The city's seismic ordinance classifies the various building code occupancies into five "relative hazard" levels. A warehouse use (an S occupancy) is classified at the very lowest hazard level, and is usually the most sparsely occupied, so a change of occupancy to almost any other use will often trigger the requirement for a seismic upgrade.
When the change in use is between occupancies at the same relative hazard, such as S (warehouse) to F (manufacturing), an upgrade is required if the change adds more than 149 occupants to the building code calculated occupant load. So it may be possible to change a warehouse building with less than 18,000 square feet of floor area to manufacturing use without a seismic upgrade.
When the change in use is to a higher relative hazard occupancy (retail, office, etc.), you are generally required to upgrade the entire building if more than 1/3rd of the area of the building is changed, or if the change results in an increase of more than 149 occupants. The triggers for seismic (area and occupant load) are cumulative over time, so if a portion of the building has already been changed without requiring an upgrade, your proposed project may cross over the allowed thresholds, so it is important to research the history of your building.
For more information on how seismic triggers would apply to your building, call a Life Safety Reviewer at (503) 823-1456.