How Much Does an ADU Cost in Boise?

A detached ADU needs its own complete foundation, roof, and structure — the most expensive path per square foot, but the most flexible in where it sits on the lot. An ADU built above an existing garage reuses the ground floor and foundation already there, which typically lowers cost, but only works where the existing garage and its foundation can actually carry the additional load — a real structural question, not a given.

Cost per square foot generally falls as size increases, because a kitchen, bathroom, and utility connections cost roughly the same whether the unit is 400 sq ft or 900 sq ft — those fixed costs are just spread over less area in a smaller unit. Boise's 900 sq ft / two-bedroom cap (see Boise ADU Rules) sets the upper bound; several of the city's free pre-approved plans are considerably smaller than that.

A site a crew and materials can reach easily costs less to build on than one requiring hand-carried materials, a narrow side-yard path, or protection for an existing structure or landscaping along the way. Access affects labor cost directly, independent of the ADU's own size or finish level.

A detached ADU usually needs its own electrical sub-panel or service, and often a new or upsized sewer and water connection. How far the ADU sits from the existing utility stub-outs, and whether the existing service has spare capacity at all, both change this cost meaningfully — sometimes more than the structure itself.

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

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

One of Boise's pre-approved ADU plans — free to use, with no design fee — skips custom design and structural engineering entirely for a qualifying project. A custom design, or any modification to a pre-approved plan, brings design and engineering costs back into the budget.

Beyond the city's permit and impact fees noted above, the review timeline itself has a real carrying cost — a pre-approved plan generally moves faster through planning review than a custom design does, which matters if financing or a rental timeline depends on it.

Removing an existing structure, tree, or paved surface to make room for an ADU — or regrading a site for drainage — is real cost that a per-square-foot construction estimate alone doesn't capture.

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