Boise ADU Laws in 2026

Where are ADUs allowed in Boise?

ADUs are allowed in most Boise residential zones — R-1A, R-1B, R-1C, R-2, and R-3 — provided the lot meets the minimum size for that zone (typically 5,000 sq ft). The City's Planning & Development Services division publishes the zoning map and the residential ADU code online, so checking your zone takes about a minute. Boise homeowners can confirm their zone via the city's interactive zoning lookup before booking a site visit.

An ADU can be detached, attached, or a garage conversion. Boise's residential code caps any single accessory dwelling at 900 square feet of finished living area, raised from 700 sq ft under the Modern Zoning Code (effective December 1, 2023). The city's six pre-approved plans still top out at the 695 sq ft Kestrel — well under the cap — because the city curated conservative designs, not because the legal maximum is smaller.

Which lots qualify?

Roughly 70% of Boise lots in the eligible residential zones can host at least one of the six pre-approved plans without a variance, based on parcel-level reviews we've run across active jobs. The remaining 30% face issues like substandard width, unusual easements, hillside review triggers in the Boise Foothills, or historic-overlay constraints in pockets of the North End and Hyde Park.

What changed about owner-occupancy in 2023?

Boise removed the strict owner-occupancy requirement for ADUs in 2023. Before the change, the property owner had to live in either the primary residence or the ADU. After the change, an accessory dwelling on a Boise lot can operate as a fully independent rental — long-term or short-term — with no owner-on-site requirement. This was the single biggest unlock for the Boise ADU rental market in the past five years.

What are the setback, height, and parking rules?

Side and rear yard setbacks

Side-yard setbacks for accessory structures in most Boise residential zones are 5 feet. Rear-yard setbacks are typically 10 feet, with reductions for one-story structures in some districts. Larger lots and corner lots have additional rules — your specific lot's site plan will show the buildable envelope. The pre-approved plan set sites cleanly within the standard 5'/10' envelope on a typical Boise infill lot.

Maximum height

ADU height is capped at the principal-structure height limit for the zone, typically 25-30 feet. In practice, most ADUs are single-story and well under the cap. Two-story accessory dwellings exist but require additional review and rarely make sense at the 900 sq ft cap — the second-floor stair eats too much usable space.

Parking

One additional off-street parking space is typically required per ADU. The space must be on the same lot, not in the right-of-way. There are exemptions in some downtown overlay districts where the city has reduced or eliminated parking minimums to encourage residential density. If your lot already has alley access, that often satisfies the parking requirement without an additional curb cut.

How does the pre-approved plan program work?

In 2023 the City of Boise launched its pre-approved ADU plan program — six plans (Goldfinch, Waxwing, Kingfisher, Kestrel, Sandpiper, and Osprey) that have already cleared design review and met the city's residential code. Building one of these plans skips the design-review stage entirely, which typically saves 4-8 weeks at the planning office.

• Goldfinch — 280 sq ft studio (Type A)
• Waxwing — 396 sq ft studio (Type B)
• Kingfisher — 491 sq ft 1-bed (Type C)
• Kestrel — 695 sq ft 2-bed (Type D)
• Sandpiper — 396 sq ft studio + garage (Type G-1)
• Osprey — 376 sq ft studio + garage (Type G-2)

You can still customize finishes, materials, and minor interior details on a pre-approved plan. Anything that changes the structural footprint, exterior elevations, or window placement risks losing the pre-approved status — at which point you're back into full design review and have lost the timeline advantage entirely. We treat pre-approved as a fixed envelope and design the interior finish package around it.

How long does Boise ADU permitting take?

A pre-approved plan typically permits in 3-5 weeks from submittal in Boise. A custom plan typically takes 8-12 weeks because of the design-review stage. Both timelines depend on planner workload (busier in spring) and how quickly your team responds to plan-checker comments. We track every Boise submittal we've made since 2023 — the median pre-approved permit comes back at 27 days; the slowest custom we've handled took 14 weeks.

What do Boise ADU fees and utility connections cost?

Boise charges a building permit fee based on construction value, plus impact fees for utility connections (water, sewer, transportation). Total city fees on a typical ADU run $8,000-$15,000. Add another $4,000-$10,000 if your lot needs a new sewer lateral or upsized water service. Idaho Power has its own fee schedule for new service drops, typically $1,500-$3,500 depending on whether trenching is required. For a full picture, see our Boise ADU cost guide and run the numbers in the cost calculator.

These fees are not negotiable, but they are predictable. We include them as line items in every fixed-price contract so the only number that varies between bid and certificate of occupancy is the final electrical service cost (which depends on whether Idaho Power decides the lot needs a transformer upgrade — that determination happens at permit, not bid).

Can I run a Boise ADU as a short-term rental?

Boise allows short-term rentals (Airbnb, Vrbo) in ADUs but requires a short-term rental permit and tax registration with the city's Finance and Administration department. State-level transient room tax also applies and is collected through the Idaho State Tax Commission. Some HOAs prohibit STRs regardless of city rules. The city's STR registry is publicly searchable, so neighbors and competitors can verify your registration.

Operationally, a permitted STR ADU in a high-demand Boise neighborhood (North End, Hyde Park, near the Greenbelt) can outperform a long-term rental on a per-night basis but underperform on stability. We've seen owners cycle a unit through a year of STR, conclude the cleaning and turnover overhead isn't worth the spread, and shift to a 12-month lease in year two. Worth modeling both before committing.

What are the most common mistakes Boise homeowners make?

1. Assuming the lot qualifies before pulling zoning. Half the calls we field start with 'we already cleared the back' — and a quarter of those lots turn out to be in a zone that doesn't allow ADUs or a parcel under the minimum size.
2. Modifying a pre-approved plan to add a window or move a door. Even a single window relocation kicks the plan out of pre-approved status and into full design review.
3. Skipping the HOA review. The city can approve your ADU and your HOA can still block it. Read CC&Rs first.
4. Underbudgeting utility upgrades. A new sewer lateral and an upsized electrical service are easy four-figure surprises if they aren't scoped into the bid.
5. Building without a clear use plan. The optimal layout for a long-term rental, an aging-parent suite, and a short-term rental are not the same. Decide before design freeze.
6. Hiring a contractor without verifying their Idaho contractor license. Idaho's Division of Occupational and Professional Licenses publishes a public lookup — use it before signing any contract over $2,000.

How do ADU rules differ in nearby Treasure Valley cities?

Boise's pre-approved plan program is unique to the City of Boise. Neighboring cities recognize the structural soundness of the plans, but each runs its own permitting process. Meridian requires a separate plan-check submittal under its Unified Development Code, with shorter timelines on average than Boise. Eagle has stricter architectural compatibility rules, often requiring custom exterior matching even on otherwise pre-approved plans. Garden City, Star, and Kuna each have their own development codes — most align broadly with Boise's structure but with city-specific setbacks, parking rules, and owner-occupancy provisions. We confirm the specific code applicable to every parcel before we contract.

Where do these rules come from?

The zoning, setback, and pre-approved-plan rules in this guide come directly from Boise's residential code and the city's published pre-approved ADU plan program. State-level rules (short-term rental tax, contractor licensing) come from Idaho's Division of Building Safety and related state agencies. Permit fees are pulled from the City of Boise's published 2026 fee schedule. We update this page when the city publishes a code change, typically within 30 days.

What should you do next?

If you've made it this far, the next step is a property-specific review: which zone you're in, whether your lot meets the minimum size, where the buildable envelope is, and which pre-approved plan fits. We'll do that walk-through for free — see the contact page. Most Boise homeowners get a yes-or-no answer in under 30 minutes.

Where to next

If you want the full picture in one place, the Complete Guide to Building an ADU in Boise covers zoning, plans, cost, financing, permits, and use cases end-to-end. For the data behind the rule changes, see the Boise ADU Market Report 2026 — including the 9× permit surge that followed the December 2023 Modern Zoning Code. If you're past the rules and ready to act, three pages move you forward: Boise's six pre-approved ADU plans (the fastest permit path), the ADU feasibility framework (everything that actually decides whether your lot qualifies), and the design-build contractor overview (one team, one contract, one price). The free ADU lot check runs the rules against your parcel.