The zoning code has a provision, Applicability §18:1-139, that permits additions to existing non-residential buildings to be exempt from a site plan submittal. The short name is usually referred to as the "10% rule".
To figure out if an addition is exempt, the zoning district must be determined. Referring to the original site plan for the subject property is usually the best way to start. Once the zone is known, the Floor Area Ratio ("FAR") for that zone must be determined and calculated. After calculating the FAR, 10% of that number is the exemption limit.
For example, in the Urban Commercial zoning district, the FAR is 40%. If the subject property is 2.5 acres, the FAR permitted will be 1.0 acre, or 43,560 square feet, and 10% of 1.0 acre is 4,356 square feet. This is the square footage that an addition, or additions cumulatively, may be permitted up to before a site plan is required.
Providing this information on your original site plan simply requires these calculations along with the amount of square footage proposed for the addition. No formal applications are required and the calculations can be written on a copy of the site plan which staff will review and sign off on.