Is an Engineer Stamp Needed for My ADU Project in New Hampshire?

New Hampshire's municipalities — Manchester, Nashua, Concord, Derry — review structural drawings for accessory dwellings, and the PE seal is the standard requirement. The seal must show the engineer's name, New Hampshire 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 frost line — ledge and till soils are common — sizes beams, headers, and the lateral system, and checks heavy snow and wind design. Garage conversions get extra scrutiny because the existing slab was rarely designed as a dwelling foundation. White Mountains snow loads are checked for northern sites. 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 New Hampshire PE license, verifiable on the state roster • Dimensioned floor plan and elevations ready for structural review • Site survey showing setbacks, easements, and existing structures • Frost-depth compliance verified for footings, plumbing, and equipment • Snow loads matched to the site jurisdiction • Garage conversions: existing slab evaluated for dwelling loads • Engineer confirms they'll respond to plan-check corrections on sealed sheets • Seal shows name, New Hampshire PE number, and date on every sealed sheet