67-6541, in effect since July 1, 2026, applies to cities of more than 10,000 people, including Boise. By February 1, 2027, covered cities must, among other things, stop limiting ADUs to "less than one thousand (1000) square feet or seventy-five percent (75%) of the size of the primary dwelling," and stop requiring owner occupancy or (with exceptions) parking.
Boise's current caps, 900 square feet and 70% of the main home, are both lower than the state figures. Boise's ADU standards already don't require extra parking, and Section 11-03-03.2.B contains no owner-occupancy rule. Until the city updates its code, ask Planning and Development Services which size limit it will apply to your application. See our Idaho ADU law page. [1][2][4]
Parking
The city's ADU packet says one code-compliant parking space is required for a single-family home and "No additional parking spaces are required for ADUs." If you convert a garage, the conversion can't reduce off-street parking below the minimum required for the lot. [1][2]
Fees
The city lists these costs for a pre-approved ADU:
- Planning application fee: $231, plus a historic district fee if applicable.
- Building fees: plan review, building permit, and erosion and sediment control if applicable.
- Sewer connection fee: collected by the city in the Boise sewer district; other districts collect their own.
- City impact fees for parks, fire and police (and a sewer impact fee). For a unit of 700 square feet or less, the parks, fire and police fees in the schedule effective October 1, 2025 total $1,384.84 in the Airport and Downtown planning areas and up to $2,992.72 in the South planning area, plus a 1% surcharge. Units of 701 to 1,400 square feet pay $2,754.80 to $5,953.27. Use the city's impact fee calculator.
- ACHD traffic impact fees, if the highway district requires them.
Idaho's new law says covered cities must amend their rules by February 1, 2027 to prohibit impact or utility connection fees for ADUs that are higher than for other single-family homes. See our ADU cost guide. [2][4][8][9]
Free pre-approved ADU plans
In March 2026 the city released a free catalog of pre-approved ADU plans designed by Boise's Parke Architecture. The city says using them can cut design and permitting time "to a matter of weeks," compared with custom designs that can cost upwards of $10,000.
The plans include the Goldfinch (280 sq ft studio), Waxwing (396 sq ft studio), Kingfisher (491 sq ft one-bedroom), Kestrel (695 sq ft two-bedroom), Sandpiper (396 sq ft with garage) and Osprey (376 sq ft studio with garage). Download them and the applicant packet from the pre-approved ADU plans page. See also our prefab ADU guide. [2][3][7]
Garage conversions
Boise allows an ADU created by converting an existing detached accessory structure or attached garage, as long as it "does not reduce off-street parking below the minimum required for that lot." The ADU still has to meet the size cap and the building code. See our garage conversion guide. [1]
Tiny homes on wheels
Boise allows a tiny home on wheels (THOW) as an accessory unit to a single-family home, separate from its ADU rules. A THOW can be no larger than 400 square feet, must be certified to the NOAH+ standard (or another standard the city adopts), must connect to city water and sewer through the house's connections, can't run on a generator, must sit behind the front of the house and at least 6 feet from other structures, and can't be used as a short-term rental. See our backyard tiny house guide. [1]
Permit steps in Boise
Check your zone and overlays
Look up your zone on the city's zoning map and confirm ADUs are allowed there. Pre-approved plans can't be used in the Flood Protection or Hillside Development overlays, and the Historic District overlay may add requirements. [2]
Size your ADU and pick a plan
Your maximum size is 70% of your home's total floor area or 900 sq ft, whichever is smaller. Choose a custom design or one of the city's free pre-approved ADU plans. [2][3]
Draw a site plan
Show lot lines, setbacks for your zone, existing buildings, parking, trees, the ADU footprint and a walkway at least 4 feet wide from the ADU to the street or sidewalk. [1][2]
Open a planning (PLN) case and attend concept review
Open a PLN case on the city's permitting and licensing site and upload your starter site plan. A planner will schedule a concept review meeting. You can also email zoninginfo@cityofboise.org. [2][10]
Submit the ADU planning application
Follow the ePlan instructions, upload your documents and pay the planning fee after prescreen. In a historic district you also need a Certificate of Appropriateness. If the application meets code, you get an approval letter by email. [2]
Apply for the building permit and pay fees
With your planning case number, apply for the building permit (type 402-B for a pre-approved ADU). You pay plan review, building permit, sewer connection and city impact fees, plus Ada County Highway District traffic impact fees if required. Contractors submit separate trade permits. [2]
Build and pass inspections
Once permits are issued, construction can start. Building staff review and inspect the work under the adopted building code. [2][10]
Pre-approved and sample plans
Free city plan sets designed by Parke Architecture, from the 280 sq ft Goldfinch studio to the 695 sq ft Kestrel two-bedroom, including designs with a garage. [3][7]
Who to call
-
Boise Planning and Development Services
Zoning, ADU planning applications and building permits; email zoninginfo@cityofboise.org for zoning questions. Monday to Friday, 8 a.m. to 5 p.m.
150 N Capitol Blvd, Boise, ID 83702
-
Ada County Highway District (ACHD)
Decides which traffic impact fees apply to your ADU; proof of payment goes with your building permit documents
5800 N Meeker Ave, Boise
Questions people ask
How big can an ADU be in Boise?
Under the current code, 900 square feet or 70% of the main home's total floor area, whichever is smaller, with up to two bedrooms. Idaho's 2026 law says cities of more than 10,000 people must amend their rules by February 1, 2027 to prohibit limiting an ADU to less than 1,000 square feet or 75% of the main home, so ask the city which limit applies to your application. [1][4]
Do I need a parking space for an ADU in Boise?
No additional parking is required for an ADU. A garage conversion can't reduce your lot's off-street parking below the minimum. [1][2]
Does Boise have free ADU plans?
Yes. Since March 2026 the city has offered free pre-approved ADU plan sets, from a 280 square foot studio to a 695 square foot two-bedroom unit. [3][7]
Can I have an ADU and a tiny home on wheels in Boise?
A lot may have up to two accessory units in any combination of ADUs and tiny homes on wheels. A tiny home on wheels has its own rules, including a 400 square foot limit and NOAH+ certification. [1]
Sources (11)
- Boise City Code 11-03-03 Use-Specific Standards (2026 S-31), City of Boise (American Legal Publishing). Accessed October 9, 2026.
- PDS Pre-Approved ADU Applicant Packet 2026, City of Boise Planning and Development Services. Accessed October 9, 2026.
- Pre-Approved ADU Plans, City of Boise Planning and Development Services. Accessed October 9, 2026.
- Idaho Code 67-6541 Accessory dwelling units, Idaho State Legislature. Accessed October 9, 2026.
- Boise Zoning Code, City of Boise Planning and Development Services. Accessed October 9, 2026.
- Current Zoning Code, City of Boise Planning and Development Services. Accessed October 9, 2026.
- City Provides Free ADU Plans to Help Increase Housing Options (March 17, 2026), City of Boise. Accessed October 9, 2026.
- Boise City Impact Fee Schedule effective October 1, 2025, City of Boise. Accessed October 9, 2026.
- Impact Fees, City of Boise Finance. Accessed October 9, 2026.
- Quick Guide to Accessory Dwelling Units, City of Boise Planning and Development Services. Accessed October 9, 2026.
- Planning and Development Services, City of Boise. Accessed October 9, 2026.