How to Estimate an ADU or Garage Conversion

HOW-TO · 2026-06-25

ADUs and garage conversions occupy a middle ground between new construction and remodel. Here is how to estimate them without missing costs.

Accessory dwelling units (ADUs) and garage conversions occupy a middle ground between new construction and remodel estimating, and mixing up which rules apply is a common source of missed costs.

Structural assessment

Structural assessment is the first step and often the biggest wildcard — an existing garage's foundation, wall framing, and roof structure weren't necessarily designed to support residential insulation, drywall, plumbing, and living loads, so a structural evaluation (not just a visual walkthrough) should precede any detailed takeoff.

Utilities

Utilities are usually the largest new-cost category, since a garage typically has minimal or no existing plumbing and limited electrical capacity — full plumbing rough-in (potentially including a new sewer lateral connection) and an electrical panel upgrade or subpanel are common necessities, not optional upgrades, and should be priced as their own trade takeoffs rather than folded into a general "utilities allowance."

Insulation and climate control

Insulation and climate control for a space that wasn't originally conditioned requires a full building envelope upgrade — wall and ceiling insulation to code minimums, and typically a dedicated HVAC solution (mini-split systems are common for ADUs) rather than extending an existing system that wasn't sized for the added load.

Permitting and code compliance

Permitting and code compliance for ADUs varies significantly by jurisdiction and has been a fast-changing area of local zoning policy — this is worth flagging as a bid assumption to confirm with local building department requirements rather than treating permit costs as a standard percentage across all markets.