Is 65 cm the maximum storage height or the maximum headroom? Many ladders
need to extend vertically above the hatch by almost the length of one
section as they are being pushed up, before then pivoting so they are
horizontal.

65cm is the space between the floor of the loft and the beam above, having looked at the spec of ordinary 3 part loft ladders they need 75cm of headroom but once fully parked they probably only take up about 30cm in height.