Does Paradise Lost suggest that Hell already existed prior to the creation of Earth?
The answer to this question can be found in Book II of this epic poem, which is when Satan confers with his minions and they decide what they should do. Notice how earth is referred to in the following quote:
There is a place
(If ancient and prophetic fame in Heav'n
Err not) another World, the happy seat
Of some new Race call'd Man, about this time
To be created like to us, though less
In power and excellence, but favour'd more
Of him who rules above;
There is no specific timing given, but it appears that Earth and man were created at about the same time as Satan and his minions were cast out of heaven and sent down to the fiery pit that is there new home. Therefore hell as a place where Satan and his other fallen angels belong seems to have been created roughly at the same time as Earth and Adam and Eve were created. This makes them interesting parallel locations in this text.