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Building an ADU in Boise, Idaho

Boise has become one of the easier cities in the West to add a backyard home. The zoning code rewrite that took effect in late 2023 raised the size limit, removed the owner-occupancy requirement and stopped requiring an extra parking space for an ADU. In 2026 the city added a catalog of free, pre-approved plans. Here's what that means for your lot.

Boise's free pre-approved ADU plans

The City of Boise's Planning and Development Services now offers eight pre-approved ADU designs, from 280 to 695 square feet, ranging from single-story cottages to units over a garage. Using one saves the cost of a custom design, which the city says can run upwards of $10,000, and simplifies plan review. You still submit a site plan and pay for utilities, impact fees and construction (City of Boise).

Lots in a Flood Protection Overlay or a Hillside Development Overlay aren't eligible for the pre-approved plans. That matters along the Boise River and in the Foothills, where a custom design and additional review are usually needed.

Size, parking and occupancy

The city's 2026 pre-approved ADU applicant packet lists the maximum ADU size as 70 percent of the house's floor area or 900 square feet, whichever is smaller, and says no additional parking spaces are required (applicant packet). Owner occupancy is no longer required. Idaho's 2026 ADU law will push the size floor higher: by February 1, 2027, cities over 10,000 people can't cap ADUs below 1,000 square feet or 75 percent of the main home (Idaho Code § 67-6541). We confirm the current limit with the city before design.

Boise neighborhoods and what they mean for an ADU

• North End and East End: older homes, alleys and detached garages make classic garage conversions and alley-facing cottages. Some areas are in historic districts, which the state ADU law exempts, so design review can apply.
• The Bench: mid-century homes on generous lots often have room for a detached unit in the backyard, with utilities close by.
• West Boise and Southwest: newer homes, sometimes with HOA covenants, and larger rear yards. We check covenants before design.
• Foothills and river lots: hillside and flood overlays add review and rule out the pre-approved plans, but a custom ADU can still work.

Permits in Boise

According to the city's applicant packet, an ADU starts with a Planning case on Boise's online permitting site. Attached ADUs then go through the residential alterations building application and detached ADUs through the new residential application. We prepare the site plan and drawings, submit the applications and schedule inspections through final approval.

Utilities and fees

Water, sewer and power for the ADU can sometimes share the main home's service and sometimes need new connections. Older North End and Bench homes may need an electrical panel upgrade. Impact and connection fees apply; under state law, cities over 10,000 people can't charge more for an ADU than for other single-family homes after February 1, 2027.

Common Boise ADU projects

• Alley cottages in the North End and East End that face the alley, keep the front of the lot unchanged and connect to utilities along the alley.
• Garage conversions of older detached garages into studios, often the quickest way to a rentable unit.
• Pre-approved plan cottages on the Bench, where larger, flatter lots fit one of the city's 280 to 695 square foot designs.
• Over-garage units where a new garage is needed anyway, using one foundation for both.

A typical Boise timeline

Most projects start with a lot check and a choice between a pre-approved plan and a custom design. A pre-approved plan shortens the design phase and simplifies plan review. Planning and building review follow, then construction: site work and utilities, foundation, framing and roof, mechanical rough-ins, insulation and drywall, finishes and final inspection. Utility work and inspection scheduling are the usual variables, so we plan them early and keep you updated at each step.

Before you call

A few things make the first conversation faster: your address, a rough idea of where on the lot you picture the unit, whether you plan to rent it or house family, and any survey or plat you already have. Photos of the backyard, the garage and your electrical panel help too.

Related: detached ADUs, garage conversions, ADU cost in Boise.