As I home owner, it is my understanding that I am allowed to change light switches, light fitting, and power points etc myself, but at what point does it become a job that you legally need an electrician to do it?

I have a single switch in the bathroom that when turns on both the light and extractor. I would like to change this to either 2 switches or a double switch so I can control the light and extractor individually.

I haven't yet looked behind the switch to see how the two things are wired. But depending on where in the circuit the extractor was wired may mean I need to run additional cable from the roof cavity to the switch position. Or if it is all already behind the switch it may be a lot more straight forward. I will not need to touch the main switchboard (except to turn off the power while I work). At what point in this sort of job does it cross the line of what I am allowed to do myself?

I want to do it myself as some of our wiring is still the old conduit stuff, and for an electrician to work on it I think they are required to replace it. As the home owner I think I get more leeway in that regard. Someone correct me if I'm wrong.