It may be a dumb question, but I don't know the answer so I'll ask it
anyway.

I'm planning on building floor-to-ceiling bookshelves completely
covering one wall (at least). The room is currently carpeted, subject
to change to hardwood in the future.

Do I build these shelves directly on the carpet, or do I go to the
subfloor and trim the carpet to match?

If it were a hardwood floor, same question: on top of the hardwood or not?

thanks,
-j