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

Colorado cities — Denver, Colorado Springs, Fort Collins, Boulder, Aurora — review structural drawings for accessory dwellings, and the PE seal is the standard requirement. The seal must show the engineer's name, Colorado PE number, and date, and the engineer must be in responsible charge of the engineering. Detached ADUs, garage conversions, and basement ADUs are treated the same at the structural level. Owner-builders don't escape the requirement: the building department asks for the seal regardless of who drafted the plans.

The engineer reviews the floor plan and elevations, designs the foundation for the site's expansive clays — drilled piers and structural slabs are common on the Front Range — sizes beams, headers, and the lateral system, and checks snow and wind design for the site elevation. Garage conversions get extra scrutiny because the existing slab was rarely designed as a dwelling foundation. Mountain snow loads are checked for high-country sites. Bring the site survey, the geotechnical report, and the architectural plans. Expansive-soil design is the item most ADU plans miss.

Use this checklist before engaging an engineer for your ADU: • Engineer of record holds an active Colorado 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 for the specific lot • Foundation designed for soil movement — drilled piers or structural slab • Garage conversions: existing slab evaluated for dwelling loads • Engineer confirms they'll respond to plan-check corrections on sealed sheets • Seal shows name, Colorado PE number, and date on every sealed sheet