Is an Engineer Stamp Required for My ADU Project in Hawaii?

Hawaii's counties — Honolulu, Hawaii County, Maui, Kauai — review structural drawings for accessory dwellings, and the PE seal is the standard requirement. The seal must show the engineer's name, Hawaii PE number, and date, and the engineer must be in responsible charge of the engineering. Detached ADUs, ohana units, and garage conversions are treated the same at the structural level. Owner-builders don't escape the requirement: the county asks for the seal regardless of who drafted the plans.

The engineer reviews the floor plan and elevations, designs the foundation for volcanic soils and lava rock — pin-down details are common — sizes beams, headers, and the lateral system, and checks hurricane wind, seismic, and flood design. Honolulu's wind speeds get extra scrutiny. Garage conversions get extra attention because the existing slab was rarely designed as a dwelling foundation. Bring the site survey, the geotechnical report, and the architectural plans. Wind design is the item most ADU plans miss.

Use this checklist before engaging an engineer for your ADU: • Engineer of record holds an active Hawaii PE license, verifiable on the state roster • Dimensioned floor plan and elevations ready for structural review • Site survey showing setbacks, easements, and existing structures • Hurricane wind design verified per the wind maps • Foundation designed for volcanic soils and lava rock • Garage conversions: existing slab evaluated for dwelling loads • Engineer confirms they'll respond to plan-check corrections on sealed sheets • Seal shows name, Hawaii PE number, and date on every sealed sheet