Boise ADU Feasibility Study

ADU Rules in Boise (2026)

The rules below reflect Boise's current code as of 2026 — verified against City of Boise zoning code Section 11-03-03 and confirmed in direct consultation with the City of Boise Planning department in May 2026.

Size and Dimensional Standards

• Maximum ADU size: ≤ 900 sq ft OR ≤ 70% of primary dwelling floor area, whichever is smaller
• Maximum bedrooms: 2
• Maximum stories: 2
• Setbacks (R-zones): Side 5 ft, Rear 5 ft (or 0 ft for alley-loaded structures under 1,000 sq ft — see below), Front matches primary dwelling
• Forms allowed: Detached, attached, or interior conversion

Sources: Boise zoning code Section 11-03-03, Tables 11-02.9 (Dimensional) and 11-04.9 (Parking), and the City of Boise ADU One-Sheet.

Number Per Lot

1 ADU per lot. Detached and attached forms are both allowed, but only one accessory dwelling unit per parcel.

Parking — where most homeowners get bad information

There is no parking requirement for the ADU itself. This is the most common misconception about Boise ADUs in 2026. The City of Boise Planning department confirmed this directly: a 2-bedroom ADU does NOT require an additional off-street parking space. The old rule that triggered an extra parking spot for larger ADUs has been removed.

Caveat: the primary residence's required parking must be maintained. If building the ADU consumes existing off-street parking for the primary house, you'll need to replace it. The ADU is exempt; the primary house's parking minimum still applies.

Alleys and the 22-Foot Backup Rule

If your lot has a rear alley, the standard rear setback drops to 5 feet — and for an alley-loaded garage or parking structure under 1,000 sq ft, the rear setback can go to 0 feet (i.e., the structure can sit on the alley line).

But here's the catch: you must prove 22 feet of backup space for the alley-loaded structure. Boise alleys are typically 12 to 15 feet wide. The alley width counts toward the 22-foot requirement, but you'll still usually need 7 to 10 feet of additional clearance from the structure to make the math work.

This combination — alley access plus 22-foot backup proof — significantly expands the buildable envelope for detached-above-garage ADUs in alley-served neighborhoods. It's why central Boise neighborhoods with intact alley networks (East End, North End, parts of Vista) are some of our most active ADU territories.

Historic Design Overlay (HD-O)

Many central Boise neighborhoods sit inside a Historic Design Overlay. If your parcel is in an HD-O, any new structure — including an ADU — requires a Certificate of Appropriateness (COA) before a building permit will be issued.

The pre-approved plans (below) do NOT apply in Historic District. Even if you pick one of the free plans, you'll still go through the full COA process. Two-story ADUs in HD-O always require Historic Commission review.

Check your overlay status at the City of Boise GIS portal before designing anything.

Owner-Occupancy and Process Sequence

No owner-occupancy requirement in most residential zones — you can build an ADU and rent it as an investment. Administrative planning review is required BEFORE you submit for the building permit. DSC's design-build coordination handles every phase: site review, planning review, COA review (if HD-O), building permit, and construction — under one project.

Impact Fees

ACHD (Ada County Highway District) increased impact fees as of March 1, 2026 under Ordinance No. 254. ADU impact fees specifically went up by approximately 68%. These are real budget items separate from City permits and utility connection charges. The feasibility study itemizes what applies to your property.

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What Affects the Cost of a Boise ADU

ADU costs vary widely from project to project — sometimes by a factor of two or three on the same lot size. Rather than publish a generic dollar range that may or may not apply to your property, DSC delivers a specific cost range as part of your free, property-specific feasibility study. Below are the factors that drive that number up or down.

• ADU type and configuration. Detached, attached, above-garage, and garage-conversion ADUs each have different cost structures. Detached-above-garage is the most complex; garage conversions are the most affordable per square foot when the existing garage has the right bones.
• Existing utility capacity. Often the single largest cost variable. Older Boise neighborhoods may need a full electrical service upgrade, new sewer connections, or larger water service drops.
• Site topography and soil conditions. Sloped lots add foundation, grading, and drainage cost.
• Finish level. Cabinet selections, countertops, flooring, lighting, fixtures, and trim move the number meaningfully.
• Structural complexity. Two-story ADUs cost more per square foot than single-story. Above-garage costs more than slab-on-grade detached.
• Historic District requirements. If your property is in an HD-O, period-appropriate exterior materials and trim cost more.
• Pre-approved plan vs. custom design. Using one of the City of Boise's free pre-approved plans saves the architecture and engineering fee — typically the single largest line item outside of construction itself.
• Permit, impact fee, and utility connection costs. City of Boise permits, ACHD impact fees (up ~68% as of March 2026), and utility connection charges are real budget items separate from construction.

Note on the "free plans" headline: the City's pre-approved plans save the architecture-and-engineering design fee, but the plans are structural and layout only — they don't specify fixtures, finishes, or trim packages. DSC is developing pre-priced finish packages aligned with the free plans to give homeowners fixed-price clarity. Ask about this in your feasibility study.

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Boise's Free Pre-Approved ADU Plans

In 2026, the City of Boise released 8 free, pre-approved ADU plans designed by Parke Architecture (led by architect Eamonn Parke, AIA). Each plan is named after a Boise-native bird and was designed specifically for the patterns of Boise's existing neighborhoods.

• Plan range: 280 to 695 sq ft | studio to 2-bedroom | single-story and over-garage designs
• Compatible options: Historic District-considerate exterior treatments available

What pre-approved plans save:

• Design fees — fully saved
• Building permit review time — significant reduction (deep building review already done)
• Planning review time — modest savings; if HD-O, you still need full COA

What they don't save:

• Fixture, finish, and trim work — you still go through finish selection
• Any modification voids the pre-approval. Move a wall, change a crawlspace? Back to full building review.
• Pre-approved plans do NOT apply in Historic Districts.

Browse Boise's Pre-Approved ADU Plans →

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How Long Does It Take to Build an ADU in Boise?

Pre-approved plan path (no HD-O)

• DSC feasibility study: 2–3 days
• Plan selection, site adaptation, finish package: 2–4 weeks
• Administrative planning review: 2–4 weeks
• Building permit (expedited): 2–4 weeks
• Construction: 4–7 months
• Total: 6–9 months

Custom design path (no HD-O)

• DSC feasibility study: 2–3 days
• Site review, 3D modeling, design contract: 2–4 weeks
• Design diligence, finish selection, structural engineering: 4–6 weeks
• Administrative planning review: 1–2 weeks
• Building permit review: 4–6 weeks
• Construction: 4–7 months
• Total: 7–12 months

HD-O properties (Historic Design Overlay): add 1–3 months to either path for Certificate of Appropriateness review, Historic Commission review (mandatory for two-story; sometimes for single-story), and any period-appropriate material substitutions.

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Frequently Asked Questions — Boise ADUs

Do I need parking for an ADU in Boise?

No. There is no parking requirement for the ADU itself, regardless of bedroom count. The City of Boise confirmed this directly in May 2026. The primary residence's required parking, however, must be maintained.

What's the maximum size for an ADU in Boise?

900 square feet OR 70% of your primary dwelling's floor area, whichever is smaller. Maximum 2 bedrooms and 2 stories.

How many ADUs can I have on one Boise lot?

One ADU per lot. Detached and attached forms are both permitted, but only one accessory unit per parcel.

Do I have to live on the property to build an ADU?

No. Boise removed the owner-occupancy requirement in most residential zones. You can build an ADU and rent it as an investment property.

What if my property has an alley?

Alleys significantly help your buildable envelope. The standard rear setback drops to 5 feet, and for an alley-loaded garage or parking structure under 1,000 sq ft, the rear setback can drop to 0 feet — but you must prove 22 feet of backup space (which the alley width partially counts toward).

What if my property is in a Historic District?

You'll need a Certificate of Appropriateness (COA) before any building permit is issued. Two-story ADUs require Historic Commission review. The free pre-approved plans do NOT apply in Historic District — you still go through the full COA process.

How much does it cost to build an ADU in Boise in 2026?

ADU costs vary widely based on type, finish level, site conditions, and existing utility capacity — sometimes by a factor of two or three on the same lot size. Rather than publish a generic range, DSC delivers a specific cost range for your property as part of your free feasibility study. Submit the form above for your property-specific number.

Are the City of Boise's free ADU plans really free?

The plans themselves are free — that's a meaningful design-fee saving. But the plans don't include fixtures, finishes, or trim. You still go through finish selection. DSC is developing pre-priced finish packages aligned with the free plans for fixed-price clarity.

How long does it take to build an ADU in Boise?

Pre-approved plan path: 6–9 months total. Custom design path: 12–18 months. Add 1–3 months for HD-O properties. DSC's design-build process compresses both timelines through milestone-driven scheduling.

What's the first step?

Submit the form above for DSC's free ADU Feasibility Study. We'll send you a property-specific report within 48 hours covering zoning, dimensional fit, buildable envelope, parking, alley analysis, overlay status, and recommended ADU configuration.

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Real Boise ADU Projects from DSC

The Boise ADUs below are DSC builds with their owners' actual Google reviews. They cover the full range of what an ADU can be — a recent primary backyard build, a rendering-and-animation-driven custom design, a garage-demolition-to-ADU conversion, and a Buildertrend-coordinated multi-phase project. Different starting points, same DSC process.

Brookman ADU (Fully Custom)

"Almost a full year, from concept to finished project, and it was all so worth it! It was a very collaborative process, and I'm so happy with my new space. It was especially great to know I was helping support so many small businesses, and families. Derek and all the subs were like extended family."

— Helen Brookman (Google review, 1 year ago)

View the Brookman ADU project →

Swift ADU

"Nothing but good things to share about Derek Sammons Construction. My wife and I originally met with him to discuss an ADU build on the back of our property. Derek was on time, professional, and used state of the art software to create renderings of the vision with video animation. Once we gave the go ahead, Derek handled all of the permitting and construction process and communicated often where we were at in the build. The end result was nothing short of amazing. The quality craftsmanship and close attention to detail were evident everywhere you look."

— Bryan Swift (Google review)

Majors ADU

"We used Derek Sammons for a backyard Accessory Dwelling build and we cannot recommend him enough. We worked closely with DSC during the entire process. From a complete demolition of our garage to creating a layout that worked with us, all while being honest and aware of our budget. It was great to get updates from Derek throughout the process as well. The ADU turned out even better than expected, we would absolutely recommend Derek to everyone!"

— Corey Majors (Google review)

Ullman ADU

"Derek did a fantastic job of managing our ADU build. He was professional, organized, and made sure that everything went smoothly. He used Builder Trend to keep everything organized, which was great for us to keep track as things moved along. I would highly recommend!"

— Shannon Ullman (Google review)

Four Boise ADU projects, four client experiences, one DSC process. The full set of DSC reviews is on our Google Business Profile — currently 5.0 stars across 11 reviews.