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 New Plymouth, Idaho is shaped by the town's defining trait: land. New Plymouth was platted in 1896 as an irrigation colony, with the Plymouth Society of Chicago and William E. Smythe laying out a horseshoe of two streets around a mile-long Boulevard park and dividing the colony into generous acre tracts meant to carry a house, garden, and pasture. That heritage, plus the working farm and ranch acreage surrounding the town, gives many New Plymouth properties exactly what ADUs need and what suburban lots rarely have: space, separation, and a logical place to put a second dwelling. With a 2020 Census population of 1,494 in a multi-generational agricultural community, the ADU demand here is genuine and practical — quarters for aging parents, housing for adult children and farm help, and flexible space tied to land that stays in families. Iron Crest Remodel (Iron Crest Remodeling Group LLC, Idaho RCE-6681702) builds New Plymouth ADUs with attention to the realities that govern them: the difference between city-limits zoning under New Plymouth and the Payette County jurisdiction that covers most farm parcels, the decisive role of private septic capacity on rural sites, frost-protected foundations to a 24-inch depth, and Payette County's 30 psf ground snow load. The town's evolving ADU rules and the rural-versus-municipal utility split make early jurisdiction and capacity analysis the single most important step in any New Plymouth ADU project.
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.
New Plymouth 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 New Plymouth:
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.
New Plymouth's housing is older and more layered than the suburban Treasure Valley: a 1896 colony-era and pre-1940 farmhouse core, a deep 1950s–1970s ranch layer, and a modest post-2000 subdivision minority. Most homes sit over vented crawlspaces.
Original colony and early-twentieth-century farmhouses around The Boulevard. Plaster-and-lath interiors, original wood siding and single-pane sash, galvanized supply lines, undersized electrical service, and crawlspace subfloors. Pre-1978 lead-paint and pre-1980 asbestos handling required.
Ranches and ramblers built as irrigated agriculture matured. Sound framing, aging copper plumbing, marginal panels, single-pane or early aluminum windows, thin insulation, and closed floor plans. Pre-1978/1980 environmental rules still apply.
Post-2000 builds such as Harvest Creek. Modern PEX plumbing, adequate electrical, and builder-grade finishes on tighter lots. No environmental-testing requirements.
Material selection affects the look, durability, and cost of your adu builder. Here are the most popular options we install in New Plymouth:
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 New Plymouth:
| 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. |
New Plymouth range: $95,000–$160,000 – $320,000–$550,000
Most New Plymouth projects: $170,000–$320,000
New Plymouth ADU costs are driven by foundation, utilities, and the build type far more than by local labor rates, which track valley norms. The low range covers a compact detached studio or one-bedroom ADU on a rural parcel with favorable existing utility capacity and simple site work. The high range covers larger two-bedroom detached units or substantial shop-apartment builds with full custom finishes, significant site and utility infrastructure, and challenging access. The average band reflects a typical New Plymouth ADU: a 500–900 square foot detached one- or two-bedroom unit with its own frost-protected foundation, full kitchen and bath, independent HVAC, and finished interior, including the utility extensions and permitting the project requires. Three local cost drivers dominate. First, septic: most rural New Plymouth ADUs are on private systems, and a second dwelling frequently requires a separate or expanded septic system and a well-capacity confirmation — often the single largest variable line item. Second, utility runs: extending water, power, and sewer or septic across a rural parcel to a detached ADU adds meaningful trenching and connection cost. Third, frost-protected foundation and 30 psf snow-load engineering add cost over mild-climate ADU construction. A garage or shop conversion can come in lower if the existing structure has sound foundation and adequate utility access.
The final cost of your adu builder in New Plymouth 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 New Plymouth homeowners:
The most common New Plymouth ADU: a single-level, accessible detached cottage for aging parents on the family's rural acreage — one bedroom, full bath designed for aging in place (zero-step entry, wide doorways, blocking-backed grab bar support), a full kitchen, and a private entrance, sited for proximity to the main home while preserving independence. Scope includes a new frost-protected foundation, a separate or expanded septic system with well-capacity confirmation, utility runs from the main service, independent HVAC, and Payette County permitting since these parcels are typically outside city limits. The accessible single-level design is the defining feature for this use case.
Many New Plymouth properties have substantial shops, barns, or outbuildings. Converting a portion into a code-compliant living unit — insulating and conditioning the space, adding a kitchen and bath, dedicated electrical, egress windows, and a separate entrance — is a cost-effective path when the structure's foundation and shell are sound. The complexity is bringing an agricultural or shop structure up to dwelling code: insulation to the 2018 IECC, proper egress, plumbing and electrical to current standards, and septic capacity for the added unit. We assess the existing structure's suitability before committing to this path.
When a New Plymouth family needs housing for adult children, a larger extended family, or year-round help, a purpose-built two-bedroom detached ADU is the answer. Scope is a full new structure: frost-protected foundation, framing engineered to Payette County snow and wind criteria, full kitchen and one or two baths, independent mechanical and electrical, and a separate or expanded septic system sized for the additional bedrooms. This is the largest typical New Plymouth ADU type and requires the most rigorous septic and well-capacity analysis up front.
On New Plymouth's in-town lots — colony Boulevard parcels and mid-century ranch streets — a detached unit is sometimes constrained by lot coverage, making a garage conversion or an attached junior ADU within the main home's footprint the practical route. Scope is converting the garage to a conditioned living space with egress, a kitchen or kitchenette, a bath, dedicated circuits, and a separate entrance, tying into municipal water and sewer where available. In-town projects fall under New Plymouth's evolving ADU regulations and the city/Fruitland permit pathway, so confirming current city ADU rules is the essential first step.
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 Payette River valley at ~2,257 ft: hot, dry, sun-intense summers and cold winters with real snow load and a 24-inch frost line, plus wind off open agricultural ground and hard water.
Payette County design criterion of 30 psf governs roof and deck structural design.
24-inch frost depth requires foundations, footings, and deck piers below grade to prevent frost heave.
115 mph ultimate wind speed and Seismic Design Category C; wind off open farmland drives infiltration and uplift on exposed structures.
Open-valley sun degrades wood siding, coatings, and decking; wide hot-to-cold swing drives material movement and air leakage.
Hard municipal and private-well water scales glass and fixtures and degrades grout and stone; drives material/glass selection.
The 1896 colony heart: two horseshoe streets around the mile-long Boulevard park with original irrigation ditches. Predominantly colony-era and pre-1940 wood-sided farmhouses on generous original acre tracts; strong period character and a protected streetscape.
Common projects in The Boulevard / Historic Horseshoe Core:
Grid streets around and behind the horseshoe filled with 1950s–1970s ranches and ramblers built as the irrigated farm economy matured. Sound framing, aging copper and marginal panels, closed floor plans, on municipal water and sewer.
Common projects in Mid-Century Ranch Streets (In-Town):
Working farm and ranch acreage surrounding the town, outside city limits and under Payette County jurisdiction. Homes range from century-old farmsteads to modern custom builds, typically on private wells and septic systems.
Common projects in Agricultural Fringe / Rural Acreage:
Post-2000 subdivision pockets representing New Plymouth's modern housing minority. Modern PEX plumbing, adequate panels, and builder-grade finishes on tighter lots; no environmental-testing requirements.
Common projects in Harvest Creek / Newer Subdivisions:
Every New Plymouth neighborhood has different housing stock, homeowner priorities, and project considerations. Here is what adu builder looks like in each area:
Permit authority: City of New Plymouth (building inspection contracted to the City of Fruitland Building Department) for properties inside city limits; Payette County Building Department for unincorporated rural parcels. Plumbing and electrical permits issued separately by the State of Idaho (Division of Building Safety / DOPL).
Online portal: npidaho.com/building-department
Here are the design trends we see most often in New Plymouth adu builder projects:
New Plymouth and Payette County home values have appreciated well above their historic norms; local market median list prices reached roughly $485,000 with an average around $449,000 in early 2026 (Redfin), against a longer-run median home value near $277,500. Inventory is limited in a small market with homes selling in roughly 70 days. With trading up locally often impractical, long-tenure, multi-generational families predominantly renovate to keep — making durable, do-it-once work the local standard and a strong resale signal in a closely-watched market.
Avoid these common pitfalls New Plymouth homeowners encounter with adu builder projects:
Better approach: On New Plymouth's agricultural fringe, a second dwelling adds bedrooms to a septic system sized by bedroom count under county and DEQ rules, frequently requiring a separate or expanded system. Analyze septic and well capacity first — it governs feasibility, siting, and the largest budget variable, and discovering a shortfall after design forces costly rework.
Better approach: In-town parcels fall under New Plymouth's evolving ADU regulations and the city/Fruitland pathway; rural parcels fall under Payette County. Small-Idaho ADU rules are actively changing. Verify the current allowances for the specific parcel and authority before any design commitment to avoid building a plan that cannot be permitted.
Better approach: Agricultural structures often lack the foundation, framing, or utility access to economically meet dwelling code. Assess the existing structure's suitability before committing to conversion; an unsuitable building can make conversion more expensive than new construction once code upgrades are tallied.
Better approach: An ADU's high surface-to-volume ratio makes envelope quality disproportionately important for operating cost in New Plymouth's wide climate range. Insulate to or above the adopted 2018 IECC and right-size an efficient system. The modest added cost pays back continuously over the unit's long family-use life.
Better approach: An ADU is a full dwelling requiring the building permit through the applicable authority plus separate state plumbing and electrical permits. Overlooking the state permits causes inspection failures and rework. We coordinate all permits across the city/county and state as standard practice.
In many cases yes, but the answer depends entirely on the parcel's jurisdiction and utility capacity, which must be checked first. Rural parcels outside city limits fall under Payette County's zoning and almost always require a separate or expanded septic system plus well-capacity confirmation for a second dwelling. In-town parcels fall under New Plymouth's evolving ADU regulations, lot coverage, and setbacks. Because small-Idaho ADU rules are actively changing, we verify the current allowances for your specific parcel and authority before any design work — this is the decisive first step.
On New Plymouth's agricultural fringe, where most ADUs are built, homes are on private septic systems sized by bedroom count under Payette County and Idaho DEQ standards. Adding a full dwelling adds bedrooms to the wastewater design load, which commonly requires a separate septic system or a documented capacity expansion, plus confirmation the well can serve added fixtures. This is typically the largest budget variable and frequently determines whether and where an ADU can be built, so we analyze it before design.
For parcels inside city limits, building permit applications route through New Plymouth City Hall with inspections by the contracted City of Fruitland Building Department (208-452-4946), under the city's zoning. For rural parcels outside city limits, Payette County's building department and zoning apply. In both cases, plumbing and electrical permits are issued separately by the State of Idaho. Confirming the correct authority and current ADU rules is the first thing we do.
Often yes, when the structure's foundation and shell are sound. Conversion means bringing an agricultural or shop building up to dwelling standards: insulation to the 2018 IECC, proper egress, plumbing and electrical to current code, and septic capacity for the added unit. It can be more cost-effective than new construction, but we assess the existing structure's suitability — foundation, framing, and utility access — before recommending this path, and lead-safe practices apply if it predates 1978.
A garage conversion or attached junior ADU runs 10–16 weeks. A shop or outbuilding conversion runs 12–20 weeks. A new detached one-bedroom casita runs 16–24 weeks. A new two-bedroom detached ADU runs 18–28 weeks. Add jurisdiction confirmation, septic design and county permitting time for rural parcels, and separate state plumbing and electrical permits. Septic system design and approval can be the longest lead item on a rural ADU, so we start it early.
Here, mostly not. Unlike suburban ADUs often built for rental yield, New Plymouth ADUs are predominantly multi-generational family housing — accessible casitas for aging parents, quarters for adult children or farm help — reflecting an agricultural community that keeps families on the land. We design accordingly, prioritizing single-level accessibility, durability, and low maintenance over rental-turnover optimization, though the unit still adds long-term property value.
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.
Get a free, no-obligation estimate for adu construction in New Plymouth, ID. We handle design, permits, and every detail of construction.
Get Your Free Estimate