Most of these websites suggest only measuring from the outside then deducting 10mm for clearance, thats ok for these red brick houses for fitting from the outside but is different for other house's, the one method for all does not work.
To be honest though. Double glazed windows aren’t worth it. The aluminium window in this video is a double glazed window and is around 40 years. I’m assuming it will be replaced with UPVC. Upvc windows aren’t that good. There aren’t any that have survived more than 30 years. Just stick with wood
Aluminium windows are so dated timber framed windows are so much better provide natural ventilation as opposed to UPVC windows which tend to cause condensation because of lack of ventilation
