Delay master's thesis: Appropriate to mention personal reason? I'm at a European university, where I'm currently writing my thesis to finish a master's degree. The deadline is in a bit over a month. Due to a serious family issue, I have not been able to work much, thus I likely need to postpone the submission deadline (which is allowed without providing a detailed reason).

I have not had much supervision (not unusual at my uni), and, apart from not being able to work much, the project is going fine. My concern is that I could be subject to unconscious lazy-student bias at grading if I dont tell him the (at least vague) reason for my delay.

Is it appropriate or relevant to mention the reason for my delay to my supervisor? Or should I simply tell the supervisor that I have to delay the submission date? 

Thanks for your input.
I don't see what would be inappropriate here. I would welcome it if my students would feel enough at ease with me to confide the reasons why their projects are running later than planned.