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

Montana's municipalities — Billings, Missoula, Bozeman, Great Falls — review structural drawings for accessory dwellings, and the PE seal is the standard requirement. The seal must show the engineer's name, Montana 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 building department asks for the seal regardless of who drafted the plans.

The engineer reviews the floor plan and elevations, designs the foundation below the deep frost line — mountain sites get extra attention — sizes beams, headers, and the lateral system, and checks heavy snow, high wind, and seismic design. Western Montana's seismic demand is checked where triggered. Garage conversions get extra scrutiny because the existing slab was rarely designed as a dwelling foundation. Bring the site survey, the geotechnical report, and the architectural plans. Snow-load errors are the item most ADU plans miss.

Use this checklist before engaging an engineer for your ADU: • Engineer of record holds an active Montana PE license, verifiable on the state roster • Dimensioned floor plan and elevations ready for structural review • Site survey showing setbacks, easements, and existing structures • Snow loads matched to the site elevation • Frost-depth compliance verified for footings, plumbing, and equipment • Garage conversions: existing slab evaluated for dwelling loads • Engineer confirms they'll respond to plan-check corrections on sealed sheets • Seal shows name, Montana PE number, and date on every sealed sheet