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

Virginia's counties — Fairfax, Prince William, Virginia Beach, Loudoun — review structural drawings for accessory dwellings under the Virginia Uniform Statewide Building Code, and the PE seal is the standard requirement. The seal must show the engineer's name, Virginia PE number, and date, and the engineer must be in responsible charge of the engineering. Detached ADUs, garage conversions, and in-law additions 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 expansive clay soils — Northern Virginia's marine clays are notorious — sizes beams, headers, and the lateral system, and checks snow, wind, and seismic design per the Virginia Uniform Statewide Building Code. Garage conversions get extra scrutiny because the existing slab was rarely designed as a dwelling foundation. Mountain snow is checked for western sites. Bring the site survey, the geotechnical report, and the architectural plans. Soil movement design is the item most ADU plans miss.

Use this checklist before engaging an engineer for your ADU: • Engineer of record holds an active Virginia PE license, verifiable on the state roster • Dimensioned floor plan and elevations ready for structural review • Site survey showing setbacks, easements, and existing structures • Geotechnical report addressing expansive clay soils for the lot • Foundation designed for soil movement, not just bearing capacity • Garage conversions: existing slab evaluated for dwelling loads • Engineer confirms they'll respond to plan-check corrections on sealed sheets • Seal shows name, Virginia PE number, and date on every sealed sheet