It's good to know that you are setting up office premises. First of all, the interiors play a vital role in any organization. After flooring is completed, you should start with the furnishing, such as tables, chairs, seating arrangements, etc. For that, you should start the process immediately so that as soon as the flooring is completed, you will be in a position to start with all seating arrangements.
Along with seating, you should start with Venetian blinds, ACs, cabin arrangements, a place for the server room, UPS room, and other facilities, etc. Since I have coordinated from construction to completion, including interiors for our own office premises of a 4-floor building, I have good knowledge of this setup.
You should also obtain copies of the building's approved drawings and occupancy certificate from the building owner.