Why infill ADUs need a single accountable crew
On a roomy suburban lot, a split between a design firm and a separate builder can survive its own seams. On a compact LA parcel it usually does not. A plan drawn without standing on the lot runs straight into a setback the designer guessed at, a sewer line in the wrong spot, or a side yard too narrow to carry materials, and on a tight site there is no slack to absorb the surprise. When one crew studies the lot, draws the unit, prices it, and then builds it, those gaps close before they cost you.
That continuity matters most where the buildable area is small and every foot is contested. The same team that measured your side yard and confirmed the access is the team that pours the foundation, frames the walls, and sets the cabinets, so the plan handed to you is one we already know fits the parcel. It keeps the project moving on a lot that punishes improvisation, and it keeps a single party answerable for the result from the first stake to the final inspection.
It also means the decisions that drive cost and livability get made together. On an infill unit the layout, the structure, the utility tie-ins, and the way the new building relates to the existing house all push on one another. Designing and building them as one project, rather than handing each piece to a different sub, is how a small unit ends up feeling like a real home instead of a collection of separately-bid compromises.