What opening size should i be leaving when i install a velux.

+45mm to height

+40-60mm to width.

these sizes are based on the size of the product you purchase - so an MK04 (78x98) would require a frame out of approx. 82x102.5.

further instructions can be found here: 454054-2016-09_EDW_Flex.indd (velux.com)