Garage Conversion · West Boise (Maple Grove area), Boise, ID
A 400 sq ft attached two-car garage off a 1990s West Boise home, converted into a 1-bed ADU — the existing slab and walls qualified, so the work was egress, insulation, MEP, and a code kitchen and bath instead of a new foundation. Roughly four months contract-to-keys.
Verified the existing slab thickness, ceiling height, and setbacks qualified before signing — the gate on every conversion
Framed in the garage door opening and added a code-compliant egress window and an exterior entry door
Insulated slab perimeter, walls, and ceiling to bring an uninsulated garage up to dwelling standard
Ran new MEP: a dedicated electrical sub-panel, plumbing supply and drain to the slab, and a ductless mini-split for heat and cooling
Built a full kitchen and a 3/4 bath roughed and finished to a true rental standard
Standard-finish interior — LVP flooring over the leveled slab, quartz counter, tile shower
The challenge
Garage conversions read as the cheap, fast ADU, but a garage is built to a lower standard than a house — uninsulated, a slab poured for cars not living space, often no plumbing within reach, and a door opening where a wall needs to be. This West Boise garage was an attached two-car off a 1990s home, so the bones were sound, but every conversion has to clear three gates first: the slab has to be in good condition and the right thickness, the ceiling has to hold legal headroom after a finished floor and ceiling go in, and the structure has to sit inside its setbacks. Plenty of garages fail one of those — a cracked or thin slab, a low ceiling, or a detached garage tight to the property line — and when they do, conversion stops being the cheaper path.
Our approach
We checked the slab, the headroom, and the setbacks before the contract was signed — that's the honest first step, because a conversion only beats ground-up when the existing slab and walls actually carry. They did here. From there the work is well-defined: frame in the garage door opening, cut a code egress window and a real entry door, insulate slab perimeter, walls, and ceiling, then run new MEP — a dedicated sub-panel, plumbing to the slab, and a ductless mini-split. The kitchen and 3/4 bath were finished to a genuine rental spec. Reusing the foundation and three existing walls is what keeps a qualifying conversion below a detached build on both cost and time; on a non-qualifying garage we'd have steered the owner to a detached plan instead.
The outcome
Because the foundation and most of the shell carried over, the build ran about four months contract-to-keys — shorter than a comparable ground-up detached ADU, and at the lower end of the turnkey range for a 1-bed. The owner uses the 400 sq ft unit as a home office now, but it was metered and plumbed as a true dwelling, so flipping it to a long-term rental is a tenant search, not a renovation. The honest caveat stands: this worked because the garage qualified. When a slab or ceiling can't make it, the conversion savings evaporate and a detached ADU is the better call.
"They told us straight that it only pencils if the slab and ceiling check out, then actually checked before we committed. The garage qualified, the office was done in about four months, and we know it's a rental when we're ready."
Paul & Diane K.
West Boise, Boise
City
Zoning, plans, and local fit for Boise ADU builds.
Cost
Real 2026 pricing by plan and finish for this market.
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