Boise building & ADU permits

Accessory dwelling units in Idaho are handled locally; with no statewide ADU mandate, requirements vary by city and county.

Idaho has no statewide upzoning law; residential density is determined by local zoning.

Boise allows one accessory dwelling unit on most residential lots under its Modern Zoning Code — the city's first comprehensive zoning rewrite since 1966, effective December 1, 2023. ADUs may be attached, detached, or interior conversions, and the rewrite removed Boise's former owner-occupancy requirement, so an owner is no longer required to live on site to have an ADU.

A Boise ADU is generally limited to 900 square feet or 70 percent of the primary home's floor area, whichever is smaller, with a maximum of two bedrooms — an increase over the old code's tighter cap — and minimum-lot-size and density standards do not apply to ADUs. Height and setback requirements follow the underlying zone rather than ADU-specific numbers. No additional off-street parking is required, and a code-compliant ADU is approved administratively, without a conditional-use permit or public hearing.

To make building easier, Boise offers a free pre-approved ADU plan program — a catalog of ready-to-use detached designs (studio to two-bedroom) that removes most of the design cost and shortens the timeline. The city has continued to refine its housing rules since the rewrite, so the per-lot allowance and related standards are worth confirming before you build.

This is an informational summary — verify current rules with the city's planning/permitting department; not legal advice.

See also: Idaho ADU law and what is an ADU?