A Structural Checklist Before You Commit to an ADU or Addition

ADUs and additions look simple from the curb, and that is exactly why they surprise people. The structural questions are the same ones a commercial building answers — loads, load paths, foundations — compressed into a small budget where a single surprise matters.

1. What is the existing foundation actually doing?

Garage conversions and second-story additions both lean on existing foundations that were never designed for the new load. A footing probe or as-built review early costs little; discovering an undersized footing after framing prices are locked costs a lot.

2. Where does the lateral load go?

Adding openings, removing walls, or stacking a story changes how wind and seismic forces travel to the ground. In seismic country, a soft or weak story is the failure mode that building departments look for hardest — expect shear wall or hardware requirements that affect your floor plan.

3. What does your soil do when it gets wet?

Expansive clays, uncompacted fill, and shallow groundwater each drive different foundation solutions. If neighbors have cracked slabs, budget for a geotechnical opinion before you budget for finishes.

4. Which code and which local amendments apply?

ADU rules have liberalized in many states, but the structural provisions still ride on the locally adopted residential or building code, including local snow, wind, and seismic amendments.

5. Is your drawing set permit-ready?

Jurisdictions increasingly require engineered drawings with calculations for ADUs. A clean, complete first submittal — plans, details, and a calculation package that anticipates the reviewer's questions — is the difference between a few weeks and a few months of plan check.