What engineering and plans does an ADU need?

I ask whether the ADU is attached to the main house, detached in the yard, converted from a garage or other space, or supplied as a system with manufacturer documents. Each configuration creates different questions about foundations, fire and property interfaces, existing framing, lateral continuity, utilities, and construction access. The responsible engineer uses the actual plans and site evidence instead of assuming that every ADU follows one standard detail.

• Attached addition, detached structure, garage conversion, or interior conversion
• Existing foundation, slab, framing, roof, openings, and alteration history
• Site plan, grades, setbacks, utilities, drainage, and access constraints
• Manufacturer drawings and installation requirements for a modular or panelized system

For a new ADU, the engineer designs the path from roof and floor loads through framing, connections, walls, and foundations. For an attached or converted unit, the engineer also evaluates how the new work meets the existing building and whether openings, floor changes, posts, or equipment alter the original system. I want the plans to identify assumptions about concealed conditions so field verification is not confused with a guess.

• Footings, slabs, stem walls, anchors, posts, beams, and bearing conditions
• Floor and roof framing, shear walls, diaphragms, collectors, and connections
• New openings, infill, stairs, balconies, decks, and concentrated equipment loads
• Existing-condition limits, inspection needs, and required exposure or measurement

ADU plans commonly involve architecture, structure, energy, civil, MEP, survey, and sometimes geotechnical or fire-related work. The engineering package should identify its sheet ownership, design criteria, calculations, details, notes, and eligible professional seal while leaving each other discipline with its own responsibility. I recommend comparing the complete coordinated set with the current authority checklist before submission, because a structural drawing cannot answer every ADU requirement.

• Structural plans, foundation details, framing schedules, and calculations
• Architectural backgrounds, dimensions, openings, and room layouts kept current
• Separate energy, civil, MEP, survey, fire, and land-use documents as required
• Sheet index, revision control, assumptions, exclusions, and submission forms

A PE seal communicates responsibility for defined engineering work; it is not a product that can be added to an unchanged plan set. If another person prepared the drawings, the responsible engineer needs enough records to independently review or redesign the eligible work and decide whether accepting responsibility is permitted. I tell owners to ask exactly which sheets and calculations are covered, who owns revisions, and what the authority—not the engineer—must approve.

• Identify the professional discipline and exact sheets within the requested scope
• Provide current plans, calculations, site evidence, and authority comments
• Document independent review, revisions, assumptions, and unresolved conditions
• Keep permit approval, construction means, and non-engineering work separate