From detached guest houses to garage conversions — we handle zoning research, design, permitting, and full construction of accessory dwelling units.
Accessory dwelling unit construction in Mountain Home, Idaho is shaped by one economic fact above all others: Mountain Home Air Force Base, twelve miles southwest, does not house its full assigned population on base, so the city's rental market is structurally tied to military demand and turnover. A well-built ADU in Mountain Home is not a speculative bet — it is a unit pointed at a real, recurring tenant pool of airmen and their families on two-to-four-year permanent-change-of-station cycles. Mountain Home is the Elmore County seat, a community of just under 16,000 at roughly 3,150 feet on the western Snake River Plain, with larger lots than dense Ada County and a meaningful share of surrounding acreage on private well and septic. Iron Crest Remodeling Group LLC, operating as Iron Crest Remodel (Idaho RCE-6681702), builds ADUs here with the zoning, utility, and climate realities the city actually imposes — verifying jurisdiction and zoning allowance for the specific parcel before promising anything, because ADU rules differ between the City of Mountain Home and unincorporated Elmore County, and because what is buildable on a Blue Sage acre with a septic system is a different project than an attached unit on a city-sewered downtown lot. This is hyper-local by necessity, not a templated ADU pitch with a city name swapped in.
Build an ADU that adds usable space, flexibility, and long-term property value.
An ADU (Accessory Dwelling Unit) is a self-contained living space on the same lot as an existing home. ADUs have become increasingly popular in the Boise area as housing demand has grown, zoning rules have evolved, and homeowners have recognized the financial and lifestyle benefits of adding a separate living unit to their property. ADU types include detached new construction (a standalone building on the lot), garage conversions (converting an existing garage into living space), attached additions (building a unit that shares a wall with the main home), and basement conversions (converting a finished or unfinished basement into a separate unit with its own entrance). Every ADU project requires careful navigation of local zoning rules, setback requirements, utility connections, parking requirements, and building code compliance. The design must balance livability, code compliance, construction cost, and long-term value. A well-built ADU adds $100,000+ in property value while generating $800-1,500+ per month in rental income in the Boise market.
Mountain Home homeowners pursue adu construction for a variety of reasons. Here are the most common situations we see:
Not every adu builder project is the same. Here are the most common project types we complete in Mountain Home:
A standalone structure built on your property — typically 400-1,000 square feet with a bedroom, bathroom, kitchen, and living area. This is the most popular ADU type and offers the most design flexibility.
Convert an existing attached or detached garage into a living space. Includes insulation, drywall, flooring, plumbing, electrical, HVAC, kitchen, and bathroom installation within the existing structure.
Build an ADU that shares one or more walls with the main home but has its own entrance, kitchen, bathroom, and living space. Similar to a home addition but designed as an independent unit.
Convert an existing basement into a separate dwelling unit with its own entrance, kitchen, bathroom, and living area. Requires egress windows, fire separation, and independent utility metering in most jurisdictions.
Mountain Home's housing spans a pre-war downtown core, a dominant 1950s-1970s air-base-era ranch belt tied to the base's Cold War growth, 1990s-2010s subdivisions, and recent custom acreage. The 2020 census recorded about 6,600 housing units.
Railroad-era and pre-war homes with galvanized plumbing, aged or knob-and-tube wiring in the worst cases, plaster and original wood, and frequent subfloor and structural deterioration. Pre-1978 lead and pre-1980 asbestos requirements apply.
The city's largest layer: simply framed ranches and split-levels built as Mountain Home AFB expanded, with original single-pane aluminum windows, galvanized supply lines, undersized electrical, minimal insulation, closed kitchens, single bathrooms, and no primary suite. Pre-1980 environmental testing required.
Production subdivision homes with modern systems and builder-grade finishes now aging out of relevance. No asbestos or galvanized concerns; straightforward upgrade candidates.
Custom homes on one-acre and rural parcels, many on private well and septic, built to modern code and high finish.
Material selection affects the look, durability, and cost of your adu builder. Here are the most popular options we install in Mountain Home:
Most detached ADUs in Idaho use a concrete slab-on-grade or stem wall foundation depending on lot conditions, frost depth, and plumbing requirements. Garage conversions may use the existing slab with modifications.
Best for: Detached ADU new construction
2x4 or 2x6 wood framing for walls, with trusses for the roof. ADU framing follows the same building codes as primary residences, including insulation requirements, fire separation, and structural standards.
Best for: All ADU types
The most common heating and cooling solution for ADUs. A ductless mini-split provides efficient heating and cooling with a small exterior compressor and one or two interior wall units. No ductwork required.
Best for: Detached ADUs and garage conversions
ADU kitchens need to be efficient. A compact kitchen typically includes a 24-inch range, apartment-size refrigerator, single-bowl sink, and upper and lower cabinets — all designed to maximize function in a smaller footprint.
Best for: Studio and one-bedroom ADUs
The ADU exterior should complement the main home. Options include matching the existing siding exactly, using a contrasting but compatible material, or using a modern material like board-and-batten or metal panel for a contemporary look.
Best for: Seamless property aesthetic
Here is how a typical adu builder project works from first contact to final walkthrough:
We research your property's zoning designation, lot size, setback requirements, maximum ADU size allowed, parking requirements, and any HOA restrictions. Not every lot qualifies for an ADU, so this step is critical before investing in design.
Based on feasibility findings, we develop a concept design including floor plan, placement on the lot, utility connection points, and exterior style. You receive a preliminary budget range to confirm the project is viable.
Detailed architectural plans are prepared including floor plans, elevations, structural engineering, mechanical systems, and site plan. These plans must meet local building codes and will be submitted for permit review.
We submit plans for permit review, coordinate utility connections (water, sewer, electrical, gas), and manage any required inspections or reviews. ADU permitting can take 4-8 weeks depending on the jurisdiction.
Excavation, grading, utility trenching, and foundation work. For detached ADUs, this typically means a new concrete foundation. Garage conversions may require foundation modifications.
Complete construction including framing, roofing, siding, windows, insulation, drywall, flooring, kitchen, bathroom, HVAC, electrical, plumbing, and all finish work. The ADU is built to the same code standards as a primary residence.
All required inspections are passed, the certificate of occupancy is issued, and the ADU is ready for use. We provide a complete walkthrough and all warranty documentation.
Here is what to expect for project duration when planning a adu builder in Mountain Home:
| Phase | Duration | Details |
|---|---|---|
| Zoning Research and Feasibility | 1–2 weeks | Confirm the property qualifies for an ADU under current zoning, identify setback and size constraints, and determine utility connection feasibility. |
| Design and Engineering | 4–8 weeks | Architectural plans, structural engineering, site plan, and mechanical design. ADU designs must meet full building code requirements. |
| Permitting | 4–8 weeks | Plan review, permit issuance, and any required revisions. ADU permitting timelines vary by jurisdiction in the Treasure Valley. |
| Site Work and Foundation | 2–4 weeks | Excavation, utility trenching, foundation pour, and curing. Weather-dependent in Idaho, especially during winter months. |
| Framing, Roofing, and Mechanical | 4–8 weeks | Framing, roof installation, windows, exterior sheathing, electrical, plumbing, HVAC, and insulation. All rough-in inspections are completed. |
| Interior Finish and Final Inspection | 4–6 weeks | Drywall, paint, flooring, kitchen, bathroom, fixtures, and all finish details. Final inspections and certificate of occupancy. |
Mountain Home range: $95,000–$145,000 – $260,000–$420,000+
Most Mountain Home projects: $150,000–$240,000
Mountain Home ADU costs run modestly below Boise-proper but the gap is narrower than home values imply because Elmore County's trade pool is thinner and crews often mobilize from the Treasure Valley. The low band covers a compact attached or garage-conversion ADU tying into existing utilities. The average band covers a freestanding one-bedroom detached ADU with its own foundation to the 24-inch frost depth, full envelope for the high-desert climate, independent systems, and a kitchen and bath. The high band covers larger two-bedroom detached units and high-finish acreage ADUs. The defining local cost variables are utilities and site: a detached ADU on a city-sewered lot ties into municipal service, but a detached ADU on a well-and-septic acreage parcel may require a septic-system expansion or a second system and a well-capacity evaluation — a Central District Health matter that can add substantial cost and timeline and that has no equivalent on city utilities. Trenching distance from the main home to a detached unit, the code-depth frost foundation, and the full insulated envelope this climate demands are the other major drivers.
The final cost of your adu builder in Mountain Home depends on several factors. Here are the biggest cost drivers:
A detached new-construction ADU costs significantly more than a garage conversion because it requires a new foundation, full framing, roofing, and all-new utility connections. Garage conversions leverage the existing structure.
ADUs range from 300 sq ft studios to 1,000+ sq ft two-bedroom units. Larger units cost more but provide more rental income potential and livability.
Connecting water, sewer, electrical, and gas to the ADU site involves trenching, new service lines, and potentially utility upgrades. Distance from the main house to the ADU affects cost.
Every ADU needs at least a bathroom and kitchen. The finish level — basic vs. mid-range vs. premium — significantly affects the mechanical and finish costs.
Sloped lots, limited access for equipment, rocky soil, or mature trees in the building area can increase site preparation and foundation costs.
ADU permit fees, impact fees, and utility connection fees vary by jurisdiction. Some Boise-area jurisdictions have reduced or waived impact fees for ADUs to encourage construction.
These are the real-world projects we see most often from Mountain Home homeowners:
The signature Mountain Home ADU: a freestanding one- or two-bedroom unit built on a homeowner's lot specifically to rent into the base-driven market. Scope includes a code-depth frost foundation, a fully insulated high-desert envelope, independent electrical and HVAC, a durable kitchen and bath specified for tenant turnover on PCS cycles, and utility connections. The design priority is durability and serviceability for a rotating tenant pool — the same total-cost-of-ownership logic that governs AFB rental housing here — rather than the cheapest possible build, which fails between tenancies. Zoning allowance for a rentable ADU on the specific parcel is verified with the governing authority before the project is committed.
On lots where a detached structure is constrained or not allowed, an attached ADU (an addition with independent entry, kitchen, and bath) or a conversion of an attached garage into a dwelling unit is the path. Scope includes structural and envelope upgrades to bring a garage to dwelling standards — proper insulation for the high-desert climate, egress, a code-compliant kitchen and bath, independent or sub-metered utilities as required, and HVAC. This is typically the lowest-cost ADU route in the city because it reuses existing structure and proximity to utilities, but the conversion has to meet full residential code, not garage standards.
A detached or attached unit built for a parent, in-law, or family member rather than rental income — common in Mountain Home's military community where households flex around deployments and elder care. Scope emphasizes accessibility: a curbless bath with blocking for grab bars, lever hardware, single-level living, and a layout that is independent but connected to the main household. On well-and-septic acreage, the added occupancy and fixtures require a Central District Health capacity review. Built to the same code and envelope standard as a rental ADU, with the finish and accessibility tuned for long-term family occupancy.
On Blue Sage's one-acre lots and surrounding rural parcels, land is rarely the constraint — utilities and jurisdiction are. A detached ADU here is typically permitted through Elmore County rather than the city, and because most parcels are on private well and septic, the project hinges on a septic-capacity evaluation (and possibly a second system) and a well-capacity assessment through Central District Health before design. Water chemistry is often hard and iron-bearing, influencing plumbing and any treatment. These ADUs can be larger and higher-finish, matched to a custom-home property, but the utility engineering is the critical path.
Solution: A detached ADU on your property generates $800-1,500+ monthly rental income while you continue living in your primary home.
Solution: An ADU with a separate entrance provides privacy and independence while keeping family close. Accessibility features can be built in from the start.
Solution: A garage conversion ADU transforms underutilized space into a functional living unit at a lower cost than new construction.
Solution: A detached ADU configured as a studio or office provides the separation remote workers need, with the commute of a backyard walk.
Solution: A well-built ADU adds $100,000+ to property value and generates ongoing rental income — one of the highest-ROI improvements a homeowner can make.
High-desert climate at roughly 3,150 feet on the open western Snake River Plain: cold winters, hot dry summers, very low humidity, large daily temperature swings, intense unobstructed UV, and strong wind.
Frequent 30+°F daily swings cycle tile, grout, caulk, siding, and waterproofing joints aggressively, making movement-accommodating detailing essential.
Open, treeless plain accelerates fading and degradation of exterior paint, decking, and cladding, and interior fading on sun-exposed rooms.
30 lb ground snow load and a 24-inch frost depth (Mountain Home area, below Tollgate) govern foundations, decks, and roofed structures; cold floors raise demand for in-floor heat.
115 mph residential design wind speed off the open plain drives siding fastening, window structural specs, and roofed-structure engineering; wind-borne grit abrades finishes.
Very dry interiors shrink and gap unacclimated wood flooring and cabinetry and reopen drywall seams; sealed winter homes still concentrate bathroom moisture.
Seismic Zone C (south of Featherville, includes Mountain Home) applies to structural and lateral detailing on additions and reconfigurations.
The oldest residential blocks around the railroad-era street grid, including landmarks like the 1910 Bengoechea building; pre-war and early-mid-century homes with aged systems.
Common projects in Downtown / Historic Core:
The city's largest housing layer, built as Mountain Home AFB expanded through the Cold War: simply framed three-bedroom, one-bath ranches with original systems and closed layouts. Split between owner-occupants and owner-landlords renting to base personnel.
Common projects in Air-Base-Era Ranch Belt (1950s-1970s):
1990s-2010s production-home build-out on the north and east edges; modern systems, builder-grade finishes aging out, frequently sold to inbound military buyers using VA financing.
Common projects in Newer Subdivisions (Silverstone, Morning View):
Blue Sage's one-acre custom-home lots and surrounding unincorporated rural parcels, many on private well and septic and permitted through Elmore County rather than the city.
Common projects in Blue Sage & Rural Acreage:
Every Mountain Home neighborhood has different housing stock, homeowner priorities, and project considerations. Here is what adu builder looks like in each area:
Permit authority: City of Mountain Home Building Department (in city limits) or Elmore County Land Use and Building Department (unincorporated)
Here are the design trends we see most often in Mountain Home adu builder projects:
Mountain Home's 2024 median home value was approximately $309,400 (Data USA), well below most of Ada County. The market is strongly influenced by Mountain Home Air Force Base: modest, fast-moving inventory, a large share of inbound military buyers using VA financing on relocation deadlines with appraisal condition review, and a substantial owner-landlord/investor segment serving base-driven rental demand. Schools are served by Mountain Home School District No. 193. This price band and buyer profile make competent, finished, defect-free remodels closer to a condition of sale than discretionary upgrades, and make durability-for-turnover the governing logic for rental work.
Avoid these common pitfalls Mountain Home homeowners encounter with adu builder projects:
Better approach: City of Mountain Home and Elmore County treat accessory dwellings differently, and subdivision covenants can be stricter than either. Confirm which authority governs the parcel and exactly what it permits — detached vs. attached, rentable vs. family-only, size and parking rules — before design or budget, never after.
Better approach: On the well-and-septic acreage common around Mountain Home, a new dwelling unit's load typically requires a Central District Health septic-capacity evaluation (often a system expansion or second system) and a well assessment. This is the critical path and a major cost item; price and confirm it before treating the ADU budget as real.
Better approach: An ADU's high envelope-to-volume ratio makes insulation and air-sealing disproportionately important. At 3,150 feet a poorly insulated unit is costly to condition and uncomfortable year-round, undermining its rental value. Specify continuous insulation, an air-sealed shell, high-performance windows, and a right-sized efficient system.
Better approach: A unit aimed at the PCS tenant pool faces repeated turnover. Cheap finishes fail between tenancies and erode the income that justified the build. Use durability-for-turnover specifications — plywood-box cabinetry, quartz or solid surface, slip-resistant flooring, serviceable fixtures — costed across tenancies, the same logic that governs base-area rental housing.
Better approach: A freestanding ADU is a complete small building with little structural redundancy. Mountain Home's published frost depth below the Tollgate line is 24 inches; a shallower foundation will heave and crack catastrophically. Build the foundation to full code depth.
It can be, and the reason is specific to this town: Mountain Home Air Force Base does not house its full assigned population on base, so there is a recurring, installation-driven rental demand from airmen and their families that turns over on PCS cycles rather than evaporating. That makes the income case more structurally grounded than a speculative ADU in a purely civilian market. The keys are verifying that the parcel's zoning (city or Elmore County) actually permits a rentable ADU before committing, and building the unit with durability-for-turnover specifications so it performs across tenancies rather than degrading between them.
Often yes — land is rarely the constraint on acreage — but utilities and jurisdiction are the gating issues. Most surrounding acreage is on private well and septic, so a new dwelling unit's added load typically requires a Central District Health septic-capacity evaluation (frequently a system expansion or a second system) and a well-capacity assessment before design. Unincorporated parcels are governed and permitted by Elmore County, not the city. We resolve the utility and jurisdiction picture first, because it determines whether and at what cost the ADU is feasible.
It depends on whether the parcel is inside Mountain Home city limits or in unincorporated Elmore County. In-limits parcels are governed and permitted by the City of Mountain Home (with a zoning permit preceding the building permit); unincorporated parcels by Elmore County. The two have different accessory-dwelling zoning treatments, and subdivisions may add covenant restrictions stricter than either. Confirming which authority governs the parcel and exactly what it allows there is the first, non-negotiable step — we do it before any design work.
Because an ADU is small, its envelope-to-volume ratio is high — proportionally more exposed surface per square foot of living space than a full house. At 3,150 feet with genuine winter cold, hot dry summers, and large daily swings, a poorly insulated ADU is expensive to heat and cool and uncomfortable year-round, which directly undercuts its value as rental or family space. We specify continuous insulation, a rigorously air-sealed shell, high-performance windows, and a right-sized high-efficiency system so the unit performs economically in this climate.
A compact attached or garage-conversion ADU typically runs $95,000–$160,000 over 10–16 weeks; a freestanding detached unit on a code-depth frost foundation with full systems runs $150,000–$260,000 over 16–26 weeks; larger or high-finish acreage units run higher and longer. The biggest cost and schedule variable is utilities: a city-sewered tie-in is straightforward, while a septic expansion or second system on an acreage parcel — a Central District Health process — adds substantial cost and time. We establish the utility path before quoting so the number is real.
Yes. ADU projects require building permits, plan review, and multiple inspections. In most Boise-area jurisdictions, ADUs also require zoning compliance review to confirm lot size, setbacks, and parking requirements are met. We handle the entire permitting process.
A detached new-construction ADU typically costs $120,000-200,000+ in the Boise area, depending on size, finish level, and site conditions. A garage conversion is typically $80,000-150,000. Costs include design, engineering, permitting, construction, and utility connections.
From start of design to move-in, a typical ADU project takes 6 to 12 months. This includes design (4-8 weeks), permitting (4-8 weeks), and construction (3-5 months). Garage conversions are faster; detached new construction takes longer.
In most Boise-area jurisdictions, yes. ADUs can be rented as long-term rentals. Short-term rental rules (Airbnb, VRBO) vary by city and may have additional restrictions. Check local regulations before planning a short-term rental strategy.
A well-built one-bedroom ADU in the Boise area can generate $800-1,500+ per month in rental income, depending on location, size, finish level, and market conditions. This income can offset or exceed the monthly cost of financing the ADU construction.
Maximum ADU size varies by jurisdiction. In Boise, detached ADUs can be up to 1,000 square feet or 10% of the lot area, whichever is less. Other cities in the Treasure Valley have different size limits. We confirm the specific rules for your property during the feasibility phase.
Owner-occupancy requirements vary by jurisdiction. Some cities require the property owner to live in either the primary home or the ADU. Others have relaxed or eliminated owner-occupancy requirements. We confirm the rules for your specific location.
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