How Much Does an ADU Cost in Boise?

The pre-approved plan path is a permitting shortcut, not a structure type of its own — it applies to specific detached and above-garage designs. Plan availability doesn't mean your lot is eligible; see Boise ADU Rules for that.

This is a qualitative grouping based on how these drivers behave in practice, not measured percentages — see the sections below for what each one actually involves.

A detached ADU needs its own full foundation, roof, and structure — the most expensive path per square foot, but the most flexible for site placement. Building above an existing garage reuses its foundation and typically costs less, but only if that garage can structurally carry the extra load.

Cost per square foot drops as size increases, since a kitchen, bathroom, and utility hookups cost about the same whether the unit is 400 or 900 sq ft. Boise's 900 sq ft cap (see Boise ADU Rules) sets the ceiling; several of the city's free pre-approved plans are smaller than that.

City of Boise's current pre-approved ADU plans. Two additional mirrored variants are listed as coming soon with no published size yet. Being pre-approved covers the design itself, not whether a specific lot is eligible to build it.

A site your crew can reach easily costs less to build on than one needing hand-carried materials or a narrow path around existing structures. Access affects labor cost directly, separate from the ADU's size or finish level.

A detached ADU usually needs its own electrical sub-panel and a new or upsized sewer/water connection. Distance from existing utility stub-outs, and whether there's spare capacity at all, can matter more than the structure itself.

Soil conditions, slope, and frost depth all affect foundation cost — and a detached ADU carries this cost in full, since there's no existing foundation to build on like an above-garage unit has.

Cabinetry, flooring, fixtures, and appliance grade swing cost meaningfully within the same footprint. A rental-grade package and a finish level matching the primary home are genuinely different budgets for the same floor plan.

A pre-approved Boise ADU plan — free, no design fee — skips custom design and engineering for a qualifying project. A custom design, or any change to a pre-approved plan, brings those costs back.

Beyond permit and impact fees, the review timeline itself carries real cost. A pre-approved plan generally moves faster through planning review than a custom design — which matters if financing or a rental timeline depends on it.

Removing an existing structure, tree, or paved surface — or regrading for drainage — is real cost a per-square-foot estimate doesn't capture.

Converting an existing garage is usually the lowest-cost path, since the shell, roof, and foundation already exist — but it can't reduce the lot's required parking (see Boise ADU Rules), and it's bound by the garage's actual condition. New construction costs more but isn't limited by an existing structure's footprint or location.