Good Morning Mary, The hallway is 48 inches wide and the shelves are 12 inches deep. The width of the hallway accommodates the width of the 3'-0" door with 4" trim at the end of the hall. We have done 9" deep shelves but this client had some larger books.

Remember that you need enough space to move furniture through the hallway...you may move or buy something that needs to be taken down the hall. 48” of open space should be enough for most pieces.