Having inside knowledge is incredibly helpful for any engagement.
I'm not saying that you should just ask the client for information.
You should try to use other means that criminals could obtain.
For example,
look on local council or state sites for planning permission applications.
These often require building schematics and can be incredibly useful.
Another method is to search for architects that may have worked on the building.
Often their sites will include internal photos that could be useful.
One of the best sources that are always up to date are fire and rescue maps that
are required to be displayed in lobbies of large buildings.
Often these are in public areas and can be photographed
before engagement for additional planning.
You may even be able to extract out ideas of other flaws using those maps.