As seen in the pictures, HOK's design is to include green roofs on many of the buildings.
These buildings "Up to 80 percent of the library’s total collection of books and the entirety of its newspaper archive..." This is a significant campus design and requires special concerns to protect the items in the collection.
HOK states, "Once completed the Boston Spa site would be the most advanced in the world and, by 2046, the whole of the Library’s collection would be stored in facilities meeting the BS5454 archival storage standard."

Here is another picture of the planned facility.