Do I Need an Engineer for an ADU?
Usually yes. Most jurisdictions require structural plans for an ADU, and many require an engineer's stamp on the foundation design, wind and seismic calculations, and energy compliance documentation. An ADU is a dwelling unit, and plan checkers review it like one.
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What the plans include
A typical ADU structural package covers the foundation design, floor and roof framing, shear walls and hold-downs for wind and seismic loads, and connection details. In California, Title 24 energy calculations ride alongside the structural set. Even modest ADUs need this documentation because they are independent living spaces, not sheds.
When a stamp is specifically required
Requirements vary by jurisdiction, but an engineer's stamp is commonly required for non-prescriptive designs, hillside or flood-zone lots, two-story ADUs, and anywhere the building official asks for calculations. Prescriptive code paths exist in some areas for simple single-story designs, but they still demand careful, complete drawings.
Conversions need engineering too
Garage conversions look simple but need structural verification: is the existing slab adequate, are the walls properly framed and anchored, can the roof structure carry new loads? Unpermitted prior work often surfaces during conversion review. An engineer sorts out what stays, what gets reinforced, and what must be replaced.
The bottom line
Budget for engineering on any ADU project. It is the norm for dwelling units, and it is what gets the permit issued.