On my side of the pond, if operating under a Part 135 (Air Taxi) certificate the use of extensions will/should be spelled out in the submitted ops specs or 135 maintenance manual. For example, in the EC135 AMM it offers a plus/minus variance in numerous inspections that do not penalize the operator if complied with X hrs before or X hrs after the standard schedule of say 400hrs. However, in order to use that program it must be indicated in the ops specs/manual of record. It is normally not a field function to use it or not unless your a private operator which is another entirely different scenario.
On a side note, some generic extensions found in AD's or in a regulatory format are usually for "scheduling" said aircraft remaining flight time to get to a facility to comply with said maintenance--but not to be used for revenue flights. But this example is a big gray area.
W1