I have a website that has a menu at the top. On some pages there is a sub menu that is at the left hand side, but this is only on some pages. How then would be best to design the page layout given that some pages wont have a left hand nav column? Should I have the column anyway but just make it empty for consistency? Or increase the page size to take into account the extra space from no navigation? Or keep the page the same size and omit a left hand column?
Don't think of it as the normal state sometimes leaving a blank space at the left, but think of it as a full width page sometimes indenting to allow for a navigation column. Suddenly all your issues of worrying about a blank area are gone. So long as your main content can flow accordingly there's no problem with this.
Use the hamburger menu item present on the top left/ top right corner of your screen to toggle a side menu/side-bar.
This way when you require the menu, clicking on the icon will display the menu. If there is no item present in the menu, then the menu can be blank (with a message saying - 'there is no action to be performed here'/ 'Nothing to see here')
This style will allow you to have a full screen page width and also maintain a consistent layout throughout your site rather than two separate layouts for pages with and without a menu.