Boise · Meridian · Eagle · Garden City
ADU Builder in Boise, Idaho
Detached backyard cottages, garage conversions and attached ADUs, designed for your lot, permitted with your city and built start to finish.
ADUs in the Treasure Valley
A second home on the lot you already own
An accessory dwelling unit, or ADU, is a small, self-contained home on the same lot as a single-family house, with its own kitchen, bathroom and sleeping space. Around Boise, homeowners build them for aging parents, adult kids, guests, a home office with a bathroom, or long-term rental income that helps with the mortgage.
The rules have moved quickly in the homeowner's favor. Boise's zoning code rewrite in late 2023 raised the size limit, dropped the requirement that the owner live on the property and stopped requiring an extra parking space for the ADU. In 2026 the City of Boise went further and began offering a catalog of free pre-approved ADU plans, eight designs from 280 to 695 square feet.
The state has stepped in too. Idaho Code § 67-6541, added in 2026, requires cities over 10,000 people to allow one ADU per single-family lot by February 1, 2027, approved administratively, without owner-occupancy rules and without size caps below 1,000 square feet or 75 percent of the main home. Meridian, Eagle and Garden City all fall under it.
What we build
Two ways to add a unit
Most Treasure Valley ADUs are one of these. Which fits depends on your lot, your budget and how separate you want the second home to feel.
Detached ADUs
A stand-alone backyard cottage with its own foundation, utilities and entry. The most privacy and the most rental value.
Detached ADUs →
Garage conversions & attached ADUs
Turn an existing garage, basement or wing of the house into a self-contained unit. Usually faster and less expensive.
Conversions & attached ADUs →Inside a Boise ADU
Small footprint, full home
A well-designed ADU has everything a house has, just closer together. These are the details we design into every unit.
Local know-how
What shapes an ADU in the Treasure Valley
The building is the easy part. These are the things that decide whether a lot works and what it costs.
Your city's rules
Boise, Meridian, Eagle and Garden City each run their own planning and building departments. Size limits, setbacks and review paths differ, and all four are updating codes ahead of the 2027 state deadline.
Utilities
An ADU needs water, sewer and power. Whether it can share the main home's lines or needs new taps, and how far the trench runs, is often the biggest swing in the budget.
Irrigation and easements
Many Treasure Valley lots carry irrigation laterals, pressurized irrigation lines and utility easements along the back or side lot lines. Those set where a detached unit can sit.
Floodplain and hillsides
Lots near the Boise River or in the Foothills can carry extra review. In Boise, properties in a flood or hillside overlay don't qualify for the city's pre-approved plans.
Climate
Hot, dry summers and cold winters with snow loads call for good insulation, a right-sized heat pump and a roof designed for the load. We spec for both seasons.
HOAs and CC&Rs
Many newer Meridian and Eagle subdivisions have covenants that address outbuildings, materials and colors. We read them before design, not after.
How it works
From feasibility to move-in
- 01
Lot check
We review zoning, setbacks, easements, floodplain and utilities for your address and tell you what fits.
- 02
Design & estimate
Start from a pre-approved Boise plan or a custom design, with a written estimate for design, permits and construction.
- 03
Permits
We prepare the drawings and site plan and handle the planning and building applications with your city.
- 04
Build & handoff
Foundation, framing, utilities, finishes and final inspection, then a walkthrough and the keys.
Cost
What an ADU costs
Angi's 2026 data puts the national average cost of an ADU around $180,000, with most homeowners spending between $60,000 and $285,000. Garage and basement conversions run roughly $60,000 to $150,000, and fully detached new construction about $110,000 to $285,000, or $150 to $300 per square foot (Angi).
In the Treasure Valley, the numbers that move a project most are the utility connections, the foundation, how far the unit sits from the street and the finish level. Starting from one of Boise's free pre-approved plans removes design fees that the city says can run upwards of $10,000 for a custom ADU, though site work, utilities and impact fees still apply.
Our Boise ADU cost guide breaks the budget down line by line, and every estimate we give is written and itemized.
| ADU type | Typical national range |
|---|---|
| Garage or basement conversion | $60,000 – $150,000 |
| Detached new construction | $110,000 – $285,000 |
| Per square foot | $150 – $300 |
Source: Angi, 2026.
Who builds ADUs
Why Treasure Valley homeowners add a unit
Family close by. An ADU lets aging parents live independently a few steps away, or gives adult children an affordable start. A single-level layout with wide doors and a curbless shower keeps it workable for years.
Rental income. Long-term rent from an ADU can cover a meaningful share of the construction loan. Boise's removal of owner-occupancy and parking requirements made rental ADUs simpler to plan.
Space to work or host. A backyard studio with a bathroom makes a quiet office, a guest suite or a creative space without remodeling the main house.
Whatever the reason, the first step is the same: a lot check that tells you what fits before you spend on design.
- AAging parents
Single-level, accessible, close to family.
- BLong-term rental
Durable finishes, good sound separation.
- COffice or guest suite
Natural light, privacy, a full bathroom.
FAQ
ADU questions
How big can an ADU be in Boise?
Boise's 2026 pre-approved ADU packet lists the maximum as 70 percent of the house's floor area or 900 square feet, whichever is smaller. Under Idaho Code § 67-6541, cities over 10,000 people can't set limits below 1,000 square feet or 75 percent of the main home after February 1, 2027, so confirm the current number before you design.
Do I have to live on the property to build an ADU?
Not in Boise, which removed its owner-occupancy rule in the 2023 code rewrite. The 2026 state law bars owner-occupancy requirements in cities over 10,000 people by February 1, 2027.
Do I need extra parking for an ADU?
Boise does not require additional parking for an ADU. The state law also bars most parking requirements, with limited exceptions such as homes without existing off-street parking.
How long does it take to build an ADU?
A garage conversion can take a few months from design to move-in. A detached ADU usually takes longer, with design and permitting first and then construction, depending on utilities, site work and city review times.
Can I rent out my ADU?
Many owners do. Long-term rental is common; check your city's rules and any HOA covenants before planning short-term rental use.