Rowling told the BBC, "I will go through a mourning period, then I will have to think of something else to write."
The author also added that writing the final book of the series was "fun in a way it hasn’t been before".
Rowling, 41, recently read extracts from her novel at a Stephen King charity event in New York. The event raised money for Doctors Without Borders and The Haven, and was Rowling’s first American appearance in six years.
In the final book, two of the main characters will die, although Rowling did not share any more details.