Detached ADU Cost · 2026 Pricing
Updated: June 2026
A detached ADU in Boise runs $115k–$345k turnkey for pre-approved plans, $200k–$345k for custom. Below: the cost plan by plan, what moves the range, and the detached-specific factors — site access, utility trenching, and the 900 sqft cap.
A detached ADU — a stand-alone backyard home, not a garage conversion or attached suite — costs $115k–$345k turnkey in Boise for the six pre-approved plans, depending on size and finish. The 280 sqft Goldfinch starts at $115k; the 695 sqft Kestrel at Premium runs to about $345k. Most homeowners land between $170k and $245k with a 1-bedroom Kingfisher at Standard finish — the highest-ROI configuration for long-term rental. Custom detached ADUs run $200k–$345k. Every figure is fixed-price and includes site work, foundation, framing, MEP, insulation, finishes, permits, and impact fees — not furniture, landscaping, or premium appliance upgrades.
All six dwelling-only and dwelling-plus-garage plans build as detached structures. Ranges use our cost-calculator math — Essential ($340/sqft), Standard ($405/sqft), and Premium ($480/sqft) with a 0.9–1.12 factor for lot conditions. Same numbers your free estimate quotes.
| Plan | Type | Size | Beds | Essential | Standard | Premium |
|---|---|---|---|---|---|---|
| The Goldfinch | Type A | 280 sqft | Studio | $86k–$107k | $102k–$127k | $121k–$151k |
| The Waxwing | Type B | 396 sqft | Studio | $121k–$151k | $144k–$180k | $171k–$213k |
| The Kingfisher | Type C | 491 sqft | 1BR | $150k–$187k | $179k–$223k | $212k–$264k |
| The Kestrel | Type D | 695 sqft | 2BR | $213k–$265k | $253k–$315k | $300k–$374k |
Prefer the studio-plus-garage plans? The Sandpiper and Osprey add a garage to a detached studio — garage square footage doesn't count toward the 900 sqft cap.
The same plan lands at the low or high end of its range depending on these four factors. The free estimate walks each one for your lot.
The biggest lever. Essential ($340/sqft) is durable builder-grade; Standard ($405/sqft) is the most common pick — quartz, LVP, mid-range fixtures; Premium ($480/sqft) adds tile baths, designer cabinets, and hardwoods. The same 491 sqft Kingfisher ranges from about $150k Essential to $264k Premium.
A detached ADU is ground-up, so the crew has to reach the backyard. Wide side-yard access for a concrete truck and excavator keeps cost down; a narrow gate or a unit behind the house means wheelbarrowing concrete, pumping, or hand-digging — a real line item. Sloped or rocky Boise lots add engineered foundations and over-excavation, $10k–$30k on a difficult lot.
Detached units connect back to the main house's water, sewer, and electrical. A shared, sub-metered setup is cheapest. Truly independent metering for a clean rental separation adds $5k–$15k in trenching and connection fees. The longer the run from the ADU to the main and the harder the dig, the higher the number.
City of Boise building, electrical, plumbing, and mechanical permits plus park, fire, school, sewer, and water fees typically total $8k–$15k for a detached ADU. Outlying jurisdictions like Garden City or Canyon County cities can run higher. We include these pass-through costs in the fixed price, but they vary by address.
Three things shape a detached build's cost and value that don't apply to a garage conversion.
Boise caps detached ADUs at 900 sqft of finished living area (raised from 700 in the 2023 zoning rewrite). Every pre-approved plan — up to the 695 sqft Kestrel — sits under the cap, so cost is driven by the size you choose, not the ceiling.
Detached ADUs generally need a lot of at least 5,000 sqft. Smaller lots can still work as a garage conversion or attached unit, which changes the cost path entirely.
Detached units rent for $200–$400/month more than an attached suite of the same size and add roughly $50k–$80k in appraised value — the cost premium over a conversion is usually earned back in rent and resale.
A garage conversion is cheaper up front — $60k–$140k versus $115k+ for a detached build — because it reuses an existing foundation, walls, and roof. A detached ADU costs more but rents $200–$400/month higher and adds more appraised value. If you have a sound detached garage you don't need for parking, the conversion is the budget play. If you want maximum rent and resale, or your lot has room for a stand-alone unit, detached usually wins on long-term return.
See the full head-to-head in the detached vs. garage conversion guide, compare the garage conversion cost breakdown, or browse all detached ADU options.
A detached ADU in Boise costs $115k–$345k turnkey for the pre-approved plans in 2026 — from the 280 sqft Goldfinch at $115k to a 695 sqft Kestrel at Premium finish near $345k. Custom detached ADUs run $200k–$345k depending on size, layout, and finish. All figures are fixed-price and include site work, foundation, framing, MEP, insulation, finishes, permits, and impact fees.
The 280 sqft Goldfinch studio, a pre-approved plan, starts around $115k turnkey at Essential finish — the lowest-cost legal, permitted detached dwelling in Boise. Anything cheaper is usually a shed conversion (not a legal dwelling) or a quote that leaves out site work, foundation, permits, or finishes.
Yes — a detached build costs more than converting an existing garage ($60k–$140k) because you're pouring a new foundation and building new walls and roof. But detached units rent $200–$400/month higher and add more appraised value, so the higher cost is often the better long-term return. We build both and can run the numbers side by side.
Not required. Boise allows a detached ADU to share the main home's water, sewer, and electrical with sub-metering or a single bill, which is the cheapest path. Fully independent metering is optional and adds $5k–$15k in trenching and connection fees — most landlords skip it unless they want a clean utility split for the tenant.
Custom detached ADUs run $200k–$345k in Boise. The per-square-foot cost is about the same as pre-approved at any finish level — what you pay extra for is the design ($8k–$15k), a longer permit cycle (4–8 extra weeks), and any unusual structural work. Custom makes sense when your lot or use case genuinely doesn't fit the six pre-approved plans.
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