Adding Walls to my Rendered Model

I have used Josh Lewis's skeleton structure model that he shared with the class as the shell for my retail store. He has used the documents that I have sourced from the Wellington Archives, as well as site visits and photographs to model the structure of the first floor and where the stairs enter the second floor..

I have used this model to then import my retail store into. It is not a complete render but it gives the idea and representation of what my store will look like with walls like if built. I am not sure why the beams holding the glass railing have gone black, but I'm sure I can work out how to fix it. I have also added the material preset of 'car paint' to the floor and walls within the store to give it a slight shine and reflectiveness.