Can the cat flap be installed in glass, and what size hole is required?

The cat flap can be installed in both single-glazed and double-glazed glass. Single-glazed glass requires a circular hole with a diameter of 215mm (8 7/16"), while double-glazed glass requires a circular hole of 260mm (10 1/4") and an Installation Adaptor sold separately. Working with glass must be carried out by a professional glazier, and it is not possible to cut holes in toughened or double-glazed units except at the time of manufacture. (Page 6)