Try Mocking Up Your Structures Before You Build Them
Before I build the other structures on the 2x4 foot N scale project layout, I mocked up all of the structures I want to add with blocks of insulation foam board. It is a lot easier and cheaper to cut and redo foam structures until you get the structure sizes and arrangement figured out before you build the actual structures. Since most of of the structures on this layout will be kit-bashed because of space constraints, mocking-up the structures first is a good way to make sure the structures will work on the layout. The foam mock-ups can also serve as a base to build the final structures around.
You can watch the video on these structure mock-ups below: