When the use of a building changes from one occupancy to another, say a store to a restaurant, you certainly need an architect to help you. The architect deals with exiting, fixture counts, layout issues and provides the stamped plans that you’ll need for the building department. When that change happens in a space that isn’t on a slab on grade, you may also need a structural engineer.
In Chapter 16 of the International Building Code there is a table that details the floor bearing capacity for different occupancy groups. Each group has a uniform distributed load that it must meet and many have an additional requirement for concentrated loads. They vary quite a bit from one occupancy to another and often the building department will want an evaluation of the system to see if it works.
As an example a restaurant requires a 100 psf distributed load capacity. If the space was previously an office it might be designed for as little as 50 psf. Worse yet if it is old enough it may be designed for even less. Again, a structural engineer needs to be brought in to evaluate and provide a solution that matches your exact needs.
As architects and engineers we can provide all the services to meet the building department requirements.