The answer provided in this question does not work with classicthesis: Double line spacing
\doublespacing does not work. Using
\linespread does but this introduces spacing in e.g. figure captions and tables too, which is not optimal.
Classicthesis can be found here: http://www.ctan.org/pkg/classicthesis
Since Classicthesis is used a lot for PhD theses, and a lot of these are required to be double spaced, I'm probably not the only one who's had this problem. Somebody had previously asked this question and it was (erroneously?) marked as a duplicate, linking back to the question above.
I'm not sure if I need to edit something in classicthesis.sty itself?