ADU Construction Cost in Boise

The North End's alley-accessed lots frequently feature detached garages at the rear of the property that are underused as vehicle storage and ideally positioned for ADU conversion. These structures — typically 20 by 24 feet or larger — can accommodate a studio or one-bedroom ADU with a full kitchen, bathroom, and private entrance without requiring new foundation construction. The conversion scope includes: structural assessment and reinforcement as needed, insulation of walls and ceiling to meet Idaho Energy Code minimums, HVAC installation (typically a mini-split system for the ADU's independent climate control), kitchen and bathroom installation, window additions for egress and natural light compliance, electrical panel upgrade or subpanel addition, and finish work to a habitability standard appropriate for rental or family use. City of Boise ADU design standards require that detached ADUs match the primary home's architectural character in key ways — siding material, roofline style, window proportion — which the North End's Craftsman aesthetic accommodates beautifully.

Bench ranches on lots larger than the typical 6,000 square feet — lots of 8,500 square feet or larger that are common in certain Bench blocks — have room to add a new detached ADU without eliminating the yard space that makes these properties desirable. A new detached ADU built from a poured concrete foundation provides complete architectural independence from the primary home while generating rental income or family housing on land already owned. Design requirements for new detached ADUs in Boise are more stringent than for garage conversions — foundation depth of 36 inches minimum, energy code compliance for all wall, roof, and window assemblies, full utility connections, and architectural compatibility with the primary dwelling — and the permit and inspection process is more involved. Iron Crest manages the full design-permit-build sequence for new detached ADUs and has a streamlined process for Bench projects that reflects experience with the specific site conditions common in that neighborhood.

West Boise two-story homes with attached two-car garages often have structural capacity for a second-story addition above the garage — an attached ADU configuration that adds living space without requiring lot coverage for a new detached structure. These projects require structural assessment of the existing garage framing, engineered beam and bearing upgrades where needed, stair construction for exterior access (to maintain the ADU's separate entrance requirement), and full finish work to meet Boise's ADU habitability standards. Above-garage ADUs in West Boise are well-suited to in-law suite use because they are connected to the main home's structure while maintaining a private entrance and independent utilities. The above-garage configuration also adds equity value to the property in a way that many lenders are beginning to recognize in appraisals.

North End bungalows on lots where a rear detached structure is not feasible — due to tree root systems, setback constraints, or lot dimensions — can often accommodate an attached ADU addition off the rear of the primary home, with its own private exterior entrance, kitchen, bathroom, and living area. These attached additions require more design creativity than detached structures to maintain the visual integrity of the primary home while providing functional independence for the ADU occupant. In the North End, where Boise's design review standards require architectural compatibility with the neighborhood's historic character, attached ADU additions must be designed with the same care given to any addition in an architecturally sensitive area. Iron Crest's experience with North End design review ensures that attached ADU additions are approved rather than flagged for redesign.

East Boise neighborhoods — foothills-adjacent properties in Harris Ranch, Southeast Boise, and the Boise Bench's eastern extension — increasingly host multigenerational ADU projects where the goal is providing quality independent housing for a family member rather than generating rental income. These ADUs are typically specified to a higher finish quality than market-rate rental units: quartz countertops, tile showers, hardwood or LVP flooring, and quality cabinetry selections that match the standard of the primary home. The scope, timeline, and cost are similar to market-rate ADU construction, but the finish selections and livability features (wider doorways, grab bar blocking, accessible shower configuration) are tailored to the specific family member who will occupy the unit.