Accurate measuring and cutting are required when planning the fitting of a new door, the fitting of an exterior PVC or composite door is always a bit more complex, a concealed timber sub-frame will help get over any existing opening problems such as the brickwork not being square.
Always measure the top, middle and bottom widths from the inside and check the height, repeat the process for the outside sizes, in most cases if the door/frame being replaced is timber it is better to measure for removing the existing timber frame so as to keep the opening sizes as large as is possible but please remember that most PVC and Composite frames/doors are made to a maximum size of 940mm overall.
Note; if the sizes between the existing timber frame are large enough (and good enough condition) to ensure that any PVC or composite door, when fitted between, it will allow a reasonable final size of opening then it could be best practice to leave the existing frame in place and fit the new door/frame within it....and yes that will mean the new actual opening door size will be smaller.
ALWAYS GIVE YOUR MANUFACTURER THE TIGHT/OVERALL SIZES LESS a total of 6mm IN HEIGHT AND WIDTH.
When fitting the new door you will need to "TOE & HEAL" the frame, this means, after getting it level on the top, the side and the face with a spirit level you may need to alter the hinges on the vertical or lateral planes, normally the hinges have a "cam" or or offset centre sleeve which pulls the door in, out or across, adjust as required, no matter what the spirit level tells you!
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How to Measure for an Exterior PVC Door.