Garage Conversion & Attached ADUs in Boise

The fastest path to a second unit

A conversion uses a structure you already have. The walls, roof and much of the foundation are in place, so the work focuses on making the space livable: insulation, windows, a proper entry, plumbing for a kitchen and bathroom, and heating and cooling. That's why Angi's 2026 data puts garage and basement conversions at roughly $60,000 to $150,000, compared with about $110,000 to $285,000 for detached new construction (Angi).

Conversions also tend to move faster through permitting and construction, because there's less site work and no new foundation for the whole unit. In Boise, attached ADUs go through the residential alterations application, while detached ADUs use the new residential application, according to the city's 2026 ADU applicant packet.

Three kinds of conversion

Detached garage conversion

A garage set back on the lot, often off an alley in Boise's older neighborhoods, can become a private cottage. The main questions are whether the slab is level and sound, whether the walls can be insulated to code, and how far the new sewer line has to run.

Attached garage conversion

An attached garage becomes a suite with its own outside door. We add a fire-rated separation where the code requires it, raise or insulate the floor, and plan the kitchen and bath around the nearest plumbing.

Basement or wing ADU

A finished or unfinished basement, or a section of the house, becomes a separate unit. Egress windows, ceiling height, a separate entrance and sound control between units are the key design points.

What changes in a conversion

• Floor: garage slabs slope toward the door. We level and insulate the floor so the unit is comfortable year-round.
• Walls and ceiling: insulation and drywall to residential standards, plus fire separation from the main house where required.
• Openings: the garage door becomes a wall with windows and a proper entry door.
• Plumbing: new supply and drain lines for a kitchen and bathroom, tied into the house or a new sewer connection.
• Electrical and mechanical: new circuits, often a subpanel, and a ductless heat pump for heating and cooling.
• Parking: Boise doesn't require a replacement parking space for an ADU, and the 2026 state law bars most parking requirements in larger cities.

Code items specific to conversions

Converting a garage or basement into a dwelling triggers the same life-safety rules as any home. Expect requirements for egress windows in sleeping rooms, smoke and carbon monoxide alarms, fire separation between an attached unit and the main house, adequate ceiling height, and insulation that meets the energy code. A separate exterior entrance is standard so the unit is truly self-contained. We design to these from the start rather than discovering them at inspection, which is where many do-it-yourself conversions stall.

Is your garage a good candidate?

Good candidates have a sound slab, a roof in decent shape, at least 8 feet of ceiling height after insulation and a reasonable path to sewer. Garages that sit low and take water during storms, or that are framed very lightly, may cost nearly as much to convert as building new. We'll tell you which way the numbers point after a site visit.

During the site visit we check the slab for cracks and level, measure ceiling height, look at framing and the roof, find the nearest sewer and water lines, and open the electrical panel. That one visit tells us whether a conversion makes sense or whether a new detached unit would be the better value.

See the full budget breakdown in our Boise ADU cost guide, or compare with a detached ADU.