How much does ADU plan engineering cost?

A detached ADU is a new structure; an attached addition transfers new reactions into the house; and a garage or room conversion may expose existing foundations and framing that were never documented for the proposed use. I ask for the plan concept, levels, dimensions, construction type, and intended permit path so the responsible engineer can define the right analysis.

• Detached, attached, addition, garage conversion, or interior conversion scope
• New floor, roof, wall, foundation, lateral, stair, and connection design
• Existing-house capacity, altered framing, and interfaces between old and new work
• Architecture, MEP, energy, civil, geotechnical, and utility coordination boundaries

ADU engineering can depend on grading, drainage, soil, setbacks, utilities, access, wind and seismic criteria, and the existing home's foundation. Drawings and photographs help, but they may not verify concealed framing or capacity. The responsible engineer identifies when a site visit, survey, geotechnical report, or additional investigation is necessary rather than filling gaps with a typical ADU assumption.

• Address, jurisdiction, adopted code, site plan, grades, and utility information
• Existing plans, foundation records, field measurements, and alteration history
• Soil, groundwater, slope, drainage, frost, wind, and seismic information
• Current architectural backgrounds and coordinated equipment or service loads

Compare ADU proposals by sheets, calculations, details, forms, coordination, site work, and revision terms. Architecture, energy compliance, civil design, utility approvals, survey, construction pricing, and permit decisions may be separate. I recommend a scope matrix that names each handoff so a low engineering number does not conceal missing work needed for the submission.

• Structural plans, sections, schedules, details, calculations, and seal limits
• Existing-building evaluation and new-to-existing connection assumptions
• Coordination with architecture, energy, MEP, civil, utilities, and geotechnical work
• Plan-check responses, revisions, exclusions, schedule, and professional responsibility