I have to adapt (or make a new one) a frame (aka liner) to an internal door. After looking through several joinery catalogues I'm totally in the dark as to what the correct allowance is to leave around the door. The catalogues only seem to give door sizes & external frame sizes - never the internal opening sizes.
The door is 1981 x 762 (ie imperial 6'6" x 2'6"). What size should the opening be? Does it vary with the door thickness?
I've checked quite a few different doors with widely varying answers.
Would the answer be the same for an external door?
TIA