Setting out, and pouring the slab
A detached ADU is a small building with all of a building's requirements. It needs its own foundation, its own framing, its own roof, and its own connections for water, waste and power.
Setting out is where the square footage becomes real. On a unit designed to a two-foot margin under a statutory threshold, the slab dimensions are not approximate — the interior livable space has to measure what the drawings say it measures when the inspector checks it.
Setbacks are confirmed at the same stage. Detached ADUs of 800 square feet or less may qualify for reduced rear and side setbacks, and that has to be verified against the ordinance governing the specific parcel before the forms go down.