To be fully insured, you’ll need to complete a separate extensions course unless they were covered in your (level 3) training.
Your insurance will only cover the units you‘ve passed an assessment on. For instance, not all hairdressers will have completed training on Perming or Chemical Straightening either. It depends on what course options you’ve taken.
On my level 3, you could choose either advanced perming or hair extensions, so I did advanced perming. You can pick up a short course in hair extensions much more easily afterwards, but I’ve not seen any advanced perming courses offered as a separate course.