Boise Launches Free Pre-Approved ADU Plan Library to Speed Housing Permits

Boise cut the cost of adding a backyard cottage by publishing a free library of pre-approved accessory dwelling unit plans, removing one of the most persistent friction points in small-unit housing: the design-and-review cycle.

The city released the catalog on April 10 in partnership with a local architect, offering standardized footprints sized between 300 and 450 square feet for detached backyard cottages, alongside configurations for garage conversions and small attached ADUs. Each plan set arrives complete with structural details, sample electrical layouts, plumbing risers, and an energy-compliance checklist, meaning a homeowner can submit a permit package without hiring a designer to produce those documents from scratch.

The practical effect is a shorter permit pipeline on both sides of the counter. By pre-engineering the configurations most common on Boise lots, the program reduces the municipal review workload while eliminating a significant share of soft costs for homeowners: architect fees and iterative plan revisions that often inflate the total price of a small ADU well beyond its construction cost.

For the tiny home and micro-ADU community, design standardization carries a compounding benefit. Builders can pre-price packages tied to the approved plans and offer fixed-cost, quick-turn installs for homeowners looking to rent a unit or age in place. The 300-to-450-square-foot detached cottage range puts Boise's standard footprints squarely in tiny home territory, and the garage conversion guidance opens a second path for lots where a freestanding structure isn't viable.

Homeowners whose lots don't match any catalog design exactly aren't shut out. The city's guidance recommends discussing needed modifications with plan review staff to determine which changes qualify as minor amendments, preserving much of the speed advantage even when some customization is required. Confirming utility capacity and setbacks before committing to any footprint remains an essential first step.

Boise's approach fits a growing pattern of municipalities using pre-approved plan libraries as a near-term tool for expanding housing supply without waiting on large multifamily projects. The program targets the soft-cost barrier directly, absorbing design expenses at the municipal level and distributing the result at no charge, shifting the economics in a way that makes a sub-500-square-foot backyard unit viable for a broader range of homeowners.

If the program delivers faster approvals and measurable ADU production growth, Boise's April 10 release may prove to be less a local housing initiative and more a replicable model for cities facing the same constraints.