Detached ADU Construction in Boise
Custom backyard cottages, guest houses, and rental units built from the ground up. The ADU type with the most privacy and the most design flexibility, sized 400 to 900 square feet, from permits to move-in.
Quick answer
A ground-up detached ADU in the Boise/Treasure Valley market runs $145,000 to $320,000 — you are essentially building a small new home with its own foundation, framing, roof, mechanicals, and finishes.
A detached accessory dwelling unit is a free-standing residential structure built on the same parcel as your primary home but physically separate from it. Unlike an attached ADU that shares a wall with the main house, a garage conversion that reuses an existing accessory structure, or a basement ADU carved out of the foundation footprint, a detached ADU sits on its own slab or crawl foundation, with its own walls, roof, exterior envelope, and dedicated entrance. It is a small, fully independent home in your backyard.
Most detached ADUs in the Treasure Valley fall between 400 and 900 square feet — large enough for a real one- or two-bedroom layout with full kitchen, full bathroom, and in-unit laundry, while staying under the size limits typically applied by City of Boise zoning. You will hear them called backyard cottages, granny flats, in-law suites, guest houses, casitas, or simply “the back unit.” The defining trait is independence: a complete dwelling that does not rely on the main house for its walls, roof, entrance, or (in many cases) utility metering.
Across the five ADU types — detached, attached, garage conversion, basement, and above-garage — detached units are consistently the most flexible from a design standpoint and the most valuable from a resale and rental standpoint. A detached ADU is the only configuration where the design starts from a blank slate. That freedom is also why it costs the most and takes the longest: every other ADU type starts with some portion of the work already done.
When a lot has room for one, a detached unit is the configuration homeowners in Boise, Meridian, Eagle, and the wider Treasure Valley ask us about most often. The reasons are consistent: privacy for both households, design freedom a conversion cannot offer, and a unit an appraiser can treat as a dwelling in its own right.
Complete Privacy for Both Households
Because the structure is physically separate, both parties get full acoustic and visual privacy. No shared walls, no footstep noise from above, no shared HVAC supply, no overlap of household routines. A detached unit is experienced as a small standalone home rather than as part of someone else's house, which is the difference tenants notice first. The same benefit applies for aging parents, adult children, or a remote-work studio.
It Rents as Its Own Home, Not Part of Yours
A standalone cottage with its own entry, its own yard space and no shared wall is a different product from a basement or garage conversion of the same square footage, and it is marketed and leased as one. We do not publish Boise rent averages or a rent premium figure — Treasure Valley rents move, and the number that matters is what comparable one-bedroom listings in your ZIP code are actually leasing for today. Ask a property manager working your neighborhood before you underwrite the project.
How Appraisers Treat a Detached Unit at Resale
Appraisers and buyers treat detached ADUs differently than finished interior space. Because the unit is a separate, code-compliant dwelling on its own foundation with its own systems, it gets recognized as a true second living unit rather than extra finished square footage. That distinction tends to drive stronger resale offers from buyers who want rental income or multigenerational housing. Exact lift is parcel- and market-specific — confirm with a local appraiser.
Full New-Build Design Control
A detached ADU is the only type where every design decision is open: floor plan, ceiling heights, window placement, orientation, roof pitch, style, indoor-outdoor relationship. Match the main house with a craftsman cottage on a North End property, build a clean modern volume on a Bench mid-century lot, or compose a casita-style unit on a newer Eagle or Meridian build. Conversion ADUs inherit the constraints of what already exists; detached ADUs do not.
Backyard Placement Preserves Curb Appeal
Detached ADUs typically sit toward the rear of the lot, behind the main house, well within setbacks. Street-facing curb appeal of the primary home stays intact and the ADU gets a private backyard enclave. In Boise's North End, many lots have improved alleys behind them, which gives the ADU a natural separate entrance, off-street parking, and utility routing without disturbing the front of the property.
Detached ADUs carry the highest upfront cost of any ADU type because the project starts from raw ground — new foundation, framing, roof, envelope, mechanicals, and finishes. The ranges below are the Boise/Treasure Valley ranges we quote for fully permitted construction, and they sit inside our overall Boise ADU range of $95,000 to $320,000. Planning ranges only, not quotes.
| Unit Type | Size (Sq Ft) | Installed Range |
|---|---|---|
| Studio / Efficiency | ~400 sq ft | $145,000 – $190,000 |
| 1-Bedroom | 500 – 650 sq ft | $145,000 – $220,000 |
| 2-Bedroom | 700 – 900 sq ft | $200,000 – $320,000 |
Installed cost per square foot: divide the ranges above by the sizes beside them and the result runs from roughly $220 per square foot on the largest, simplest units to roughly $475 on the smallest. A detached ADU is functionally a small new home with all the same code, inspection, and finish requirements, so the per-square-foot figure tracks small-scale new construction rather than remodeling. Ultra-small footprints land at the top of that spread because fixed costs (foundation mobilization, utility trenching, permit work, kitchen and bath fixtures) are spread across fewer square feet.
What Actually Moves the Cost Up or Down
- Foundation type and soil: Slab-on-grade or frost-protected shallow foundations are the most common — lowest cost. Stem-wall crawl space adds accessible mechanical space at higher cost. Full basements rarely pencil under a small unit. Bench and foothills lots sometimes need deeper footings or engineered solutions for expansive or sloped soils — a soils report drives final design.
- Utility trenching distance: Every foot between the main house and the ADU adds to water, sewer, electrical, and gas trenching cost. On deep lots where the ADU sits 60-100+ feet back, utility runs alone can absorb $5,000-$15,000. Alley access often shortens these runs.
- Electrical service capacity: A 100A main service often needs upgrade to 200A or a second meter/service drop, coordinated with the utility. Both a cost and timeline driver.
- Site access: Narrow side yards, fences, mature landscaping, sloped grade, or limited equipment access push hand-work hours and stretch the schedule. A lot where a small excavator drives straight to the build pad is cheaper than one where every yard of concrete is carried.
- Finish level: Cabinetry, countertops, tile, fixtures, lighting, and flooring are the most flexible line items. A standard finish package keeps you at the lower end of each range; a fully custom package can add $30,000-$60,000+ on a 700-900 sq ft unit.
- Tree, hardscape, and demo: Removing mature trees, relocating irrigation, demolishing sheds, or breaking up patios and driveways show up on most established Bench, North End, and Southeast Boise lots. We mark these on the site plan during design so they sit in the contract, not a change order.
- Permit and impact fees: The City of Boise (or applicable jurisdiction) charges building, trade, and plan-review fees, and in some cases impact or connection fees. These change with code cycles — verify current fee schedules with the permitting jurisdiction.
The most capable ADU type, also the most significant investment. Reading the trade-offs honestly before design begins is the difference between a project that delivers on the plan and one that runs into avoidable friction halfway through.
Advantages
- Complete privacy and separation — no shared walls, floors, or ceilings
- Leases as a standalone cottage rather than as part of the main house — pull current comparables for your ZIP code rather than a published average
- Strongest resale recognition — appraised as a separate dwelling, not extra finished space
- Full new-build design control — any floor plan, roofline, orientation, or style
- Independent metering possible — separate utility accounts for tenant billing
- Main house largely undisturbed during construction — work happens in the backyard
Limitations
- Highest upfront cost — $145,000 to $320,000 for ground-up construction
- Requires adequate lot — small or narrow city lots may not have a buildable envelope
- The zone's setbacks define the footprint (Boise sets no maximum lot coverage percentage) — verify current Boise code for your parcel
- Longest timeline — generally 5-8 months from contract to Certificate of Occupancy
- May require removing mature trees, sheds, or hardscape inside the build envelope
- Property tax reassessment likely after adding a permitted second dwelling
Boise has become one of Idaho's more ADU-friendly jurisdictions, and code in this area has been moving over recent cycles. The constraints that actually bind a detached unit are the ADU size cap, the setbacks of the underlying zone and the two-story height limit — Boise sets no maximum lot coverage percentage in its residential districts. Verify the current City of Boise or Ada County code against your parcel with Planning & Zoning at (208) 608-7100, and confirm whether your property is inside Boise city limits or in unincorporated Ada County, Eagle, Meridian, Garden City, Star, or Kuna — each jurisdiction sets its own ADU rules.
Eligible Zones and Lot Size
Detached ADUs are generally permitted across most Boise residential zoning designations on parcels with an existing single-family home. Boise does not apply a minimum lot-area or density standard to ADUs (Zoning Code 11-03-03.2.B(1)), so lot size is not a threshold test — what decides feasibility is whether the zone's setbacks leave a buildable rectangle once the existing house and other structures are plotted. Smaller, narrower, or irregularly shaped lots may still qualify but require a careful build-envelope study. Verify the underlying zoning, allowed uses, and ADU-specific allowances against your parcel before assuming a detached unit is feasible.
Setbacks and the Build Envelope
A Boise ADU has no setback standard of its own — it honors the dimensional standards of the underlying zone (Zoning Code 11-03-03.2.B(6)): 5 feet at the interior side and 15 feet at the rear in R-1C, 10 feet and 20 feet in R-1A and R-1B, plus that zone's front and street-side setbacks. Boise's residential districts set no maximum lot coverage percentage, so the setback envelope and the ADU size cap — not a coverage calculation — are what define the physical rectangle where the unit can sit. We pull your parcel record, overlay the current setbacks, and produce a build-envelope drawing before settling on a floor plan. Verify the current numbers against your parcel: setbacks change with code updates and overlay districts.
Maximum Size and Height
Boise caps an ADU at 900 sq ft or 70% of the primary building, whichever is smaller, so many detached ADUs end up sized in the 400-900 sq ft range. Height is capped at two stories (Zoning Code 11-03-03.2.B(3)) within the zone maximum — 35 ft in R-1A and R-1B, 3 stories / 40 ft in R-1C — which supports a single-story unit with vaulted ceilings or a compact two-story design on a smaller footprint. Note that staying under 14 ft is what qualifies a detached unit of 121-500 sq ft for the reduced setbacks in Table 11-04.4. Historic overlay districts, design-review areas, the North End and Hillside zones, and some neighborhood plans add additional rules on height, massing, roof pitch, exterior materials, and window placement. Always verify the current size cap, height limit, and any overlay rules against your address before locking in a footprint.
Alley Access, Driveways, and Parking
Alley access is one of the most useful features a lot can have for a detached ADU. Many North End blocks and some Bench blocks have improved alleys behind them, which gives the back unit a natural separate entrance, off-street parking that does not eat into the front yard, and shorter utility runs from pole-mounted services. On lots without alley access — common in Southeast Boise, parts of the Bench, and most newer Eagle and Meridian neighborhoods — the ADU entrance and any required parking come off the same driveway as the main house, which influences siting. Boise has updated its ADU parking rules: the Modern Zoning Code does not require an added off-street space for an ADU, and Idaho Code § 67-6541(2)(b) limits when a city may require one at all. Confirm what applies to your parcel with Planning & Zoning at (208) 608-7100 before finalizing the site plan.
Owner-Occupancy, STR, and Idaho Code Adoption
Boise dropped its ADU owner-occupancy requirement in the 2023 Modern Zoning Code, and Idaho Code § 67-6541 — enacted in 2026, in force July 1, 2026, with cities over 10,000 required to comply by February 1, 2027 — prohibits one. The Harris Ranch Specific Plan is the exception to watch: it still carries an owner-occupancy requirement and one parking space until Boise conforms its code. Short-term-rental rules are set separately and still change. On the building side, Idaho has adopted the 2018 IRC, the 2018 IECC, the 2017 Idaho State Plumbing Code and NEC 2023, with the Idaho Division of Occupational and Professional Licensing (DOPL) setting the framework and local jurisdictions enforcing it. Confirm the editions in force at the time of your permit submittal, and the current STR rules, before locking in the plan.
Systems decisions on a detached ADU follow the same logic as a small new home in the Treasure Valley — just at smaller scale.
Foundation
For most Boise detached ADUs, the answer is slab-on-grade with frost-protected footings, or a stem-wall crawl space if the design benefits from accessible underfloor mechanical space. Full basements under a 400-900 sq ft unit rarely pencil. Footings in Ada County are designed to a 24-inch frost depth, and the structure is designed for Boise's 20 psf ground snow load, 90 mph wind and Seismic Design Category C. On Bench, foothills, or sloped lots, a soils report is worth the small upfront cost — expansive or sloped soils sometimes need deeper footings or engineered solutions.
Framing, Envelope, and Energy Code
Conventional stick framing with engineered roof trusses is standard. Plate heights of 9 feet and a vaulted ceiling over the living area make a small footprint feel larger. Insulation values, window U-factor, and air-sealing are driven by the Idaho-adopted IECC edition in force at the time of your permit — currently the 2018 IECC, alongside the 2018 IRC. Confirm the edition before submittal, because adopted editions do update. Practically, Boise ADUs benefit from well-insulated walls, R-49-class attic, properly detailed rim joists, and quality dual-pane low-E windows. Small buildings reward tight air sealing.
HVAC — Ductless Mini-Splits Dominate
The dominant HVAC solution is a ductless mini-split heat pump: one outdoor condenser feeding one to three indoor heads, providing both heating and cooling. Efficient, quiet, fast to install, no full ductwork required. We specify cold-climate-rated heat pumps for Boise winters and confirm sizing with a proper load calculation. Optional electric resistance backup or a small gas furnace where the owner prefers it.
Plumbing & Electrical — Sub-Panel vs. Separate Service
Plumbing: a new sewer lateral tied to the main building sewer or directly to the City main, a domestic water tap from the existing service, and a properly sized water heater (small tank or compact tankless). Electrical: most detached ADUs run off a sub-panel pulled from the main house service. Older 100A services often need an upgrade to 200A; in some cases the utility recommends a separate service drop and second meter. A second meter is the right call when you want fully separated tenant billing.
Because a detached ADU is a new structure designed from scratch, the architectural style is fully your choice. The three most common directions we design for in the Boise/Treasure Valley market are summarized below. None of them is automatically right or wrong — the goal is fit between the ADU, the main house, the neighborhood, and how you plan to use the unit.
Craftsman / North End Cottage
Lap or shingle siding, a covered front porch, exposed rafter tails, divided-light windows, and a steeper roof pitch — designed to read as a small craftsman cottage that sits comfortably behind a 1920s North End or older Bench main house. This is the style that integrates most quietly on established Boise blocks and works well where a historic overlay or neighborhood character is in play.
Modern / Clean-Line
Standing-seam metal or simplified gable roof, board-and-batten or fiber-cement siding, larger expanses of glass, a single front entry sequence, and a restrained material palette. Works well on Bench mid-century lots, newer Eagle and Meridian properties, and as a deliberate counterpoint to a more traditional main house when the owner wants the ADU to read as its own building rather than a copy.
Accessory Match
The ADU is intentionally designed to echo the main house: same siding material, same roof pitch, same window proportions, same trim details, scaled down. This approach reads as “the two buildings belong together” and tends to be the safest resale story because it telegraphs intention rather than improvisation. We use this approach more often on Southeast Boise and newer subdivision lots where the main house has a strong architectural identity.
Finish levels span a wide range. A clean rental-grade interior uses durable LVP, painted MDF cabinetry, modest quartz counters, simple tile, and recessed lighting on dimmers. An owner-occupied or premium-rental interior steps up to engineered or solid hardwood, custom cabinetry, real quartz or natural stone, larger-format tile, and designer lighting. Pick the finish level honestly against how the ADU will be used — over-finishing a long-term rental rarely pays back in rent.
Realistic end-to-end timelines for Boise detached construction are roughly 5 to 8 months from signed contract to Certificate of Occupancy — the sum of the phases below. Weather, plan-review queues, utility coordination, and finish-package lead times can shift the schedule in either direction.
Design, Engineering, and Permitting — 7 to 14 Weeks
Site assessment, parcel research, build-envelope study, floor plans, elevations, structural design, MEP plans, energy compliance, and final construction documents (4-8 weeks), then plan submittal to the City of Boise (or applicable jurisdiction), plan-check comments, revisions, and permit issuance. The City publishes no guaranteed plan-review turnaround; across our own Boise ADU projects it has typically run 3-6 weeks, and an incomplete submittal restarts part of the clock. Confirm the current queue with Planning & Development Services at (208) 608-7100 before you build a schedule around it.
Site Prep and Foundation — 2 to 3 Weeks
Tree and hardscape removal where required, layout, excavation, footing placement, slab pour or stem-wall construction, and foundation inspection. Foundation work prefers above-freezing weather, which in Boise generally means April through October. Concrete needs proper cure time before framing begins.
Framing and Dry-In — 3 to 4 Weeks
Wall framing, roof truss set, sheathing, weather-resistant barrier, roofing, window and exterior door installation. By the end of this phase the building is dried in and protected from weather, which matters because the rest of the work happens inside.
Rough-In, Insulation, and Drywall — 4 to 6 Weeks
Electrical wiring, plumbing supply and DWV, HVAC line sets, ventilation, and utility tie-ins to the main house and City services, followed by rough-in inspections. Then insulation to the active Idaho-adopted IECC values, air sealing, insulation inspection, drywall hanging, taping, and texturing.
Finishes, Final Inspections, and CO — 4 to 6 Weeks
Interior paint, flooring, cabinetry, countertops, tile, plumbing and light fixtures, appliances, trim, exterior siding and paint, then landscaping restoration. Utility activation, final MEP and building inspections, and Certificate of Occupancy. We walk the punch list, close open items, and hand you keys. The 3-year workmanship warranty starts at substantial completion.
Seasonal strategy for Boise: Starting design and engineering in late summer or fall positions plan review through winter and groundbreak in early spring. That sequence puts foundation work in the dry-weather window, framing through summer, and finishes into the fall — the most efficient seasonal cadence for Treasure Valley detached construction.
Detached ADUs are feasible on qualifying lots throughout the Treasure Valley, but certain neighborhoods consistently make the math easier — through lot size, alley access, zoning posture, or proximity to rental demand.
North End
Improved alley access on most blocks gives a detached ADU a natural separate entrance, off-street parking, and shorter utility runs. Established craftsman character supports cottage designs that match the neighborhood. Deep lots and mature trees create natural privacy. Historic overlay rules apply on many North End blocks — verify current overlay requirements before settling on exterior materials or massing.
Southeast Boise
Some of the largest residential lots in the city, which gives a detached ADU room to breathe with generous setbacks and landscaped buffers. Many lots have established trees that need to be designed around. Most lots lack alley access, so the ADU entrance and any parking come off the main driveway.
Boise Bench
Reasonable lot sizes, mostly flat terrain that simplifies foundation work, and a short commute to downtown. Many Bench properties already have a detached garage or accessory structure in the backyard, which sets precedent and utility layout for an ADU. Alley access varies street by street.
West Boise, Eagle, Meridian & Beyond
Newer West Boise, Eagle, and Meridian lots often have the size and modern utility infrastructure to support detached ADUs comfortably. Wider side yards lower site-prep cost. ADU regulations in Eagle, Meridian, Garden City, Star, Kuna, and unincorporated Ada County differ from Boise — verify the applicable jurisdiction's current ordinance for your parcel.
A detached ADU is a six-figure capital project. Two value streams matter: monthly rental income while you hold, and lift in resale value at sale. They are related but not the same number, and treating them as one is a common planning mistake.
We do not publish market rent averages or projected ROI tables for Boise ADUs — Treasure Valley rents move enough that any figure we printed would be stale, and the honest source for a rent number on your property is current comparable listings from a property manager working your ZIP code. What we can show you is the recorded outcome of a real project we completed.
Recorded figures — North End detached ADU
- 600 sq ft, one bedroom, full kitchen and bath, covered porch
- Budget range: $120,000–$150,000
- Project duration: 16 weeks
- Appraised added value: $155,000
- Rental potential recorded for this unit: $1,400–$1,600 per month
These are this project's own recorded figures, not a Boise market average, and they do not predict what another lot will produce. Full budget, phasing and photos: 600 sq ft Detached ADU Build in North End Boise.
Whatever rent figure you land on, subtract the four costs gross rent hides before you call it a return: vacancy and property management if you use it, a maintenance reserve, the insurance increment, and the property tax increase Ada County assesses once the certificate of occupancy is issued (talk to the Ada County Assessor). Build the pro forma with a local property manager and a CPA, not from a contractor's website.
On the resale side, value lift varies with the local market, parcel, neighborhood, main house, and finish level, and a detached unit is the configuration an appraiser can most readily treat as a separate dwelling rather than extra finished square footage. Confirm parcel-specific impact with a local appraiser. One more item to settle early: short-term-rental regulation, if your plan depends on STR income rather than a long-term lease — verify current City of Boise STR rules, plus your HOA covenants, lender and insurer, before underwriting that revenue stream.
Detached ADUs are the most capable ADU type, but not always the right one. We turn down detached scopes on a regular basis when a different ADU type fits the lot, budget, or timeline better. Honest cases where detached is the wrong call:
The Lot Is Too Small, Narrow, or Steep
Small city lots, narrow flag lots, or sloped properties may not have a code-compliant build envelope once the zone's setbacks are plotted. Forcing a detached unit onto a marginal lot produces an awkward structure that takes the whole backyard. A garage conversion or basement ADU usually delivers more livable space for less money on those lots.
The Budget Is Below the Ground-Up Range
Under roughly $145,000, a ground-up detached build will not pencil at code-compliant quality. A garage conversion is a realistic permitted ADU at meaningfully lower cost, a basement ADU often delivers the best dollars-per-square-foot, and above-garage sits between. Forcing a detached unit into a too-small budget typically shows up as compressed scope or finishes.
The Main House Already Has Usable Basement or Garage
A main house with adequate basement ceiling height, egress potential, and moisture control already has the bones of a basement ADU. A detached garage with adequate ceiling height and slab condition already has the bones of a conversion. Building a second structure when an existing one can host the unit is rarely the most efficient use of capital.
The Holding Period Is Short
A detached ADU carries the largest upfront capital outlay of any ADU type. If you plan to sell within a couple of years, a lower-cost conversion may yield a stronger net return after transaction costs and reassessment effects. Run the numbers with your CPA and a local appraiser.
When detached is not the best fit, the in-home estimate ends with an honest recommendation toward the ADU type that does match your goals. Full pages on garage conversion, basement, above-garage, and attached ADUs, plus a detached vs. garage conversion comparison.
Common questions Boise homeowners ask about detached ADU construction.
How much does a detached ADU cost to build in Boise?
A ground-up detached ADU in the Boise/Treasure Valley market runs $145,000 to $320,000 — you are essentially building a small new home with its own foundation, framing, roof, mechanicals, and finishes. As a planning guide: a studio near 400 sq ft starts around $145,000-$190,000, a one-bedroom of 500-650 sq ft runs $145,000-$220,000, and a two-bedroom of 700-900 sq ft runs $200,000-$320,000. The biggest drivers are foundation type, utility trenching distance, finish level, and lot access. We build a fixed-scope estimate after a free in-home site visit.
Do I need a permit to build a detached ADU in Boise?
Yes. A detached ADU is a new permitted dwelling, so the City of Boise requires a building permit, architectural and structural plans, and separate trade permits for plumbing, electrical, and mechanical work. Plan-review timelines and fees change — confirm the current City of Boise fee schedule and review queue for your application month with the Building Division at (208) 608-7070 or Planning & Zoning at (208) 608-7100. Iron Crest Remodel prepares and submits the plan set, responds to plan-check comments, schedules inspections, and delivers the Certificate of Occupancy. If your parcel is in unincorporated Ada County, Eagle, Meridian, Garden City, Star, or Kuna, the permitting jurisdiction differs — we confirm jurisdiction before drawing anything.
What are the setback, height, and size limits for a detached ADU in Boise?
A Boise ADU has no setback or height rules of its own — it must meet the dimensional standards of the underlying zoning district (Zoning Code Section 11-03-03.2.B(6)). In R-1C that means a 5-foot interior side and 15-foot rear setback; in R-1A and R-1B it is 10 feet and 20 feet. A detached accessory structure of 121 to 500 square feet and under 14 feet in height may instead use reduced setbacks of 3 feet at the side and 9 feet at the rear (Table 11-04.4). Height is capped at two stories for the ADU (Section 11-03-03.2.B(3)) within the zone's own maximum — 35 feet in R-1A and R-1B, three stories not to exceed 40 feet in R-1C. Size is capped at 900 square feet or 70 percent of the primary building, whichever is smaller, and is not tied to lot area. Overlay districts can add design review, so verify the current code against your specific parcel before locking in a footprint.
How big can my detached ADU actually be on my specific lot?
Two rules govern the maximum footprint: the ADU size cap (900 square feet or 70 percent of the primary building, whichever is smaller) and the setback requirements of your zone, which carve a build envelope inside the lot. Boise's residential districts do not set a maximum lot coverage percentage, so the envelope — not a coverage calculation — is what constrains a detached unit. On many Boise residential lots the result is a buildable footprint in the 400 to 900 sq ft range. A 5,000 sq ft city lot generally supports a smaller cottage; a 9,000+ sq ft lot in Southeast Boise or the Bench frequently supports the largest allowed size. Always verify current code against your parcel, and note that Idaho Code § 67-6541 requires Boise to raise the size floor to 1,000 square feet or 75 percent by February 1, 2027.
Do detached ADUs need separate utilities from the main house?
Not always. Boise detached ADUs are commonly built with a mix: a dedicated electrical sub-panel fed from the main service (or, on older 100A services, a service upgrade or second meter), a tap off the existing water service, a sewer line tied to the main building sewer or a new lateral to the City main, and either a gas tap or all-electric design. Fully independent meters cost more upfront but make tenant billing cleaner. Shared services are cheaper but mean the homeowner is the utility customer of record. We size the existing service and recommend the right approach for your rental plan.
How are detached ADUs heated and cooled in Boise's climate?
The dominant HVAC choice in the Treasure Valley is a ductless mini-split heat pump: one outdoor condenser and one or two indoor heads handle both heating and cooling for a 400-900 sq ft unit. They install fast, avoid full ductwork, and meet Idaho energy code expectations. Boise winters are cold enough that we specify cold-climate-rated heat pumps paired with proper envelope insulation, air sealing, and tight windows. All-electric ADUs simplify utility design. Idaho is currently on the 2018 IECC and the 2018 IRC; confirm the edition in force with the Boise Building Division at (208) 608-7070 before your permit submittal, because adopted editions do change.
Can I rent out a detached ADU in Boise — long-term or short-term?
Long-term rental of a permitted detached ADU is generally allowed in Boise. Short-term rentals (Airbnb, Vrbo) are subject to the city's separate short-term-rental rules and registration process, which are updated periodically — verify current STR code, registration fees, and any occupancy or licensing requirements before counting on STR revenue. We do not publish Boise market rent averages: Treasure Valley rents move enough that any figure we printed would be stale, and the honest source for a rent number on your property is current comparable listings from a property manager working your ZIP code. Outside the city limits, rules vary — Eagle, Meridian, Garden City, Star, Kuna, and unincorporated Ada County each have their own ADU and rental ordinances.
How long does a detached ADU project take from contract to move-in?
Plan on roughly 5 to 8 months end to end, adding up the phases below. Design and engineering run 4-8 weeks. The City of Boise publishes no guaranteed plan-review turnaround; across our own Boise ADU projects plan review has typically run 3-6 weeks, and an incomplete submittal restarts part of the clock — confirm the current queue with Planning & Development Services at (208) 608-7100. Active construction is 13 to 19 weeks: foundation 2-3 weeks, framing and dry-in 3-4 weeks, rough-in through drywall 4-6 weeks, finishes and final inspections 4-6 weeks. Foundation work prefers above-freezing weather, so many Boise homeowners design through winter and break ground in early spring.
What is the realistic ROI on a detached ADU in the Treasure Valley?
Two streams matter: monthly rental income and added resale value. We do not publish market rent averages or projected ROI tables for Boise ADUs — Treasure Valley rents move enough that any figure we printed would be stale, and the honest source for a rent number on your property is current comparable listings from a property manager working your ZIP code. What we can show you is the recorded outcome of one real project: our 600 sq ft one-bedroom detached ADU in the North End ran a $120,000-$150,000 budget over 16 weeks, appraised at $155,000 in added value, and the rental potential recorded for that unit was $1,400-$1,600 per month. Those are that project's own figures on that one lot, not a Boise market average. For your own numbers, speak with a local appraiser and your CPA about depreciation, property tax reassessment, and capital gains treatment. We don't quote ROI guarantees; we walk you through the inputs honestly.
How does a detached ADU compare to a garage conversion or basement ADU?
Detached ADUs cost the most and take the longest, but deliver the most privacy and full architectural freedom, and they are the configuration appraisers most readily treat as a separate dwelling rather than extra finished square footage. Garage conversions are the cheapest and fastest because the shell exists, but ceiling height, slab elevation, and zoning treatment of the original garage can constrain what's possible. Basement ADUs reuse existing foundation and walls for the lowest cost-per-square-foot, but require egress windows, code ceiling height, and careful moisture control in Boise's clay soils. See the garage conversion, attached, basement, above-garage, and detached-vs-garage comparison pages linked below.
Will building a detached ADU require removing trees or hardscape?
Sometimes — yes. A detached ADU needs a buildable rectangle inside your setbacks, equipment access during foundation and framing, and trenching corridors for utilities. On established North End, Bench, and Southeast Boise lots, mature trees, irrigation, patios, sheds, and fences sometimes sit in the build envelope. We try to design around significant trees because they are worth preserving, but in some cases a tree, an old detached garage, or a section of driveway has to come down to make the ADU feasible. We mark all of this on the site plan during design so there are no surprises at framing.
When is a detached ADU NOT the right choice?
Several real situations: if your lot is too small, too narrow, or too steep to fit a code-compliant build envelope, a garage conversion or basement ADU will almost certainly cost less and finish faster. If your budget is below roughly $145,000, a conversion type is more realistic than ground-up construction. If you don't want to lose backyard space or mature trees, an attached or above-garage configuration may preserve more usable yard. If the main house already has a usable basement with adequate ceiling height and egress potential, a basement ADU often delivers the lowest dollars-per-square-foot. We will tell you honestly at the site visit if a different ADU type fits your goals better.
Is Iron Crest Remodel registered and insured to build detached ADUs in Idaho?
Yes. Iron Crest Remodeling Group LLC (DBA Iron Crest Remodel) is registered with the Idaho Division of Occupational and Professional Licensing (RCE-6681702) and insured — Idaho registers contractors through DOPL rather than issuing a contractor's license — building throughout Boise, Meridian, Eagle, Nampa, Caldwell, Kuna, Star, Garden City, and the surrounding Treasure Valley. Every detached ADU project includes a fixed-scope contract, permit handling, code-compliant construction, and a 3-year workmanship warranty. Free in-home estimates. Call (208) 779-5551 Monday-Friday 7 AM to 6 PM.
A detached ADU is not the only option. Depending on your property, budget, and goals, another ADU type may be a better fit. Explore all of Iron Crest Remodel's ADU options below or visit our ADU Construction hub for a complete overview.
Garage Conversion ADU
Convert an existing garage into a living space — lowest cost, fastest timeline
Basement ADU
Transform unused basement space into a finished dwelling unit
Above-Garage ADU
Build a living unit above a new or existing garage structure
Attached ADU
Home addition with separate entrance and independent systems
Detached vs. Garage Conversion
Side-by-side comparison of Boise's two most-asked-about ADU types
ADU Construction Overview
Full ADU service hub — process, types, and where to start
Planning a detached ADU touches design, budget, code, and rental strategy. A few other places on the site that help.
ADU Construction Hub
All ADU types, process, and Boise-specific guidance in one place
Service Regions
Boise, Meridian, Eagle, Nampa, Caldwell, and the rest of the Treasure Valley
Guides
Long-form remodeling and ADU guides from Iron Crest Remodel
Contact Iron Crest
(208) 779-5551 — Mon-Fri 7 AM to 6 PM, free in-home estimates
Get a Free Estimate
Request a fixed-scope detached ADU estimate for your Boise parcel
Detached vs. Garage Conversion
Direct comparison of the two most-asked-about ADU types
The following government agencies, industry organizations, and official resources provide additional information relevant to your remodeling project.
Ready to Build a Detached ADU?
Every backyard is different. Call Iron Crest Remodel at (208) 779-5551 Mon-Fri 7 AM to 6 PM, or request a free in-home site assessment online — we will tell you honestly whether a detached unit, a conversion, or a different ADU type fits your Boise parcel best. Registered with Idaho DOPL (RCE-6681702) and insured, with a 3-year workmanship warranty.