What Structural Engineering Does a Detached ADU Really Require?

A detached ADU requires complete structural engineering — foundation, framing, and lateral design with stamped drawings — just like a small single-family home. There's a persistent myth that ADUs get a lighter engineering standard. They don't. The code doesn't care that the building is accessory — it cares that the foundation bears on competent soil, the framing carries the loads, and the lateral system works. The engineering package for an ADU is simply a house package at a smaller scale.

Foundation plan with footing sizes, slab details, and any grade beams the soils require. Floor, wall, and roof framing plans with member sizes — complete, not schematic. Lateral design: shear walls or braced frames with hold-downs and a continuous load path to the foundation, designed for the site's wind and seismic demands. Two-story ADUs get the full multi-story treatment: stacked bearing walls, floor diaphragm design, and overturning checks. Garage-conversion ADUs get the existing-structure evaluation — the garage's slab, walls, and roof reframed and verified for the new use. Every ADU type has its own structural story, and the drawings tell it completely.

ADU permits move fast when the submittal is complete and coordinated. Here's what keeps yours out of the correction cycle. This is the formula that works.

• Get the soils input early — even a small ADU needs to know what it's sitting on
• Submit architectural and structural as a coordinated set, not piecemeal
• Show utility connections and trenching on the plans so reviewers see the whole site
• Verify local ADU ordinances for size, height, and setback limits before engineering starts
• Respond to plan check comments with revised calculations, not just revised drawings