Accessory Dwelling Unit (ADU) Construction in Boise, Idaho

Accessory Dwelling Units offer a flexible solution for growing families, aging parents, rental income, or home offices. These secondary dwellings on your property provide independence while keeping loved ones close, generate passive income, or create dedicated workspace separate from your main home.

At EarthCraft Construction, we design and build custom ADUs that match the quality and environmental performance of our full-sized homes. Whether you're looking for a 400-square-foot studio or a 1,200-square-foot two-bedroom unit, we bring the same commitment to energy efficiency, healthy materials, and thoughtful design.

What Is an ADU?

An Accessory Dwelling Unit is a secondary housing unit on a single-family residential

lot. It can be detached as a separate structure in your backyard, attached to your main

house with a separate entrance, converted from existing garage space, or built above a

new or existing garage.

ADUs have their own kitchen, bathroom, and living space. They're complete, selfcontained homes, just smaller.

Why Build an ADU in Boise?

  • Multi-Generational Living

    Keep aging parents close while maintaining privacy and independence for everyone. An ADU allows family members to live on the same property without living in the same space.

  • Rental Income

    Boise's rental market is strong. An ADU can generate $800 to $1,500 per month in rental income, helping offset mortgage costs or fund retirement.

  • Home Office or Studio

    Remote work has made dedicated workspace more valuable than ever. An ADU provides professional space separate from home distractions.

  • Guest Accommodation

    Host visitors without losing privacy or cramming everyone into your main home.

  • Aging in Place

    As you age, downsizing to the ADU while renting the main house can provide income and a more manageable living space.

  • Property Value

    ADUs typically increase property values by adding functional square footage and income-generating potential.

ADU Regulations in Boise

Boise updated its ADU ordinances in recent years to make them easier to build. Current regulations allow ADUs up to 800 square feet (or 40% of primary dwelling, whichever is greater), both detached and attached ADUs, one ADU per single-family lot, and owner occupancy in either the main house or ADU. Standard setback and height requirements apply.

We handle all permitting and ensure your ADU meets city codes and building standards.

Our Approach to ADU Design

Every property is different, as are the needs of each family. We aren't clipboard contractors. We start by understanding how the ADU will be used. Is it for rental, family, or office space? Who will occupy it? What's your budget and timeline? How does it fit on your property?

Then we design an ADU that's tailored to your site, matches or complements your main home's architecture, and meets your functional needs. We never build the same structure twice.

Energy-Efficient ADU Construction

Just because it's smaller doesn't mean we compromise on performance. Our ADUs include high-performance insulation (R-30 walls minimum, higher if desired), passive solar orientation when the site allows, energy-efficient windows and doors, LED lighting throughout, and high-efficiency heating and cooling. We often use mini-split heat pumps because they're incredibly efficient for smaller spaces. We use low-VOC materials for healthy indoor air and design with potential for net-zero energy performance with solar.

Many of our ADUs operate for $20 to $40 per month in utilities due to their small size and high-performance construction. That's comparable to the $30 per month utilities we achieve in our full-sized straw bale homes.

Sustainable ADU Materials

We can incorporate the same green building principles in ADUs as our full-sized homes. That means locally sourced materials when possible, reclaimed or recycled content, natural finishes with low environmental impact, durable materials that last, water-efficient fixtures, and potential for rainwater collection.

An ADU is also inherently sustainable. It densifies existing lots without sprawl, uses existing infrastructure, and provides housing without consuming greenfield land. As Ron puts it, green building is a synthesis of natural elements and good design. That applies whether we're building 3,000 square feet or 600.

ADU Design Options

  • Studio Layout (400 to 600 square feet) Open-concept living and sleeping area, kitchenette, full bathroom. Perfect for single occupant or guest suite.

  • One-Bedroom (600 to 800 square feet) Separate bedroom, full kitchen, living area, bathroom. Ideal for rental income or aging parents.

  • Two-Bedroom (800 to 1,200 square feet) Two bedrooms, full kitchen, living area, one or two bathrooms. Suitable for couples, small families, or premium rentals.

  • Office or Studio (300 to 500 square feet) Work space, bathroom, optional kitchenette. For professionals working from home.

The ADU Construction Process

Phase 1: Design & Permitting (6 to 10 weeks)

Site analysis and feasibility study, custom design tailored to your needs, utility coordination for water, sewer, electric, and gas, then building permit submission and approval. Phase 2: Construction (3 to 6 months) Site preparation and foundation, framing and roofing, rough-in for plumbing, electrical, and HVAC, insulation and drywall, finish work including flooring, cabinets, and fixtures, then final inspections and certificate of occupancy. Total timeline from concept to completion is 6 to 9 months.

Phase 2: Construction (3 to 6 months)

Site preparation and foundation, framing and roofing, rough-in for plumbing, electrical, and HVAC, insulation and drywall, finish work including flooring, cabinets, and fixtures, then final inspections and certificate of occupancy.

Total timeline from concept to completion is 6 to 9 months.

How Much Does an ADU Cost in Boise?

ADU costs vary based on size, finishes, and complexity. A basic ADU runs $150 to $200 per square foot. A mid-range ADU costs $200 to $275 per square foot. A high-end ADU is $275 to $350 or more per square foot.

A typical 600 square foot one-bedroom ADU might cost $120,000 to $165,000 including design, permits, and construction.

When coming to an estimate for an ADU, it's less about the square footage than it is about how the unit is built. Our cost-plus transparent pricing means you know exactly where money is going. No hidden markups.

Financing Your ADU

Options for financing include cash-out refinance on your primary mortgage, home equity line of credit, construction loan that converts to permanent loan after completion, personal savings, or ADU-specific loan programs that are increasingly available.

Some jurisdictions offer incentive programs for ADUs that address affordable housing. We can help you explore financing options.

Common ADU Questions

Do I need to live on the property?

Yes. Boise requires owner occupancy in either the main house or the ADU. You can't rent both.

Can I build an ADU before the main house?

No. The primary dwelling must exist first. However, if you're building both new, we can design them together.

Will an ADU affect my property taxes?

Yes, adding square footage will increase assessed value and therefore taxes. However, rental income typically more than offsets the increase.

Can I use an ADU as a short-term rental like Airbnb?

Check current Boise ordinances, as short-term rental regulations change. As of recent years, there are restrictions on short-term rentals in residential zones.

How long does an ADU take to build?

Six to nine months total from design start to completion, including permitting.

Can I connect to existing utilities?

Usually yes. We coordinate with your existing water, sewer, gas, and electric service. Some upgrades may be needed depending on capacity.

What's the return on investment for an ADU?

If renting, ADUs typically achieve 8 to 12 percent annual return on investment through rental income, plus property value increase.

Can you build a green ADU with passive solar design?

Absolutely. We orient ADUs for passive solar when the site allows, use the same high-performance insulation and air sealing, and can incorporate straw bale construction principles at smaller scale.

Start Your ADU Project

Whether you're accommodating family, generating income, or creating dedicated workspace, an ADU can transform how you use your property.

EarthCraft Construction, Inc | Treasure Valley, Idaho

Serving Boise, Oregon & Surrounding Areas (208) 391-3754 | [email protected]

3618 East Newby St, Nampa, ID 83687

EarthCraft Construction, Inc | Treasure Valley, Idaho

Serving Boise, Oregon & Surrounding Areas (208) 391-3754 | [email protected]

1415 South Owyhee St, Boise, ID 83705

Licensed and insured in Idaho RCE-27334 and Oregon CCB-186023

© 2025 EarthCraft Construction. Built with conservation and integrity since 1978.