Hi Tina, you talk about speaking to your supervisor, but it's probably worth contacting your research student office/postgrad office at your uni. These things usually tend to be decided at a university-wide level. IE: they'll have general guidelines, then look at individual cases to see if extensions are granted.
Our research registrar is quite flexible: we are supposed to hand in by 1st Oct, but as this is a weekend, they will let us hand in on Monday or Tues if we wish to use this weekend. Bottom line is, they will give afew days - anything extending into weeks needs authorisation. Hope that helps!