California ADU Engineering Requirements: Complete 2026 Guide

California's ADU laws have made it easier than ever to add a granny flat, garage conversion, or detached unit. But every ADU still requires stamped structural engineering plans approved by your local building department.

Types of ADUs We Engineer

• Detached new construction ADU
• Garage conversion ADU
• Attached addition ADU
• Second-story ADU above a garage
• Junior ADU (JADU) within an existing home

Structural Engineering Requirements

Every ADU project in California requires stamped structural calculations and plans. Key engineering components include:

• Foundation design — conventional spread footings, post-tensioned slab, or drilled piers for hillside
• Framing plans — floor, wall, and roof framing with member sizes
• Lateral analysis — seismic and wind load resistance
• Connection details — holdowns, strap ties, and anchor bolts
• Energy compliance coordination — with your Title 24 consultant

Cost and Timeline

ADU structural engineering typically costs $2,500–$6,000 for a detached new build, and $1,500–$3,500 for a garage conversion. Plans are delivered within 1–2 weeks. Building department plan review takes 2–6 weeks depending on the city.

Do I Need a Geotechnical Report?

Many California jurisdictions require a soils report for new ADU construction, especially on hillside lots or in areas with expansive clay soils. We coordinate with licensed geotechnical engineers and incorporate their recommendations into the foundation design.