Site work & excavation
Lot prep, grading, utility trenching from main house, and final grading at completion.
Boise ADU Cost · 2026 Pricing
How much does a Boise ADU cost?
Updated: May 2026
Real 2026 pricing — $115k–$345k turnkey for pre-approved plans depending on size and finish. Custom builds run $200k–$450k. Below: the plan-by-plan numbers, what's included, and what actually moves the price.
Boise ADU cost — the headline number
A pre-approved Boise ADU costs $115k–$345k turnkey in 2026, depending on plan size and finish level. The 280 sqft Goldfinch starts at $115k; the 695 sqft Kestrel at Premium finish runs to $345k. Most homeowners land in the $170k–$245k zone with a 1-bedroom Kingfisher at Standard finish — the highest-ROI configuration for long-term rental. Those numbers are turnkey, fixed-price, and include site work, foundation, framing, MEP, insulation, drywall, finishes, paint, permits, and impact fees. They do not include furniture, landscaping, or premium appliance upgrades. Custom (non-pre-approved) ADUs typically run $200k–$450k depending on size, layout complexity, and finish — with 6–10 weeks added to the timeline for design and review.
Cost by pre-approved plan size & finish
Pricing methodology: The per-sqft rates below are derived from current Boise market builds and reconciled against the City of Boise ADU code and fee schedule. Numbers are real bid figures from our active jobs, not allowance ranges.
Ranges below come straight from our cost calculator math — Essential ($340/sqft), Standard ($405/sqft), and Premium ($480/sqft) with a typical 0.9–1.12 range factor for lot conditions. Same numbers your free estimate will quote.
| 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 |
| The Sandpiper | Type G-1 | 396 sqft | Studio | $121k–$151k | $144k–$180k | $171k–$213k |
| The Osprey | Type G-2 | 376 sqft | Studio | $115k–$143k | $137k–$171k | $162k–$202k |
Want a customized number? Use the cost calculator to slide square footage and finish — same math we use to quote.
What's included in the fixed price
Eight scope items every Boise ADU contract includes. If a competing bid is meaningfully cheaper, check whether it actually covers all of these — most surprise change orders come from missing scope on day one.
Foundation
Engineered concrete foundation appropriate for your soil, including footings, stem walls, and slab.
Framing
Floor system, walls, roof system, sheathing, and weather-tight enclosure.
MEP rough-in
Electrical, plumbing, and mechanical (HVAC) rough-in to current Idaho residential code.
Insulation
Wall, ceiling, and floor insulation meeting 2018 IECC + Boise amendments. Air sealing included.
Drywall & paint
Drywall hung, taped, textured, and primed. Two coats of finish paint on walls, ceilings, and trim.
Finishes
Cabinets, counters, flooring, fixtures, appliances (range, fridge, dishwasher, washer/dryer), and hardware per finish level.
Permits & inspections
All city permits (building, electrical, plumbing, mechanical), inspection fees, and final certificate of occupancy.
What's not included
Honest about the line items that fall outside a turnkey ADU build contract. Plan and budget for these separately.
Furniture & decor
Beds, sofas, tables, art, window treatments. Assume $5k-$15k to fully furnish a 1BR rental at midrange.
Premium appliance upgrades
Standard finish includes a working appliance package. Sub-Zero, Wolf, or other luxury packages run $8k-$25k extra.
Landscaping & hardscape
We backfill and rough-grade around the building. Sod, plantings, irrigation, patios, and walkways are separate (typically $5k-$20k).
Fences & privacy walls
If you want a new fence between the ADU and the main house yard, that's a separate scope. Plan $3k-$8k for a typical Boise lot.
Other Treasure Valley cities
ADU cost in your specific city
Per-sqft labor + materials run similar across the Treasure Valley. What changes city-to-city is the size cap, owner-occupancy rule, permit + impact fees, and rent ceiling.
What drives the price range
The same plan can land at the low end or the high end of its range depending on these four factors. The free estimate walks through each of them for your specific lot.
Finish level
The biggest cost lever. Essential ($340/sqft) gets you durable, builder-grade everything. Standard ($405/sqft) is the most common pick — quartz counters, LVP floors, mid-range fixtures. Premium ($480/sqft) adds tile baths, designer cabinets, hardwoods, upgraded windows. The same Kingfisher floor plan can range from $150k Essential to $264k Premium.
Lot conditions & excavation
Flat lots with easy utility access build cleanest. Sloped lots need engineered foundations. Lots with poor soil need over-excavation and engineered fill. Distant utility connections add trenching cost. Most Boise lots fall in the middle — but a difficult lot can add $10k-$30k.
Permit & impact fees
City of Boise building permit fees, school impact fees, sewer connection fees, and water connection fees typically total $8k-$15k for a Boise ADU per the City's published ADU program. Garden City and outlying jurisdictions can run higher. These are pass-through costs we include in the fixed price but they vary by address.
Plan customization
Sticking with a pre-approved plan as designed is the cheapest path. Material upgrades and minor interior tweaks are typically fine. Anything that changes the structural footprint or exterior elevations risks losing pre-approved status — which adds 4-8 weeks and roughly $8k-$15k in design fees.
Custom ADU pricing in Boise
Custom (non-pre-approved) ADUs typically run $200k–$450k turnkey in Boise depending on size, layout complexity, and finish level. The cost per square foot is roughly the same as a pre-approved plan at any given finish tier — what you pay extra for is the design itself ($8k–$15k architectural fees), the longer permit cycle (4–8 weeks added for full design review), and unusual structural details (cantilevers, complex rooflines, oversized openings).
Custom typically makes sense in three situations: (1) your lot has a constraint the pre-approved plans can't accommodate (steep slope, narrow access, oddly shaped buildable area), (2) your use case is specific (accessible aging-in-place layout, specific kitchen or workshop requirements, integrated home office), or (3) you genuinely want a one-of-a-kind design that's worth the time and money premium. For everyone else, the six pre-approved plans cover the use cases at materially lower cost.
Boise ADU cost — frequently asked
Impact fees and permit costs referenced below come from the City of Boise ADU program and the current Boise fee schedule.
What's the cheapest ADU you can build in Boise?
The smallest pre-approved plan, the Goldfinch (280 sqft studio, Type A), starts around $115k turnkey at Essential finish. That's the cheapest legal, permitted, code-compliant ADU in Boise. Any 'cheaper' number you see is either a shed conversion (not legal as a dwelling) or a number that excludes site work, permits, foundation, or finishes — none of which are optional.
What's the most expensive pre-approved plan?
The Kestrel (695 sqft 2BR, Type D) at Premium finish runs roughly $300k-$374k turnkey. It's the largest pre-approved plan and the only true 2-bedroom in the City of Boise's pre-approved set. For most landlords, Standard finish on a Kestrel ($253k-$315k) hits the best ROI — the Premium upgrades don't move rent enough to pay back.
How much more does a custom ADU cost vs pre-approved?
Custom ADUs in Boise typically run $200k-$450k depending on size, complexity, and finish. The cost-per-square-foot is roughly the same as pre-approved at any given finish level — what you pay extra for is design ($8k-$15k), the longer permit timeline (4-8 extra weeks), and the layout flexibility. Custom makes sense when the pre-approved set genuinely doesn't fit your lot or use case.
What are Boise impact fees on an ADU?
City of Boise charges impact fees for new dwelling units to fund parks, fire, and police. As of 2026, total Boise impact fees on an ADU run roughly $4,500-$7,000 depending on size. Add West Ada or Boise School District impact fees ($2,500-$4,000), sewer connection ($1,500-$3,000), and water connection ($1,000-$2,500). Total permit + impact fees usually land between $8k-$15k. We include them all in the fixed price.
Are there hidden costs with a Boise ADU build?
Not in our contracts — we quote a single fixed price covering site work through final inspection, including allowances for finishes you've selected. The places homeowners get surprised on other contractors' jobs: surprise excavation (rocky or wet soil), utility upgrades the main house wasn't ready for, design revisions after permit submittal, and finish allowances that turn out to be too tight. We pre-walk all four during the free estimate.
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Start with a free lot check — plan fit, lot conditions, and the right cost path for your goals. Or call (208) 297-2036.