Finishing a commercial interior almost always happens while the building is in use: neighbouring offices keep working, and access is limited in time.
Order of works
First-fix and services come first, then walls, ceilings and floors. Final finishes wait until the end, so damaged work does not have to be redone.
Common areas
Lobbies, stairs and toilets are done in separate bays, so the building stays passable through the whole refit.