We’ve done this type of project before. What did it cost last time? Is that the final cost or was it a budget?
Is it a hospital, refinery, or treatment plant? What information is available? They call these construction documents? Who is the client? The design team? The contractor? They want it when?
Typically a project goes from concept to design to construction. Sometimes it goes from concept to construction to design to change orders.
Compare it to baking something. You grab a recipe, gather ingredients, mix, and bake. Even using the same recipe, some ingredients might change and the temperature and time to bake may vary. Do you expect the same outcome? Maybe not.
If you are dealing with the same project type and the same phase, don’t expect the costs to match unless the same story is being told.