ADU Construction in Boise & the Treasure Valley
ADU Construction in Boise & the Treasure Valley
If you’re considering ADU construction in Boise or anywhere in the Treasure Valley, you’re making one of the smartest investments available to Idaho homeowners today. Accessory dwelling units — self-contained living spaces built on the same lot as your primary home — offer a rare combination of rental income potential, multigenerational flexibility, and lasting property value. At Eliezer Custom Homes, we design and build ADUs from the ground up, handling every detail from initial design through final inspection so you can focus on what matters most.
Whether you’re in Star, Boise, Eagle, Meridian, or the surrounding communities, our team brings decades of hands-on construction experience — rooted in an Eastern European carpentry tradition — to every accessory dwelling unit we build. This page covers everything you need to know about building an ADU in Idaho: the types of ADUs available, local regulations, what to expect during the process, and how to make your investment count.
What Is an Accessory Dwelling Unit (ADU)?
An accessory dwelling unit is a secondary residential structure located on the same lot as a primary single-family home. ADUs are fully self-contained, meaning they include their own kitchen, bathroom, sleeping area, and private entrance. They go by many names — granny flats, in-law suites, backyard cottages, casitas — but the function is the same: a complete, code-compliant living space that operates independently from the main house.
In Idaho, ADUs have gained significant traction over the past several years as housing costs in the Treasure Valley have risen sharply. Cities like Boise, Meridian, and Eagle have updated their zoning codes to make ADU construction more accessible, recognizing that these units help address housing density without changing the character of established neighborhoods.
Unlike apartment complexes or duplexes, an ADU preserves the single-family feel of your property while giving you an entirely new living space — one you can use for aging parents, adult children, guests, a home office, or rental income.
Types of ADUs We Build
Every property is different, and the right ADU type depends on your lot, your budget, and how you plan to use the space. We build five distinct types of accessory dwelling units across the Treasure Valley, and we’ll help you determine which approach makes the most sense for your situation.
Detached ADUs
A detached ADU is a standalone structure built separately from the main house — often in the backyard. This is the most popular ADU type in the Boise area because it offers maximum privacy for both the homeowner and the occupant. Detached ADUs can be designed to match your home’s architecture or take on their own character, and they typically range from 400 to 900 square feet. They require their own foundation, utility connections, and full permitting.
Garage Conversion ADUs
A garage conversion transforms an existing attached or detached garage into a livable dwelling unit. Because the shell of the structure already exists, garage conversions are often the most cost-effective path to an ADU. The work involves insulating walls and ceiling, adding plumbing for a kitchen and bathroom, upgrading electrical service, installing flooring, and ensuring the space meets residential building codes. If you have a two-car garage you rarely use to its full capacity, this is worth serious consideration.
Basement ADUs
If your home has an unfinished or underutilized basement, a basement ADU lets you create a fully independent living unit without adding any footprint to your lot. Basement ADUs in the Treasure Valley work particularly well because many homes in Star, Eagle, and parts of Boise were built with full or walkout basements. Key considerations include egress windows (required by code), ceiling height, moisture management, and independent utility metering.
Above-Garage ADUs
An above-garage ADU adds a living unit on top of an existing or new garage structure. This approach is ideal when you want to keep your garage functional while still adding an ADU — and it makes efficient use of vertical space. Structural engineering is critical here: the garage below must support the weight of a full living space, and stairway access, fire separation, and sound insulation all need careful planning.
Attached ADUs
An attached ADU is built as an addition to your existing home but includes a separate entrance and fully independent living quarters. Think of it as a self-contained wing of your house. Attached ADUs share at least one wall with the primary dwelling, which can reduce construction costs compared to a fully detached build. They work especially well for multigenerational families who want proximity with privacy.
ADU Regulations and Zoning in Idaho
Understanding local regulations is one of the most important steps before starting ADU construction in Boise or the surrounding area. Zoning rules vary by city and even by neighborhood, and getting them right at the outset prevents costly delays or redesigns later.
Boise ADU Zoning Rules
The City of Boise allows ADUs on lots zoned R-1 (single-family residential) under specific conditions:
- Maximum size: 900 square feet of finished living space
- Location: Must be on the same lot as the primary dwelling
- Setbacks: ADUs must meet standard rear and side yard setbacks (typically 5 feet from side lot lines and 10–15 feet from the rear property line, depending on the zone)
- Height: Generally limited to one story for detached ADUs, or matching the height limit of the primary residence
- Parking: One additional off-street parking space may be required, though exceptions exist near transit corridors
- Owner occupancy: The property owner must occupy either the primary dwelling or the ADU (specific requirements vary and should be confirmed with the city)
- Separate entrance: The ADU must have its own entrance that does not face the same street as the primary dwelling’s main entrance
Other Treasure Valley Communities
If you’re looking to build an ADU in Star, Eagle, Meridian, Nampa, or other Treasure Valley cities, regulations differ. Star, Idaho — where Eliezer Custom Homes is based — and Eagle have been increasingly accommodating of ADU construction, though specific lot size minimums, setback rules, and permitting requirements apply. We handle the zoning research and permitting for every project, so you’ll know exactly what’s possible on your property before we break ground.
Benefits of Building an ADU
Homeowners across the Treasure Valley are building ADUs for a variety of compelling reasons. Here are the most common:
Rental Income
A well-built ADU in the Boise metro area can generate $1,200 to $1,800 or more per month in long-term rental income, depending on size, finishes, and location. That income can offset your mortgage, fund retirement, or pay for the ADU itself over time. With Boise’s strong rental market driven by continued population growth, vacancy rates for quality rental units remain low.
Multigenerational Living
Many of our clients build ADUs so aging parents can live close by while maintaining their independence — or so adult children can have affordable housing while they save for their own home. The separate entrance and self-contained layout give everyone the privacy they need, while the proximity provides peace of mind.
Property Value Increase
A professionally built ADU adds real, appraised value to your property. National data shows that ADUs can increase property values by 20–30%, and in a high-demand market like the Treasure Valley, that number can be even higher. It’s one of the few home improvements that both generates income and appreciates in value.
Flexible Use
Beyond renting or housing family, ADUs serve as dedicated home offices, art studios, guest houses, or short-term rental units. The flexibility to adapt the space as your life changes makes it a uniquely versatile investment.
Designing Your ADU for the Idaho Climate
- Insulation: We use high-performance insulation in walls, ceilings, and floors — typically R-21 or higher in walls and R-49 in ceilings — to keep heating and cooling costs manageable year-round.
- Heating systems: Efficient mini-split heat pumps are a popular choice for ADUs in Idaho because they provide both heating and cooling in a compact, energy-efficient package. For basement ADUs, in-floor radiant heating is an excellent option.
- Windows and doors: We specify double-pane or triple-pane Low-E windows to minimize heat loss in winter and heat gain in summer. Proper orientation of windows can maximize natural light while reducing energy demand.
- Moisture management: Idaho’s freeze-thaw cycles and occasional heavy snowfall require proper drainage, vapor barriers, and foundation waterproofing — especially for basement ADUs and ground-level structures.
- Roofing and snow load: Detached and above-garage ADUs must be engineered to handle Idaho’s snow load requirements, which vary by elevation and location. In the Boise foothills, snow load requirements are higher than in the valley floor.
The ADU Build Process With Eliezer Custom Homes
We’ve refined our ADU construction process to be straightforward, transparent, and stress-free. Here’s what to expect when you build an ADU with us:
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Consultation and Site Assessment
Every project starts with a free consultation. We visit your property, assess the lot, discuss your goals and budget, and determine which ADU type is the best fit. We’ll also review preliminary zoning requirements so you have a clear picture of what’s possible before any commitments are made.
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Design & Planning
Once we agree on direction, we move into design. We work with experienced architects and designers to create a layout that maximizes your available space, meets all code requirements, and reflects your personal style. You’ll see 2D floor plans and 3D renderings before anything is finalized.
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Permitting
We handle the full permitting process with your local jurisdiction — Boise, Star, Eagle, Meridian, or wherever your property is located. This includes the building permit application, structural engineering, and coordination with plan reviewers. Permitting in the Treasure Valley typically takes 4–8 weeks, depending on the complexity of the project and the jurisdiction’s review backlog.
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Site Preparation and Foundation
For detached ADUs, this means excavation, grading, and pouring the foundation. For conversions (garage or basement), we prepare the existing structure — reinforcing framing, adding footings if needed, and ensuring the space is structurally sound for residential occupancy.
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Construction
This is where our team’s craftsmanship really shows. Framing, roofing, electrical, plumbing, HVAC, insulation, drywall, flooring, cabinetry, fixtures — every element is built to the same standard we apply to our custom homes. We keep you informed at every stage with regular updates and open communication.
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Final Inspections and Handoff
Once construction is complete, we coordinate final inspections with the building department. After the certificate of occupancy is issued, we do a thorough walkthrough with you, address any punch-list items, and hand over the keys to your new ADU.
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Typical Timeline
Most ADU projects take 4 to 8 months from permit approval to completion. Garage conversions and basement ADUs tend to fall on the shorter end (4–5 months), while ground-up detached builds typically take 6–8 months. We’ll provide a detailed timeline during the planning phase so you can plan accordingly.
What Does ADU Construction Cost in Boise?
ADU cost in Boise varies significantly depending on the type of unit, size, site conditions, and the level of finish you choose. Here are general ranges to help you budget:
| ADU Type | Typical Cost Range |
|---|---|
| Garage Conversion | $100,000 – $200,000 |
| Basement ADU | $120,000 – $220,000 |
| Attached ADU | $150,000 – $300,000 |
| Detached ADU (new build) | $180,000 – $350,000+ |
| Above-Garage ADU | $175,000 – $325,000 |
These ranges reflect complete, turnkey construction including design, permitting, all labor and materials, and final finishes. Higher-end finishes, complex site conditions (sloped lots, difficult soil, limited access), and custom features will push costs toward the upper end.
We provide detailed, transparent estimates during the planning phase — no hidden fees, no surprise change orders. That’s a core part of how we operate at Eliezer Custom Homes.
Why Choose Eliezer Custom Homes as Your ADU Builder
There are several general contractors in the Treasure Valley who offer ADU construction, but few bring the combination of skill, transparency, and personal attention that defines our work.
Our founders learned the building trade in Eastern Europe, where craftsmanship is taught through years of hands-on apprenticeship — not just classroom instruction. That foundation shows in the quality of our framing, our finish carpentry, and the durability of every structure we build. When we moved to Idaho and established Eliezer Custom Homes in Star, we brought that tradition with us.
We also believe in doing business the right way. Our faith-based values guide how we treat our clients: with honesty, respect, and a genuine commitment to delivering what we promise. We communicate openly throughout every project, we don’t cut corners, and we stand behind our work.
Whether you’re building a detached ADU in Boise, converting a garage in Eagle, or finishing a basement unit in Meridian, we treat your project with the same care and attention we’d give our own home. Check out our portfolio to see the quality of our work firsthand.
Frequently Asked Questions
How much does it cost to build an ADU in Boise, Idaho?
ADU construction costs in Boise typically range from $150,000 to $350,000 or more depending on the type, size, finishes, and site conditions. Detached new-build ADUs generally cost more than garage conversions or basement ADUs. A detailed consultation with your builder is the best way to get an accurate estimate for your specific property and goals.
Are ADUs legal in Boise, Idaho?
Yes. The City of Boise permits accessory dwelling units on lots zoned R-1 (single-family residential) and certain other residential zones. ADUs must be located on the same lot as the primary dwelling and cannot exceed 900 square feet. Setback, height, and parking requirements also apply. Other Treasure Valley cities have their own ADU ordinances, and we can help you navigate the rules for your area.
How long does it take to build an ADU in the Treasure Valley?
Most ADU projects take between 4 and 8 months from permit approval to final inspection. Garage conversions and basement ADUs tend to be on the shorter end, while ground-up detached ADUs typically take closer to 6–8 months. Permitting adds an additional 4–8 weeks before construction can begin.
Do I need a permit to build an ADU in Idaho?
Yes. All ADU construction in Idaho requires a building permit from your local jurisdiction. This includes new construction, garage conversions, and basement conversions. You will also need separate electrical, plumbing, and mechanical permits. Eliezer Custom Homes handles the full permitting process on your behalf so you don’t have to navigate it alone.
Can I rent out my ADU in Boise?
Yes, long-term rentals of ADUs are permitted in Boise. Short-term rental regulations vary and may require additional permits or licensing depending on your zone. Many Boise homeowners earn $1,200–$1,800 per month renting their accessory dwelling unit, making it one of the strongest return-on-investment improvements you can make to your property.
What is the maximum size for an ADU in Boise?
In the City of Boise, ADUs are capped at 900 square feet of living space on R-1 zoned lots. The unit must also comply with lot coverage limits, setback requirements, and height restrictions. Other Treasure Valley municipalities may have different size limits, and some allow up to 1,000 square feet or more.
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If you’ve been thinking about building an ADU on your property, we’d love to talk. Our free consultation gives you a clear picture of what’s possible on your lot — including the best ADU type for your goals, a preliminary budget, and a realistic timeline. No pressure, no obligation — just honest, expert guidance from a team that builds ADUs right.